GRUPPO ITALIANO DI GEOLOGIA STRUTTURALE
RIUNIONE ANNUALE 2002
Pisa, 11 - 12 giugno 2002
Architettura interna e cinematica delle zone di taglio
ELENCO DEI CONTRIBUTI
R. Anczkiewicz , G. Viola
VERTICAL EXTRUSION OF THE DAY NUI CON VOI MASSIF,
SEISMICPUMPING AND NE-SW ORIENTED EXTENSION WITHIN THE RED
RIVER SHEAR ZONE IN N-VIETNAM ................................................................................. p.
1
A. Billi
SOLUTION SLIP AND SEPARATIONS ON STRIKE-SLIP FAULT ZONES ...................... p.
2
A. Billi, F. Salvini, F. Storti
THE TEMPORAL AND SPATIAL TRANSITION FROM DAMAGE ZONES TO
FAULT CORES IN CARBONATE ROCKS .......................................................................... p.
2
A. Bistacchi, G.V. Dal Piaz, M. Massironi
ORTHORHOMBIC NORMAL FAULTS AND LATE-COLLISIONAL POST-NAPPE
EXTENSION IN THE AOSTA VALLEY................................................................................ p.
2
S. Carruba, C. Perotti, R. Casnedi, A. Ravaglia
TECTONIC SETTING OF THE PLIO-PLEISTOCENE PERIADRIATIC FOREDEEP
IN THE ABRUZZO AREA (ITALY)....................................................................................... p.
4
G. Cello, C. Invernizzi, L. Marchegiani, L. Mattioni, S. Mazzoli, E. Tondi
FAULT ZONE EVOLUTION FROM CONJUGATE BRITTLE-DUCTILE SHEAR
ZONES IN LIMESTONES: A CASE STUDY FROM MONTE CUGNONE (PZ),
SOUTHERN APENNINES .................................................................................................... p.
5
M. Cesarano, P.P. Pierantoni, G. Santarelli, E. Turco
ESTENSIONE TIRRENICA NELLA ZONA DI INTERAZIONE DEI DUE ARCHI
APPENNINICI (M. LEPINI, M. AUSONI E M. AURUNCI, ITALIA CENTRALE) ................ p.
6
i
C. Collettini, M.R. Barchi
THE ENIGMATIC LOW-ANGLE NORMAL FAULTS: THE CASE OF THE
NORTHERN APENNINES .................................................................................................... p.
7
E. Costa, F. Speranza
VERTICAL AXIS ROTATION ALONG STRIKE-SLIP FAULTS .......................................... p.
8
L. Crispini, G.Capponi
CONTROL OF METASOMATIC ALTERATION ON THE DEVELOPMENT OF
SHEAR ZONES IN ULTRAMAFIC ROCKS: A CASE STUDY FROM THE VOLTRI
MASSIF (LIGURIAN ALPS) ................................................................................................ p.
9
G.V. Dal Piaz, S. Martin, I.M. Spalla
PERMIAN AND TRIASSIC MAGMATISM AS A MARK OF TECTONO-THERMAL
EVOLUTION OF THE ADRIATIC LITHOSPHERE ............................................................. p. 10
G. Di Toro , D.L. Goldsby , T.E. Tullis
DRAMATIC HIGH SPEED VELOCITY DEPENDENCE OF QUARTZ FRICTION
WITHOUT MELTING ............................................................................................................ p. 12
G. Di Toro, G. Pennacchioni
SUPERHEATED FRICTIONAL MELTS IN ZONED PSEUDOTACHYLYTES OF
THE GOLE LARGHE – V. GENOVA FAULT ZONE (ADAMELLO MASSIF,
ITALIAN SOUTHERN ALPS) .............................................................................................. p. 13
M.G. Fellin, S. Martin, M. Massironi
POLYPHASE NEOGENE FAULT KINEMATICS AT THE HANGING WALL OF
THE NORTH GIUDICARIE FAULT (WESTERN TRENTINO, CENTRALEASTERN ALPS) ................................................................................................................ p. 13
V. Festa, A. Caggianelli, D. Liotta, G. Prosser, A. Del Moro
ASPETTI PETROLOGICI E STRUTTURALI DI UNA ZONA DI TAGLIO DUTTILE
IN GRANITOIDI TARDO-ERCINICI (SILA, CALABRIA) .................................................... p. 14
,
A. Frigeri G. Minelli, C. Pauselli
STRESS DISTRIBUTION IN AN ANTICLINE; A NUMERICAL APPROACH..................... p. 15
R. Gambillara , F.Quattrocchi , M. Massironi e S. Martin
RELATIONSHIPS BETWEEN GROUNDWATER SYSTEMS AND TECTONIC
STRUCTURES IN THE WESTERN VENETO FOOTHILLS ............................................... p. 15
F. Ghisetti, M. Grasso, R. Maniscalco, L. Vezzani
PLIO-PLEISTOCENE STRUCTURAL EVOLUTION OF THE GELA NAPPE AND
ASSOCIATED FOLD STRUCTURES IN CENTRAL-SOUTH SICILY................................. p. 16
M. Grasso, R. Maniscalco, R.W.H. Butler
STRATIGRAPHIC EVIDENCE FOR THE TECTONIC EVOLUTION OF THE
MOUNT ALTESINA THRUST SYSTEM, CENTRAL SICILY............................................... p. 17
ii
M. Malusà, S. Martin, R. Polino
STRUCTURAL SETTING OF THE “GRAN SAN BERNARDO NAPPE” ALONG
THE SOUTHERN AOSTA VALLEY TRANSECT (WESTERN ALPS, ITALY) ................... p. 18
S. Martin
THE ULTEN UNIT: AN EXAMPLE OF STRENGHT COMPETITION IN
SUBDUCTION ZONE ........................................................................................................... p. 19
M. Massironi, A. Bistacchi, R. Brandner,G.V. Dal Piaz, B. Monopoli, A. Schiavo
THE SPRECHENSTEIN-VAL DI MULES LINE: A TECTONIC LINKAGE
BETWEEN BRENNER AND PUSTERIA FAULT SYSTEMS .............................................. p. 20
S. Mazzoli, D. Di Bucci, C. Invernizzi
EVOLUTION OF “BRITTLE-DUCTILE” SHEAR ZONE ARCHITECTURE,
DISPLACEMENT AND SHEAR STRAIN DURING NORMAL FAULT
NUCLEATION IN LIMESTONE: STRUCTURAL AND FLUID INCLUSION
ANALYSIS ............................................................................................................................ p. 21
F. Piana, P. Perello
FAULT ZONES ARCHITECTURE: HOW TO SEPARATE FAULT SEGMENTS
INTO HIERARCHIC ORDERS? - FIELD EXAMPLES......................................................... p. 21
F. Storti, A. Billi, F. Salvini
PARTICLE SIZE DISTRIBUTIONS AND THE EVOLUTION OF CATACLASIS IN
CARBONATE FAULT ROCKS ............................................................................................ p. 22
S.Tumiati, S. Martin, P. Nimis
DEFORMATION IN THE ULTRAMAFIC ROCKS FROM A SUBDUCTION ZONE.
CASE STUDIED: THE HOCHWART PERIDOTITES (ULTEN ZONE) ............................... p. 23
G. Viola, R. Anczkiewicz
MYLONITIZATION PROCESSES IN THE RED RIVER SHEAR ZONE,
NORTHERN VIETNAM: MICROSTRUCTURAL EVIDENCE AND TECTONIC
IMPLICATIONS..................................................................................................................... p. 24
D. Zampieri, M. Massironi, V. Sparacino
CENOZOIC KINEMATIC EVOLUTION OF A MAJOR STRIKE-SLIP FAULT: THE
SCHIO-VICENZA FAULT, NORTH ITALY........................................................................... p. 25
iii
VERTICAL EXTRUSION OF THE DAY NUI
CON VOI MASSIF, SEISMIC PUMPING
AND NE-SW ORIENTED EXTENSION
WITHIN THE RED RIVER SHEAR ZONE IN
N-VIETNAM
1
2
R. Anczkiewicz , G. Viola
Institute of Geological Sciences, Polish
Academy of Sciences, ul. Senacka 1,
Krakow, Poland
2
Department of Geological Sciences,
University of Cape Town, 7701 Rondebosch,
South Africa
1
Synopsis:
The Red River Shear Zone (RRSZ) has
been described as a left-lateral fault
absorbing significant amount of postcollisional
India-Asia
convergence
by
accommodating lateral extrusion of the
Indochina block. The most valuable
information on the RRSZ comes from studies
of the metamorphic massifs exposed within
the shear zone itself. Geochronological and
PT data obtained for the Ailao Shan (Yunnan,
S China) and the Day Nui Con Voi (N
Vietnam) massifs indicate a period of rapid
cooling (100° C/Ma), which was attributed to
uplift and exhumation in a transtensional
setting. Yet, no structural evidence had been
found supporting this interpretation. In this
study we present localized structural
evidence for a period of major extensional
movement along the RRSZ, which postdated
strike-slip motion. However, despite this
major transtensional phase, the regional
structural setting requires a more effective
exhumation mechanism in order to exhume
the amphibolite facies metamorphic massifs
in this flat-lying stretching lineationdominated strike-slip fault. We present strong
evidence for a seismic pumping-driven
exhumation.
Moreover, the timing of shearing, despite
its crucial importance for evaluating the
significance
of
lateral
extrusion
in
accommodating India-Asia convergence, is
still debatable. We thus provide Sm-Nd
garnet chronology, which yields an upper age
limit for the beginning of left lateral shearing
along the fault zone in N Vietnam.
Structural setting:
The NW-SE striking Day Nui Con Voi
(DNCV) massif bears record of the RRSZ
activity in N Vietnam. The massif is bounded
by the SW dipping Song Hong fault in the
southwest and by the NE dipping Song Chay
fault in the northeast. The massif comprises
mainly mylonitic gneisses, stromitic gneisses,
proper
migmatites,
amphibolites
and
abundant late tectonic leucogranites. Early
deformation of DNCV is related to left-lateral
NW-SE oriented shearing, which resulted in a
complex antiformal structure with axis parallel
to the subhorizontal stretching lineation. This
phase of shearing took place under high
temperature conditions. The foliation attitude
is very steep to subvertical in the limbs of the
antiform (bound by the two discrete faults
described above) and tends to flatten out in
the middle part of the antiform to very gentle
dip angles. There is a clear strain gradient
with
the
strongest
mylonitic
fabric
concentrated on the limbs of the antiform.
Most of the stromatic gneisses, migmatites
and leucogranites are located in the core of
the structure. The kinematics is consistently
sinistral. Symmetric upright folds were
produced during this deformation phase.
A second, ductile phase of deformation is
associated with extensional movement
accommodated by the two faults bounding
the DNCV massif. This is mostly inferred
from asymmetric NW-SE trending folds with
gently to moderately NE or SW dipping axial
planes. The fold vergence is in accordance
with the dip direction of the two faults (i.e. SE
for the Song Hong and NE for the Song Chay
fault), which, together with stretching
lineation oriented at high angle to fold axes,
indicate normal sense of shear. Kinematic
indicators are consistent with the sense of
shear deduced from fold vergence. We
interpret these folds as resulting from vertical
shortening associated with rapid uplift of the
DNCV during northeast-southwest oriented
extension. Locally, superposition of earlier
upright folds with later “drag or collapse” type
folds resulted in a mushroom interference
type
pattern,
which
supports
our
interpretation of a two-stage evolution.
Recent field work showed the existence of
peridotitic lenses (of lithospheric mantle
isotopic composition) in the core of the DNCV
antiform. There is also common evidence of
hydraulic fracturing phenomena and intense
fluid activity in the core of the DNCV
antiformal structure. Based on these
observations, which point to a lithospherically
rooted RRSZ, we suggest that seismic
pumping might have been the trigger for the
extremely
rapid
exhumation
of
the
metamorphic massifs in the shear zone. The
fault zone, which acted as a narrow, steep
channel,
represents
a
crustal-scale
preferential pathway for the “intrusion” (driven
by sudden changes in the lithostatic vs.
iv
hydrostatic pressure ratio) of amphibolite
facies rocks and even mantle slices and for
their continuous shearing in the left-lateral
regime. The lateral distribution of rocks
across the shear zone is such that the
peridotitic slivers and the migmatites (which
originate from the deepest region sampled by
the fault zone) are found in the centre of the
DNCV antiform, reproducing the expected
velocity distribution (highest in the middle) for
a fluid in a narrow pipe. The seismic cycle
along this continental scale fault would drive
this process in time and space. Instead, the
extensional phase described above seems to
accommodate locally the extremely rapid
uplift of the rocks to shallower levels.
Preliminary results of Sm-Nd garnet
dating on sillimanite gneisses from the fault
zone gave a mineral isochron age of 32 ± 3
Ma.
Peak
metamorphic
temperature
established at T = 690 +30/-60° C is below
closure temperature for Nd diffusion in garnet
for fast cooling rocks. Therefore, we interpret
this age as approximating the time of garnet
crystallization. Textural relationships indicate
that garnet is syn-prekinematic relative to the
left-lateral shearing; hence, our date provides
a maximum age for the beginning of left –
lateral shearing along the RRSZ in NVietnam. The new age, together with new,
preliminary zircon and apatite fission-track
ages, establish a time interval from ~ 32 Ma
to 23-25 Ma for cooling from 700 to 250-110°
C. This places rather tight time constraints on
the main period of RRSZ activity, comprising
both, strike-slip movement and a major
extensional phase, during which vertical
extrusion of the DNCV massif took place.
fault zones. The fault zone shortening
produces an "apparent slip" and possible
separations
of
reference
stratigraphic
surfaces across the fault zone. Solution
related slip and separations can differ in
magnitude and have either the same or the
opposite sense. These discrepancies depend
upon the amount of fault zone shortening and
upon the angles between the fault and the
shortening axis, and between the fault and
the
reference
stratigraphic
surface.
Separations can be considerable at any
scales even for very low amount of fault zone
thinning. Apparent slip can be appreciable for
large amount of fault zone thinning and/or
high fault-to-cleavage incidence angles. With
the proper geometrical conversions, the
relationships here presented can apply to any
fault type.
The application of this technique to the
left-lateral
Mattinata
Fault,
Italy,
demonstrated that both left- and right-lateral
strike separations can occur along the fault
even for low amounts of fault zone
contraction by rock dissolution.
SOLUTION SLIP AND SEPARATIONS ON
STRIKE-SLIP FAULT ZONES
1
Andrea Billi
Dipartimento di Scienze Geologiche,
Università "Roma Tre", L.go S.L. Murialdo 1,
00146, Rome, Italy ([email protected])
1
I present a set of relationships to
determine the component of slip and
separations generated by the cleavagecontrolled volume contraction in strike-slip
fault zones. The fault walls can translate
toward each other along the (cleavagenormal) axis of maximum shortening as rock
is dissolved by pressure solution along
patterned cleavage surfaces within strike-slip
v
THE
TEMPORAL
AND
SPATIAL
TRANSITION FROM DAMAGE ZONES TO
FAULT CORES IN CARBONATE ROCKS
1
1
Andrea Billi , Francesco Salvini , Fabrizio
1
Storti
1
Dipartimento di Scienze Geologiche,
Università "Roma Tre", L.go S.L. Murialdo 1,
00146, Rome, Italy ([email protected];
[email protected]; [email protected])
We studied the nucleation and growth of
cataclastic fault cores from fractured damage
zones in extensional and strike-slip fault
zones in carbonate rocks. Analysed fault
zones have similar lithology and sedimentary
fabric of the protolith, but different geometry,
kinematics, size, tectonic environment and
deformation history. Orthorhombic rock
lithons, a few decimetres in size, characterise
the structural fabric of damage zones.
Lithons derive from the intersection of a
dominant fracture/cleavage set with bedding
and/or joints. At the damage zone-fault core
transition, orthorhombic lithons reduce in size
and approach an isometric shape. Their
cross-sectional aspect ratio has an average
value of 1.4. Analysed fault cores have
similar
rock
textures,
sorting
and
comminution
degree.
Particle
size
distributions of fault core rocks show linear
trends in log-log graphs. Their average fractal
dimension is 2.5. Our results on rock fabrics
suggest that fault core development initiates
from rock masses in damage zones, where
the shape anisotropy of orthorhombic lithons
favours additional fracturing at high angle to
their long axes. Eventually, smaller, nearly
isometric lithons generate from repeated
fracturing of orthorhombic lithons. When the
aspect ratio of these lithons approaches the
threshold value of about 1.4, particle rotation
is favoured and cataclastic flow starts. Owing
to the granular nature of the damage zonefault core transitions in carbonate rocks,
analogies with the nucleation of deformation
bands in sandstones can be established.
ORTHORHOMBIC NORMAL FAULTS AND
LATE-COLLISIONAL
POST-NAPPE
EXTEN-SION IN THE AOSTA VALLEY
Andrea Bistacchi, Giorgio V. Dal Piaz, Matteo
Massironi
Dipartimento di Geologia – Università di
Padova – via Giotto, 1, 35173, Padova
The Oligocene final exhumation of the
Penninic-Austroalpine wedge in the middle
and lower Aosta Valley (Dal Piaz, 1999),
Italian NW-Alps, caused progressive cooling
(fission tracks evidence) of the collisional
nappe stack, which progressively changed its
rheological behaviour from “ductile” to
“brittle”. In the last few years a complex
network of normal faults has been recognized
by satellite image interpretation and field
work. These faults dissect the nappe stack
into fault-bounded blocks showing differential
exhumation histories (Bistacchi & Massironi,
2000, Bistacchi et al., 2001). Fault-related
rocks, developed on different metamorphic
cover and basement protoliths, highlight a
ductile-to-brittle evolution mainly related to
progressive cooling, but that could also be a
result of increasing strain localization.
Post-nappe faults, which developed in the
Oligocene (isotope dating of concurrent
hydrothermal activity and lamproitic dykes;
Bistacchi et al., 2001, and refs. therein), can
be grouped in four systems according to their
attitude:
(1) Low-angle NW- and SE-dipping brittleductile detachment horizons, developing in
relatively weak rocks, such as the Piedmont
calcschists. These faults are marked by wide
horizons
of
penetrative
extensional
crenulation cleavage and more localised
shear zones characterised by 20-200 cmthick layers of clayey-chloritic fault-gouge.
(2) Low-angle N-dipping detachments
showing the same features as set 1. This
system mainly developed in the southern
slope of the middle Aosta Valley, reactivating
the steep N-dipping greenschist facies
regional foliation.
(3) Intermediate to high-angle, NW- and
SE-dipping conjugate faults, homogeneously
developed all over the Aosta Valley region.
These faults show very different features,
depending on the host-rock. In carbonate
rocks (e.g. Piedmont calcschists), they
display a high frequency and polished
slickensides, always overprinting the lowangle structures (1 and 2), typically with
calcite fibrous steps, Riedel-type joints and
calcite-filled veins. In harder rocks, such as
gneisses
(Austroalpine
and
Penninic
basement), fault zones are more localised
and characterised by thick cataclastic
horizons (up to 500 m-thick) with abundant
pseudotachylites. In places, veining is
noticeably developed, indicating a focus of
hydrothermal fluid flow owing to the
enhanced permeability of fault zones. The
synkinematic gold-bearing quartz veins of the
vi
Brusson area result from this hydrothermal
activity, as well as a rather peculiar kind of
hydrothermal metasomatism that took place
where large faults cut through serpentinites.
The results of this process are fault breccias
(called listvenites) characterised by strong
enrichment in Ca, K and OH, with respect to
the original composition, and by a high
variability in Si content up to 70% (Diamond,
1990; Richard, 1981; Dal Piaz and Omenetto,
1978).
(4) Intermediate to high-angle N- and Sdipping conjugate faults, mainly along the
middle Aosta Valley, showing the same
features as set 3, but with strong asymmetry.
The master fault (Aosta-Ranzola system)
dips to the N and is characterised by strong
hydrothermal fluid flow; conversely, antithetic
S-dipping faults always show minor
displacements and veining. Focus of
hydrothermal fluids (with a rather deep
source) along the
Aosta-Ranzola fault
suggests that this great system probably
acted as a continuous high-permeability
channel at the crustal scale. Note that the
concurrent lamproitic dykes have to be feed
by mantle sources.
As we have seen, all these fault sets
show the same deformation mechanisms and
are associated with the same type of
hydrothermal fluid flow; hence they may be
considered coeval. In fact, given that thermal
conditions continuously changed during
exhumation,
similarity
of
deformation
mechanisms and hydrothermal mineral
assemblages may be considered as
evidence for simultaneity.
No systematic cross-cutting relationships
are associated with the strike of different
sets. Structures belonging to sets 1 and 2 are
developed in different areas, depending on
the distribution of sufficiently weak rocks
(mainly calcschists). Faults belonging to sets
3 and 4 cut each other repeatedly, and
therefore must be considered coeval from a
geometrical point of view.
Steep faults (3 and 4) consistently crosscut low-angle detachments (1 and 2) in weak
rocks, but the principal extension direction,
inferred from congruent kinematic indicators,
is everywhere horizontal NW-SE. These
relationships are interpreted as the effect of
the transition from brittle-ductile to colder,
truly brittle conditions during continuing NWSE extension. In some favourable exposures,
this
continuous
evolution
is
really
straightforward.
Taking into account these evidences, the
four sets listed above developed in response
to a single deformation phase, characterised
by an overall NW-SE extension (with vertical
shortening) and intense hydrothermal activity,
focused along two major faults: the AostaRanzola and Ospizio Sottile fault systems.
If the extensional activity may be referred
to a single tectonic phase, synchronous
displacements along these four conjugate
sets (N-, S-, NW-, SE-dipping) can be
interpreted with the 3D faulting model of
Aydin and Reches (1982). Accordingly,
conjugate displacements along four sets,
arranged in orthorhombic symmetry, take
place in response to a “truly triaxial” strain
field (Reches, 1978; Aydin and Reches,
1982; Reches and Dieterich, 1983; Reches,
1983). This model is more general than the
Anderson (1951) classic model, which
predicts displacement along two conjugate
faults in the restrictive condition of plane
strain; therefore, finding evidence of triaxial
strain is not surprising.
Applying the triaxial model to the middle
Aosta Valley, the maximum extension axis is
inferred to be
NNW-SSE oriented, the
intermediate axis is positive (extensional) and
ENE-WSW oriented, and the shortening axis
is vertical. Therefore, the Oligocene
“regional” strain ellipsoid is oblate, with a
NNW-SSE-directed maximum
extension
(perpendicular to the axis of the NW Alps).
REFERENCES
Anderson, E.M., 1951. The Dynamics of
Faulting and Dyke Formation with
Application to Britain, 2nd ed. Oliver &
Boyd, Edinburgh, 206 pp.
Aydin, A., Reches, Z., 1982. Number and
orientation of fault sets in the field and in
experiments. Geology, 10, 107-112.
Bistacchi, A., Dal Piaz, G.V., Massironi, M.,
Zattin, M., Balestrieri, M.L., 2001. The
Aosta-Ranzola extensional fault system
and Oligocene-Present evolution of the
Austroalpine-Penninic wedge in the northwestern Alps. Int. J. Earth Sciences (Geol.
Rundsch.) 90, 654-667.
Bistacchi, A., Eva, E., Massironi, M.,
Solarino, S., 2000. Miocene to Present
kinematics of the NW-Alps: evidences
from remote sensing, structural analysis,
seismotectonics and thermochronology. J.
Geodynam. 30 (1-2), 205-228.
vii
Bistacchi, A., Massironi, M., 2000. Postnappe brittle tectonics and kinematic
evolution of the north-western Alps: an
integrated approach. Tectonophysics,
327, 267-292.
Dal Piaz, G.V., 1999. The AustroalpinePiedmont nappe stack and the puzzle of
Alpine Tethys. In G. Gosso et al. (Eds):
Third Meeting on Alpine Geol. Studies,
Mem. Sci. Geol., 51, 155-176.
Dal Piaz, G.V., Omenetto, P., 1978. Brevi
note su alcune mineralizzazioni della falda
piemontese in Valle d'Aosta. Ofioliti 3
(2/3), 161-176.
Diamond, L.W., 1990. Fluid inclusion
evidence for P-V-T-X evolution of
hydrothermal solutions in Late-Alpine
gold-quartz veins at Brusson, Val d'Ayas,
northwest Italian Alps. Am. J. Sci., 290,
912-958.
Reches, Z., 1978. Analysis of faulting in
three-dimensional
strain
field.
Tectonophysics 47, 109-129.
Reches, Z., Dieterich, J.H., 1983. Faulting of
rocks in three-dimensional strain fields. I.
Failure of rocks in polyaxial, servo-control
experiments. Tectonophysics, 95, 111132.
Reches, Z., 1983. Faulting of rocks in threedimensional strain fields. II. Theoretical
analysis. Tectonophysics, 95, 133-156.
Richard, A., 1981. Le district aurifère de
Challant (Val d'Aoste; Italie): litologie,
gèochimie et metallogènie de l'or. Thèse
de doctorat de spècialitè, Universitè
Scientifique et Mèdicale de Grenoble.
TECTONIC SETTING OF THE PLIOPLEISTOCENE
PERIADRIATIC
FOREDEEP IN THE ABRUZZO AREA
(ITALY)
1
1
Stefano Carruba , Cesare Perotti , Raffaele
1
1
Casnedi , Antonio Ravaglia
1
Dipartimento di Scienze della Terra Università di Pavia, via Ferrata 1, 27100
Pavia, Italy
The Periadriatic foredeep basin lies
between the front of the Apenninic Thrust
Belt and the foreland zone of the Central
Adriatic Sea and is genetically linked to the
continental collision between the European
and the Adria plate, which underthrusts the
Apenninic chain.
The siliciclastic turbiditic sediments of the
Periadriatic foredeep in the Abruzzo sector
(Pliocene Cellino Fm. - Casnedi, 1983 -, and
more recent clastic sediments) were
deposited onto the Marche carbonatic
sequence, ranging from late Triassic
dolostones and evaporites (Burano Fm.) to
middle Messinian evaporites (GessosoSolfifera Fm.). These deposits were affected
by Plio-Pleistocene compressional events
related to the migration of the Apenninic
thrust front towards the foreland. (Bally et al.,
1986).
The foredeep deposists have been
intensively studied for several decades, being
one of the hydrocarbon exploration targets in
Italy (Casnedi, 1991; Ori et al., 1993). New
realised subsurface data (2D seismic lines
and geophysical well logs) provided
constraints for a better characterization of the
structural setting of the area.
At the present day, the substratum of the
outermost
sector
of
the
foredeep,
represented by the top of Messinian
evaporites, generally dips W and can be
divided into four blocks characterized by
different depth, ranging from more than 7000
m in the NW sector up to 4000-4500 m in the
SE sector. The substratum is affected by an
array of middle Messinian extensional faults
striking N170°-175°. The faults are conjugate,
arranged in en echelon pattern and form an
horst and graben system, 10 km wide. The
throw of the faults is quite variable, up to
1500 m. The faults do not involve the
overlying
siliciclastic
sequence.
The
parallelism
between
the
Apenninic
compressional front and the Messinian
extensional structures suggests that the
extensional event may be the response of the
flexural folding of the W-dipping subduction
viii
of the Adriatic lithosphere under the
Apenninic chain.
The
Plio-Pleistocene
compressional
deformation of the foredeep sediments
produced two main thrust systems, named
Internal Structure and Coastal structure. The
Internal Structure consists of two thrust
sheets, whose basal detachment level is
probably located in the Triassic evaporites
and which comprise the carbonatic
succession. The geometry of the Internal
Structure has been validated with a balanced
cross-section. The basal detachment level of
the Coastal Structure corresponds to the
Messinian evaporites. The Coastal Structure
is an imbricate fan; it is parallel to the
extensional faults and lies above them. The
internal geometry of both the Internal
Structure and the Coastal Structure changes
rapidly along the structure axes. In particular,
the position, the geometry and the
development of the thrust faults and the
associated anticlines of the Coastal Structure
appear to be influenced by the substratum
morphology, as confirmed by analogue
experiments performed in sandboxes. The
deformational history of the tectonic units is
quite complex, being characterized by out-ofsequence
thrusting
and
non-coeval
deformation within each thrust sheet.
REFERENCES
Bally A.W., Burbi L., Cooper G. and
Ghelardoni R., 1986. Mem. Soc. Geol. It.,
35, 257-310.
Casnedi R., 1991. In: A. Bouma and B.
Carter
(eds.):
Facies
Models
in
Exploration
and
Development
of
Hydrocarbon and Ore Deposits. VSP,
219-233.
Ori G.G., Serafini G., Visentin C., Ricci
Lucchi F., Casnedi R., Colalongo M. and
Mosna S., 1993. In: A.M. Spencer (ed.):
Generation, Accumulation and Production
of Europe's Hydrocarbons III, Spec. Publ.,
of E.A.P.G. n° 3, 233-258.
Casnedi R., 1983. AAPG Bull., 67, 359-370.
FAULT
ZONE
EVOLUTION
FROM
CONJUGATE BRITTLE-DUCTILE SHEAR
ZONES IN LIMESTONES: A CASE STUDY
FROM
MONTE
CUGNONE
(PZ),
SOUTHERN APENNINES
1
1
1
G. Cello , C. Invernizzi , L. Marchegiani , L.
1
2
1
Mattioni , S. Mazzoli , E. Tondi
1
Dipartimento di Scienze della Terra,
Universita’ di Camerino
2
Istituto di Dinamica Ambientale, Università
di Urbino
In this note, we report on detailed
structural analysis carried out in the Upper
Triassic micritic limestones of the Mesozoic
Lagonegro Basin succession outcropping in
the Marsico Nuovo area (Basilicata). The
limestones are cross-cut by several, regularly
spaced “brittle-ductile” shear zones of
extensional type which are nicely exposed in
a quarry at Monte Cugnone. These
structures, characterised by arrays of en
échelon tension gashes, are the main subject
of this work.
The study area is located in the crestal
region of the gently northwest plunging
Monte Cugnone anticline, in a sector of the
major fold that is characterised by subhorizontal to gently dipping bedding. The host
rock consists of a strongly anisotropic but
rather homogeneous multilayer, made up of
regular limestone beds (mostly 10 to 40 cm
thick) containing chert lenses and nodules.
The portion of the Calcari con selce Fm
exposed in the quarry is completely lacking in
the marly/clayey intercalations that are typical
of this formation. This rules out the possibility
that the localisation of “brittle-ductile” shear
zones and discrete faults may be controlled
by competence contrasts between layers of
differing composition.
Three different types of structures have
been distinguished: (i) “brittle-ductile” shear
zones, characterised by arrays of en échelon,
calcite-filled tension gashes; (ii) shear zones
showing
incipient
development
of
discontinuous shear-parallel fractures cutting
through the vein array; and (iii) faulted shear
zones, in which a continuous, discrete normal
fault breaks through – and clearly developed
from – an original en échelon vein array.
Environmental (P-T) conditions existing
during the development of the analysed
shear zones were mostly controlled by the
tectonic burial resulting from the previous
contractional episodes. Fluid inclusions from
calcite veins sampled from the different sets
of structures, from (i) to (iii) above, indicate
that environmental conditions remained
constant during the different stages of shear
zone evolution and normal fault development.
Maximum homogenisation temperatures from
primary fluid inclusions are all consistently
within the range of 130°-140°C.
The en échelon vein systems have been
classified by means of a triangular plot
designed for the representation of conjugate
ix
vein arrays (Srivastava, 2000). In this plot,
three angular parameters are used (dihedral
angle q, vein array angle d, inter vein angle
y). Based on this graph, the vast majority of
the measured vein arrays can be
geometrically classified as “Type 1” (adopted
as a strictly descriptive term, and not
referring to the mode of formation of the vein
arrays) (Beach, 1975). They mostly range
from the “weakly convergent” to the “strongly
convergent” types of Smith (1996).
To understand the kinematic meaning of
the vein arrays, values of vein (a) vs. shear
zone boundary (d) angles are also plotted
(Rothery, 1988; Kelly et al., 1998). The graph
shows that only few arrays occur in the
extensional domain, whereas most of them
are clustered in the simple shear domains
and can be interpreted, according also to
field observations, as “brittle-ductile” shear
zones. In particular, incipiently faulted or
completely ‘broken’ vein arrays are roughly
clustered into the simple shear field of the
graph. This suggests a progressive rotation
of the veins inside the arrays, producing: (i)
strain localisation, (ii) overlapping, and
eventually (iii) linkage of the veins producing
brittle deformation (Olson, 1991). A similar
evolution is also suggested by a graph in
which the average angle between the shear
zone boundary and the associated veins
(vein array angle d) is plotted vs.
displacement. In this graph, a trend of larger
offsets for increasing d can be recognised.
REFERENCES
Beach A., 1975. The geometry of en-échelon
vein array. Tectonophysics, 28, 245-263.
Kelly P.J., Sanderson D.J. and Peacock
D.C.P., 1998. Linkage and evolution of
conjugate strike-slip fault zones in
limestones of Somerset and Northumbria.
J. Struct. Geol., 20, 1477-1493.
Olson J. E. and Pollard D. D., 1991. The
initiation and growth of en échelon veins
J. Struct. Geol., 13, 595-608.
Rothery E., 1988. En échelon vein array
development in extension and shear. J.
Struct. Geol., 10/1, 63-71.
Srivastava
D.C.,
2000.
Geometrical
classification of conjugate vein arrays. J.
Struct. Geol., 22, 713-722.
Smith J.V., 1996. Geometry and kinematics
of convergent conjugate vein array
systems. J. Struct. Geol. , 18, 1291-1300.
ESTENSIONE TIRRENICA NELLA ZONA DI
INTERAZIONE
DEI
DUE
ARCHI
APPENNINICI (M. LEPINI, M. AUSONI E M.
AURUNCI, ITALIA CENTRALE)
1
2
2
M. Cesarano , P.P. Pierantoni , G. Santarelli
2
& E. Turco
1
Università degli Studi del Molise
2
Dipartimento di Scienze della Terra –
Università degli Studi di Camerino
Il sistema M. Lepini – M. Ausoni – M .
Aurunci (Catena dei Volsci) rappresenta la
zona di interazione tra i due archi
appenninici. Esso è costituito da una dorsale
disposta in senso appenninico il cui limite
nord-orientale
è
caratterizzato
dall’accavallamento dei carbonati mesozoici
sui depositi terrigeni altomiocenici della Valle
Latina in un’età compresa tra il Messiniano e
il Pliocene inferiore, mentre quello sudoccidentale è caratterizzato un sistema di
faglie estensionali plio-quaternarie che
ribassano l’edificio strutturale principalmente
verso il Mar Tirreno (ACCORDI, 1966;
CERISOLA & MONTONE, 1992; CIPOLLARI &
COSENTINO, 1991 e 1995; CIPOLLARI et al.,
1995; NASO & TALLINI, 1993; PAROTTO, 1980;
PAROTTO & PRATURLON, 1975; ROSSI, 2001).
Le strutture tettoniche principali dell’area
di studio sono state rilevate utilizzando la
comparazione tra immagini telerilevate
(Landsat TM5), foto aree e geologia di
superficie. Sono state poi distinte le strutture
quaternarie da quelle più antiche utilizzando
sia i criteri morfologici che geologici. In
particolare, per quanto riguarda i criteri
geologici, i bacini intramontani hanno fornito
una buona datazione delle strutture
tettoniche che li bordano. I lineamenti
tettonici identificati dalle immagini telerilevate
e dalle foto aeree sono stati raggruppati in
quattro sistemi con direzioni: N-S, NE-SW,
NW-SE e E-W.
I dati di geologia di superficie hanno
portato, analogamente a quanto osservato
dalle immagini telerilevate e ed aeree, alla
definizione dei quattro sistemi di faglie già
evidenziati.
L’analisi
degli
indicatori
cinematici
osservati sui piani indica un movimento
prevalentemente trascorrente destro per le
faglie orientate N-S e trascorrente sinistro su
quelle orientate NE-SW. I piani di faglia di
questi due sistemi, ad ulteriore conferma
della loro natura trascorrente, sono tutti ad
x
alto angolo. Lungo questi piani tuttavia
spesso si osservano due generazioni di strie:
alle chiare cinematiche trascorrenti seguono
e si sovrappongono riattivazioni con
componenti normali.
Le faglie orientate NW-SE sono le più
evidenti e frequenti, ed immergono sia a SW
che a NE; esse mostrano delle chiare
cinematiche estensionali di dip-slip e
individuano delle strutture tipo horst e
graben. Le faglie con direzione E-W
presentano un movimento prevalentemente
di tipo transtensivo, sia destro che sinistro.
Questi ultimi due sistemi di faglie, infine,
sembrano tagliare e dislocare gli altri due.
Questi dati, unitamente al fatto che lungo i
piani orientati NW-SE spesso si rinvengono
strie che evidenziano movimenti da
trascorrenti a transtensivi sia destri che
sinistri, suggeriscono per quest’area una
distensione plio-quaternaria con una doppia
direzione: una principale orientata NE-SW e
una secondaria NW-SE.
BIBLIOGRAFIA
ACCORDI B. (1966) – La componente
traslativa nella tettonica dell’Appennino
Laziale-Abruzzese. Geologica Romana,
V, 355-406.
CERISOLA R. & MONTONE P. (1992) – Analisi
strutturale di un settore della catena dei
Monti Ausoni-Aurunci (Lazio,
Italia
centrale). Bol. Soc. Geol. It., 111, 449457.
CIPOLLARI P. & COSENTINO D. (1991) –
Considerazioni sulla strutturazione della
catena dei M. Aurunci: vincoli stratigrafici.
Studi Geol. Camerti, Vol. speciale 1991/2,
CROP 11, 151-156.
CIPOLLARI P. & COSENTINO D. (1995) – Il
sistema
Tirreno-Appennino:
segmentazione litosferica e propagazione
del fronte compressivo. Studi Geol.
Camerti, Vol. speciale 1995/2, 125-134.
CIPOLLARI P., COSENTINO D. & PAROTTO M.
(1995) – Modello cinematico-strutturale
dell’Italia centrale. Studi Geol. Camerti,
Vol. speciale 1995/2, 135-143.
NASO G. & TALLINI M. (1993) – Tettonica
compressiva e distensiva dei M. Aurunci
occidentali (Appennino centrale): prime
considerazioni. Geologica Romana, 29,
455-462.
PAROTTO M. (1980) – Apennin central. In:
Introduction a la geologie generale
d’Italie. 26° Congr. Geol. Intern., Parigi
1980, 33-37.
PAROTTO M. & PRATURLON A. (1975) –
Geological summary of the Central
Apennines, in: Structural Model of Italy.
Quad. Ric. Scient., C.N.R., 90, 257-300.
ROSSI D. (2001) – Analisi geometrica e
cinematica dei M. Ausoni-Aurunci (Lazio
meridionale). Tesi di Dottorato.
THE ENIGMATIC LOW-ANGLE NORMAL
FAULTS: THE CASE OF THE NORTHERN
APENNINES
Cristiano Collettini, Massimiliano Rinaldo
Barchi
Università degli Studi di Perugia
The presence of Low-Angle Normal
Faults, LANFs, (dip < 30°) has been
extensively documented in areas of
continental extension. LANFs were firstly
recognised in the Basin and Range province
and than documented in other worldwide
areas. Though the LANFs subject has been
well covered in literature, it is still full of
controversies.
First, how do they form (i.e. their lowangle attitude is an original feature or the
result of rotation)? According to AndersonByerlee frictional faults mechanics, normal
faults initiate at dips ~60° than domino rotate
to frictional lockup angles, 40°-30°, (Sibson,
1985). Dips lower than lockup would be
achieved by domino rotation produced by
successive normal faults sets (Proffett,
1977), or isostatic adjustments producing
footwall flexure and uplift (Wernicke and
Axen, 1988). In marked contrast some field
observations
constrain
initiation
and
movements along LANFs at dips similar to
their present attitude (Scott and Lister, 1992)
. Very low dips have been also explained as
the result of dramatic departures from the
Andersonian state of stress induced by
severe topography (Abers et al., 1997) or
high shear stress at the base of the brittle
crust (Westaway, 1999).
Second,
can
displacement
be
accommodated by LANFs and in which way
(seismic, microseismic aseismic)? Dip
estimates for M>5.5 continental normal-fault
earthquakes,
compiled
from
focal
mechanisms
with
rupture
planes
unambiguously
identified,
yielded
a
predominance of moderate-to-steep dips,
30°-65°(Collettini and Sibson, 2001); this
result being in agreement with active normal
faults which follow the Anderson-Byerlee
frictional faults mechanics. On the contrary,
three possibly low-angle ruptures, though
xi
without positive discrimination, have been
adduced to a LANF active in the Papua New
Guinea region and the lack in the
contemporary seismic record of moderate
and large ruptures on LANFs has been
interpreted as due to their long recurrence
intervals (Wernicke, 1995).
The presence of a microseismically active
LANF, dip 15°, has been suggested beneath
the Gulf of Corinth in Greece (Rigo et al.,
1996), and a similar seismic behaviour
characterises the Altotiberina fault in the
Northern Apennines (Boncio et al., 2000;
Collettini et al., 2000).
Here we focus our attention on
movements on LANFs. The characteristics of
extensional tectonics in the Northern
Apennines: (1) continuum migration with time
from west (Tyrrhenian sea) to east
(Apenninic chain); (2) presence of crustal
scale LANFs which have different ages and
degree of exhumation; (3) location of the
easternmost LANF in the active region of the
Northern Apennines, provide an example
where active processes affecting the axial
region of the Northern Apennines can be
observed and studied in the exhumed
outcrops exposed in the Tyrrhenian islands.
Geophysical data, seismic reflection
profiles and seismology, from the eastern
active region are compared with structural
data from the western exhumed region and
both are used to investigate movements on
the studied faults at very low-angle dips
(<30°) and under vertical s1 trajectories.
Proffett, J.M., 1977. Cenozoic geology of the
Yerington
district,
Nevada,
and
implications for the nature of Basin and
Range faulting. Geol. Soc. Am. Bull. 88,
247-266.
Rigo, A., Lyon-Caen, H., Armijo R.,
Deschamps A., Hatzfeld D., Makropoulos
K., Papadimitriou P., Kassaras I., 1996. A
microseismic study in the western part of
the Gulf of Corinth (Greece): implications
for
large-scale
normal
faulting
mechanisms. Geophys. J. Int. 126, 663688.
Scott, R. J., Lister G. D., 1992. Detachment
faults: Evidence for a low-angle origin.
Geology 20, 833-836.
Sibson, R.H., 1985. A note on fault
reactivation. J. Struct. Geol. 7, 751-754.
Wernicke, B., 1995. Low-angle normal faults
and seismicity: A review. J. Geophys.
Res. 100, 20159-20174.
Wernicke, B., Axen G. J., 1988. On the role
of isostasy in the evolution of normal fault
systems. Geology 16, 848-851.
Westaway, R., 1999. The mechanical
feasibility of low-angle normal faulting.
Tectonophysic, 308, 407-443.
REFERENCES
INTRODUCTION
Vertical axis rotations have shown to be a
crucial component of continental deformation,
mainly in such geodynamic contexts having a
component of simple shear deformation.
Various kinematic models have been
proposed to account for the vertical axis
rotations found in crustal blocks bounded by
strike slip faults at various. The most followed
kinematic model was the “block or domino
model” (McKenzie and Jackson, 1983 among
others) which proposes that rotations are
spatially constant within blocks having near
the same dimensions of the width of the
shear zones. More recently a “small-block
(quasi-continuum) model” was proposed by
Sonder et al. (1994). In this model, blocks
much smaller than the shear zone
dimensions rotate independently in response
to shearing and rotations increase toward the
faults. This distribution of rotations implies
the existence of a plastic layer in the upper
crust allowing a mechanical decoupling from
the deeper crust. In this model rotations are
Abers, G.A., Mutter C. Z., Fang G., 1997.
Shallow dips of normal faults during rapid
extension: Earthquakes in the WoodlarkD'Entrecasteaux rift system, Papua New
Guinea. J Geophys. Res. 102, 1530115317.
Boncio, P., Brozzetti F., Lavecchia G., 2000.
Architecture and seismotectonics of a
regional low-angle normal fault zone in
central Italy. Tectonics 19, 1038-1055.
Collettini, C., Barchi M. R., Pauselli C.,
Federico C., Pialli G., 2000. Seismic
expression of active extensional faults in
northern Umbria (central Italy). In: The
Resolution of Geological Analysis and
Models for Earthquake Faulting Studies
Edited by G. Cello and E. Tondi. Jour. of
Geodyn. 29, 309-321.
Collettini, C., Sibson R. H., 2001. Normal
Faults Normal Friction?. Geology 29, 927930.
VERTICAL AXIS ROTATION
STRIKE-SLIP FAULTS
1
ALONG
2
Elisabetta Costa , Fabio Speranza
1
Università degli Studi di Parma, Italy.
2
INGV Istituto Nazionale di Geofisica e
Vulcanologia, Roma, Italy.
xii
controlled by the rheological properties of the
decoupling layer, the length of and the offset
across the shear zone. Rotations are CW in
dextral shear zones and CCW in sinistral
shear zones.
Freund, (1970) showed, on the basis of
2D kinematic analysis, that a strike-slip zone
contains several faulted domains each
having an independent sense of fault slip and
rotation. Both blocks and their bounding
faults rotate away from the direction of
shortening and toward the direction of
elongation. Many other Authors (Garfunkel,
1974; Ron et al., 1984; 1986) found the same
results on the basis of structural and
paleomagnetic analysis carried out in various
areas (northern Israel; Lake Mead, Nevada;
Mojave desert, California). In this model,
rotations are CW for sinistral shear areas and
CCW for dextral shear areas.
Thus, even the sense of rotation is
controversial, mainly at a scale less than 100
km.
The above simplistic and defective
summary shows that both kinematics and
sense of rotations in shear zones are still
controversial, depending on the different
scales of observation and on the various
approaches to the problem (either simply
geometric or rather dynamic).
NEW INSIGHTS FROM MODELING
Vertical axis rotations, theoretically
predicted in shear zones, had been
tentatively reconstructed in the field by the
shape of the deformation structures formed
at any scale. Nevertheless some doubts
remained if either material rotations or stress
field rotations had occurred, until rotations
have been determined paleomagnetically by
discrepancies between expected declinations
and tilt-corrected observed declination.
The same happens in physical models
addressing to the problem of rotations due to
strike-slip faulting (Schreurs, 1994, among
many others).
We try a new approach to this problem by
using a new method: the analog magnetic
modeling. Very simple scaled physical
models have been prepared by using sand
previously mixed with magnetic (magnetitedominated) powder. Before being deformed,
the models were magnetized by means of
two permanent magnets generating in the
models a quasi-linear magnetic field of
intensity variable between 20 and 100 mT
and parallel to the shortening direction. After
deformation the models were cut into
sections normal to the fault direction and
sampled using 1x1-cm cylinders. Afterwards,
the natural remanent magnetization of these
samples was measured using alternating
field demagnetization. The strike-slip fault
cutting each model has been obtained either
by moving the basement blocks underlying
the models (Fig.1) or by directly moving a
block bounded by a fault pre-cut in the sandy
cover and filled by glass micro-beads, which
make easier the slip along the fault (Fig.2).
Both kinds of models were made of sand
detaching either on the Plexiglas base of the
box or on a viscous decollement layer made
of silicone which allows a mechanical
decoupling between the cover and the
underlying basement.
RESULTS
a) fault cutting only the cover (Fig.2)
b) fault in the basement (Fig.1)
Fig.1- section cut in the
model with the sinistral
fault in the basement.
Note the fault in the cover
rotate CCW compared
with the basement fault
Fig.2 - map view of the
model with a dextral fault
only in the cover. The
small sinistral conjugated
fault
formed
during
deformation and CCW
rotations occurred along it.
Paleomagnetic data show that rotations in
the model with right-handed strike-slip fault
are CCW far from the fault. The CCW
rotations gradually decrease toward the fault
plane and become slightly CW along the fault
itself. This means that two superimposed
rotations of opposite sense took place in the
model detaching on Plexiglas. The rotation
induced by slip along the faults on the two
sides, are CW for dextral shear and CCW for
sinistral shear and they increase toward the
fault. The other rotation of opposite sense
affects the faults themselves, along with the
blocks they bound.
1) The model detaching on Plexiglas
show only CW rotations, due to dextral shear,
that increase toward the fault
xiii
2) The model detaching on silicone
shows only CCW rotations, due to sinistral
shear. The rotation trend is less regular than
the previous one. In this context also the fault
in the cover rotate CCW in respect to the
basement fault (Fig.1) and the rotation center
is located at half of the fault length.
CONTROL
OF
METASOMATIC
ALTERATION ON THE DEVELOPMENT OF
SHEAR ZONES IN ULTRAMAFIC ROCKS:
A CASE STUDY FROM THE VOLTRI
MASSIF (LIGURIAN ALPS).
Crispini L. and Capponi G.
DIPTERIS Università degli Studi di Genova
REFERENCES
Freund, R., 1970a. Rotation of strike-slip
faults in Sistan, Southern Iran. J.Geol.,
78, 188-200.
Garfunkel, Z.,1974. Model for the late
Cenozoic tectonic history of the Mojave
Desert, California and for its relation to
adjacent regions. Geol.Soc.Am.Bull., 85,
1931-1944.
McKenzie, D. and Jackson, J.,1983. The
relationship between crustal thickening,
paleomagnetism, finite strain and fault
movement within a deforming zone. Earth
and Planetary Science Letter, 65, 182202.
Ron, H., Freund, R. and Garfunkel, Z. and
Nur, A., 1984. Block rotation by strike-slip
faulting: structural and paleomagnetic
evidence. J. Geophys. Res., 89 (B7),
6256-6270.
Ron, H., Aydin, A. and Nur, A., 1986. Strikeslip faulting and block rotation in the Lake
Mead fault system. Geology, 14, 10201023.
Schreurs, G., 1994. Experiments on strikeslip faulting and block rotation. Geology,
22, 567-570
Sonder, L.J., Jones, C.H., Salyards, S.L. and
Murphy, K.M.,1994. Vertical axis rotations
in the Las Vegas Shear Zone, southern
Nevada: Paleomagnetic constraints on
kinematics and dynamics of block
rotations. Tectonics, 13, 769-788.
In the Voltri Massif late orogenic events (late
Eocene-early Miocene) are characterized by
a complex superposition of syn-metamorphic
(greenschist to zeolite facies) and non
metamorphic structures linked to the
exhumation of the subducted slabs. This
work deals with the role of fluids and of
metasomatic alteration in the nucleation and
development of shear zones in the
ultramafites (lherzolites and dunites) of the
Voltri Massif.
Two main systems of superposed reverse
shear zones (RSZ1 and RSZ2) are
considered. These shear zones are
significant at the regional scale; they are
associated with shear joints and vein
networks and are characterized by opposite
sense of shear and by different thickness,
fault rocks, veins and type of wallrock
alteration.
RSZ1
developed
during
greenschist facies metamorphism, RSZ2 are
syntectonic to a zeolite facies phase.
In the lherzolites of the Voltri Massif
greenschist facies shear zones (RSZ1) show
brittle to semibrittle structures, with large
development of cataclasite and crush breccia
(10-50 mt thick) at places localized ductile
structures may develop. RSZ2 are more
complex and show heterogeneous features.
We focus on cases where the overprint of the
two systems (RSZ1 and RSZ2) is evident
and ductile deformations overprint brittle
ones. Metasomatic alteration consists mainly
of replacement of the lherzolite minerals by
Mg- and Fe-carbonate, quartz, chalcedony
and phyllosilicates. Greenschist facies shear
zone (RSZ1) nucleate mainly along preexisting spaced cleavage of decametric open
folds and the concurrent occurrence of
networks of quartz/carbonate veins enhance
the development of the zone. Metasomatic
alteration is limited to the border of the veins
or very close to the fault plane, where
wallrock lherzolite is increasingly affected by
chemical alteration and transformed in
serpentinite,
talc-chlorite
schist,
Mgcarbonate and Mg-silicate rocks. Progressive
deformation
proceeded
by
plastic
deformation of new "softer" minerals and
facilitate development of ductile structures.
RSZ1 show a continuous evolution from
xiv
greenschist
to
subgreenschist
facies
metamorphism, with a top to W-NW sense of
shear. They are characterized by thin
serpentinite gouges and cataclasites evolving
to chlorite-tremolite-talc mylonites, with
structures such as Riedel shears, asymmetric
boudins and porphyroclasts, shear bands.
Cataclasites are cut by gold bearing Mgcarbonate-quartz veins (in places exploited
until the end of 1800) and Fe-Mg-carbonate
veins.
Ductile RSZ2 nucleate at the expense of preexisting RSZ1 and have a top to E-NE sense
of shear. Strain softening may be essentially
related to hydraulic weakening and/or mineral
alteration
and
reaction
softening.
Metasomatic alteration is more massive with
respect to RSZ1 phase and is characterized
by Ca- and CO2- rich fluids. This huge Cametasomatic alteration can be associated
with the zeolite facies metamorphism
described in other sectors of the Voltri Massif
(T≈150-300 °C and very low pressure). In the
early stage of this deformational event,
cataclasite and hydraulic breccia develop at
the expanse of RSZ1 lherzolite wallrocks;
progressively they are transformed into
carbonate-rich rocks by metasomatic fluids.
In the middle of the shear plane levels of
deformed sepiolite levels occur, decimetric
fibers of brown serpentine minerals are
ubiquitous along discrete slip planes.
Mechanical weakening and the growth of
new weaker minerals favoured the formation
of mylonites and ductile structures at the
expense of the older brittle structures.
Whole rocks and REE chemical analyses
reveal at least three metasomatic events
linked to RSZ, which are characterized by
peculiar alteration of the ultramafic host
rocks. Alterations can be clustered in three
main types: type 1 series gives serpentinite,
carbonatized serpentinite, Mg-Fe-carbonate
and phyllosilicate-rich rocks ("listvenites");
alteration type 2 series: serpentinite,
carbonatized
serpentinite,
saponite
+
dolomite-rich rocks; alteration type 3:
sepiolite levels. Alteration 1 is linked to
RSZ1, alteration 2 and 3 to RSZ2.
The
results
of
field
observation,
microstructural and chemical analyses
through the shear zones outline the key-role
of syntectonic fluids and metasomatic
alteration in the development of ductile or
brittle structures.
PERMIAN AND TRIASSIC MAGMATISM
AS A MARK OF TECTONO-THERMAL
EVOLUTION
OF
THE
ADRIATIC
LITHOSPHERE
1
2
Giorgio V. Dal Piaz , Silvana Martin , Iole M.
3
Spalla
1
Università di Padova
2
Università dell’Insubria
3
Università Statale Milano
In the Alpine domain, the Variscan
continental collision and related Barrovian
metamorphism were accomplished in the
Lower Carboniferous. From the Westphalian,
the collapsed Variscan belt began to be
unconformably covered by clastic deposits
and later was deformed by transtensive fault
systems. The Permian evolution of the
Austroalpine-Southalpine Adriatic crust was
dominated by a bimodal to calc-alkaline
magmatic activity which was interpreted as
evidence of: i) a late-Variscan volcanic arc
generated by subduction of a now totally lost
southern ocean (e.g., Finger and Steyrer,
1990); ii) post-collisional collapse of the
Variscan
belt
by
delamination
and
detachment of lithospheric roots (Bonin et al.,
1993); iii) post-Variscan asthenosphere
upwelling,
thermal
perturbation
and
asymmetric attenuation of lithosphere by
deep normal detachments, as a mark of the
Lower Permian onset of a new regime of
diverging plates (Dal Piaz and Ernst, 1978;
Lardeaux and Spalla, 1991; Dal Piaz, 1993;
Trommsdorff et al., 1993). The former
reconstruction is contrasted by the absence
of an oceanic and collisonal suture, the
presence of continental tholeiitic melts, global
(Arthaud and Matte, 1977; Ziegler, 1988) and
regional (Perotti and Cassinis, 1994)
tectonics, and other remarks discussed in Dal
Piaz and Martin (1998). The latter two may
both be reliable, even if the last is favoured
by the extent of igneous underplating below a
thinned continental crust, its delay from the
Variscan collision (≥ 50 Ma), and other points
here in the following. The Adriatic crust was
noticeably accreted by Permian gabbro
batholiths (asthenospheric melts) which
emplaced at the base of or inside the lower
crust. The Permian magmatic event is also
represented by roughly coeval tholeiitic
basalt dykes, abundant anatectic granites
and calc-alkaline products represented by
shallower granitoids and andesite-rhyolite
volcanics filling graben or half-graben. Some
Late Permian-Triassic ages are probably
representative of cooling or younger
magmatic pulses. The principal lower crust
xv
gabbros of dated or supposed Lower
Permian age occur in the western Southern
Alps (Ivrea zone: Rivalenti et al., 1984;
Voshage et al., 1987; Vavra et al., 1999;
Mayer et al., 2000) and the Austroalpine
domain, from the Sesia-Lanzo zone (Dal Piaz
et al., 1971; Venturini, 1995; Rebay and
Spalla, 2001) and Dent Blanche nappe
(Collon, Matterhorn; Dal Piaz, 1976; Dal Piaz
et al., 1977; Mt. Emilius: Dal Piaz et al., 1983;
Benciolini, 1996; Pennacchioni, 1996) to the
central (Sondalo, Fedoz; Tribuzio et al.,
1999; Müntener et al., 2000) and eastern
(Koralpe; Thöni and Jagoutz, 1992)
basement units. The emplacement depth of
mafic bodies is documented by their primary
crystallisation in granulite facies conditions;
the subsequent evolution of mafic bodies and
surrounding crust by a sequence of
subsolidus re-equilibrations at decreasing
pressure,
consistent
with
extensional
exhumation in a pre-Alpine high-T regime
(Diella et al., 1992; Gardien et al., 1994;
Rebay and Spalla, 2001). The Ivrea zone
regionally shows that the Lower Permian
igneous activity began with underplating of
the main gabbro batholith, derived from
tholeiitic melts and emplaced below rising
sections of attenuated lower continental crust
(Voshage et al., 1990; Quick et al., 1992;
Sinigoi et al., 1994). In the meantime, the
roofing kinzigitic crust, rapidly heated by
basic melts, was re-equilibrated in granulite
facies and generated abundant anatectic
products (Barboza and Bergantz, 2000).
Other tholeiitic metagabbros are presently
located in upper crust fragments: the best
example is provided by the Collon-Matterhorn
bodies, which are included in the upper
Austroalpine thrust system and associated to
dominant gneissic granitoids (Arolla series)
from calc-alkaline protoliths of Lower
Permian age (Dal Piaz et al., 1977; Bussi et
al., 1998; Monjoie et al., 2001). Isotope
dating (293-284 Ma) and mineral fabrics
suggest the existence of nearly concurrent
intrusions at different crustal levels, i.e.
gabbro batholiths at depth (granulite relics)
and granitoids at shallower crustal levels.
The gabbro-granitoid contact is tectonic,
marked by Alpine shear horizons which
probably reactivated pre-Alpine detachments
that allowed the extensional uprising of
metagabbros and their ultimate juxtaposition
to granitoids. The Permian calc-alkaline
products are mainly represented by shallow
plutons, dykes and volcanics in the central
and eastern Southern Alps (Bonin et al.,
1993, and refs. therein), whereas faultbounded asymmetric basins filled by the
Collio fm and coeval volcanics evidence
brittle tectonics (Cassinis and Perotti, 1994).
The origin of the Lower Permian magmatism
is constrained by geochemical features
pointing for partial melting of asthenosphere
(tholeiitic gabbro) and lithosphere (calcalkaline suite) mantle sources. We know that:
i) lithospheric mantle slices are associated to
the Southalpine (Balmuccia, Finero) and
Austroalpine metagabbros (Godard et al.,
1996; Wogelius and Finley, 1989), but these
mafic and ultramafic rocks do not display cogenetic relationships; ii) asthenospheric
slices in the Inner Ligurian ophiolite show
trace of Permian partial melting and later
accretion to the subcontinental lithosphere
(Rampone et al., 1996, 1998), but are never
associated to coeval basic melts. By
integration of these data, it may be concluded
that there is field evidence of Permian
tholeiitic melts and active asthenospheric
sources at the Alpine-Apennine scale, even if
in places now far away. The calc-alkaline
suite derived from previously enriched
(Variscan subduction) lithospheric mantle
sources, later activated during the Permian
extension. Calc-alkaline magmas emplaced
in shallow magmatic chambers where
fractionation and crustal contamination
developed to various extents. A new
magmatic event occurred in the Middle
Triassic. It is mainly recorded in the
Southalpine upper crust by calc-alkaline to
shoshonitic lavas, dykes and shallow plutons
(Dal Piaz and Martin, 1998, and refs.
therein), just before the appearance of rifting
features in cover sequences (Bertotti et al.,
1993). Also in this case, the orogenic
chemical affinity was a delayed feedback of
previously enriched mantle sources (Variscan
subduction), whereas partial melting was
made active by Triassic lithosphere extension
and the related melts ascended along open
fractures (dykes). Early Jurassic (Costa and
Caby, 2000) and possibly older gabbros with
MORB composition occur inside the ophiolitic
Piedmont nappe, as evidence of a persistent
emplacement of asthenospheric melts into
the continental lithosphere before mantle
denudation and ocean spreading (Elter,
1971; Lombardo and Pognante, 1982; Dal
Piaz and Lombardo, 1985; Lardeaux and
Spalla, 1991; Dal Piaz, 1999). No surface
evidence supports the existence of a single
and long lasting Permian to Triassic
extensional process. Most authors are
inclined to favour two independent thermomechanical and magmatic events, because
of the temporal gap between them.
Nevertheless, the absence or paucity of
xvi
magmatic traces between the Lower Permian
and Mid-Triassic peaks may simply reflect a
changed rheological behaviour of the Adriatic
continental crust: formerly fragmented,
accreted and softened in the Lower Permian,
the Adriatic crust was re-compacted and
hardened since the Permian intrusions
ultimately cooled, long preventing further
uprising of magmas eventually trapped or
generated at depth. In this view, it cannot be
excluded that the same driving forces might
potentially have been active from the Lower
Permian extension to the Mesozoic
continental rifting, even if in absence of a
continuous superficial record in the
Austroalpine-Southalpine crust.
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Arthaud, F., Matte, P., 1977. Geol. Soc. Am.
Bull. 88, 1305-1320.
Barboza, S.A., Bergantz, G.W., 2000. J.
Petrol. 41, 1307-1327.
Benciolini, L., 1996. Mem. Sci. Geol. 48, 7391.
Bertotti, G., Picotti, V., Bernoulli, D.,
Castellarin, A., 1993. Sedimentary Geol.
86, 53-76.
Bonin, B., Brändlein, P., Bussy, F., et al.,
1993. In von Raumer J. & Neubauer F.
(Eds):
Pre-Mesozoic Geology in the Alps. SpringerVerlag, 171-202.
Bussy, F., Venturini, G., Hunziker, J.C.,
Martinotti G., 1998. Schweiz. Min.
Petrogr. Mitt. 78, 163-168.
Cassinis, G., Perotti, C.R., 1994. Boll. Soc.
Geol. It. 112, 1021-1036.
Costa, S., Caby, R., 2001. Chemical Geol.
175, 449-466.
Dal Piaz, G.V., 1976. Mem. Ist. Geol. Min.
Univ. Padova 31, 60 pp.
Dal Piaz, G.V. 1993. In von Raumer J. &
Neubauer F. (Eds): Pre-Mesozoic geology
in the Alps.
Springer-Verlag, 327-344.
DAL PIAZ, G.V., 1999. In Gosso, G. et al.
(Eds): 3td Workshop on Alpine Geological
Studies,
Biella-Oropa 1997, Mem. Sci. Geol. 51, 155176.
Dal Piaz, G.V., De Vecchi, Gp., Hunziker,
J.C., 1977. Schweiz. Min. Petrogr. Mitt.
57, 59-88.
Dal Piaz, G.V., Ernst, W.G., 1978.
Tectonophysics 51, 99-126.
Dal Piaz, G.V., Gosso, G., Martinotti G.,
1971. Mem. Soc. Geol. It. 10, 257-276.
Dal Piaz, G.V., Lombardo, B., 1985. Rend.
Soc. It. Min. Petr. 40, 125-138.
Dal Piaz, G.V., Lombardo, B., Gosso G.,
1983. Am. J. Sci. 283A, 438-458.
Dal Piaz, G.V., Martin, S., 1998. Mem. Soc.
Geol. It. 53(1996), 43-62.
Diella, V., Spalla, I.M., Tunesi, A., 1992. J.
Metam. Geol. 10, 203-219.
Elter, G., 1971. Géol. Alpine 47, 147-169.
Finger F., Steyrer, H.P., 1990. Geology 18,
1207-1210.
Gardien, V., Reusser, E., Marquer, D., 1994.
Schweiz. Min. Petrogr. Mitt. 74, 489-502.
Godard, G., Martin, S., Prosser, G., Kienast,
J.R., Morten, L., 1995. Tectonophysics
259, 3313-341.
Lombardo, B., Pognante, U., 1982. Ofioliti 2,
371-394.
Mayer, A., Mezger, K., Sinigoi S., 2000. J.
Geodynamics 30, 147-166.
Monjoie, Ph., Bussy, F., Schaltegger, U.,
Lapierre, H., Pfeifer, H-R., 2001. EUG
Abstr., MS07, 475.
Müntener, G., Hermann, J., Trommsdorff, V.,
2000. J. Petrol. 41, 175-200.
Pennacchioni, G., 1996. J. Structural Geol.
18, 549-561.
Quick, J.E., Sinigoi, S., Negrini, L., Demarchi,
G., Mayer, A., 1992. Geology 20, 613616.
Rampone, E., Hofmann, A.W., Piccardo,
G.B., Vannucci, R., Bottazzi, P., Ottolini,
I., 1996.
Contrib. Mineral. Petrol. 123, 61-76.
Rampone, E., Hofmann, A.W., Raczek, I.,
1998. Earth Planet. Sci. Lett. 163, 175189.
Rebay, G., Spalla, I.M., 2001. Litho 58, 85104.
Rivalenti, G., Rossi, A., Siena, F., Sinigoi, S.,
1984. TMPM 33, 77-99.
Sinigoi, S., Quick, J.E., Clemens-Knott, D.,
Mayer, A., Demarchi, G., Mazzucchelli,
M., Negrini, L., Rivalenti, G., 1994. J.
Geophys. Res. 99, 575-590.
Thöni, M., Jagoutz, E., 1992. Geochim.
Cosmochim. Acta 56, 347-368.
Tribuzio, R., Thirlwall, M.F., Messiga, B.,
1999. Contrib. Mineral. Petrol. 136, 48-62.
Trommsdorff, V., Piccardo, G.B., Montrasio,
A. 1993. Schweiz. Min. Petr. Mitt. 73, 191203.
Vavra, G., Schmidt, R., Gebauer, D., 1999.
Contrib. Mineral. Petrol. 134, 380-404.
Venturini, G., 1995. Mém. Géol. Lausanne
25, 148 pp.
Voshage, H., Hunziker, J.C., Hofmann, A.W.,
Zingg, A., 1987. Contrib. Mineral. Petrol.
97, 31-49.
Voshage, H., Hofmann, A.W., Mazzucchelli,
M., Rivalenti, G., Sinigoi, S., Raczek, I.,
xvii
Demarchi, G., 1990. Nature 347, n. 6295,
731-736.
Wogelius, R.A., Finley, F.C., 1989. Geology
17, 995-998.
Ziegler, P.A., 1988. AAPG Mem. 43, 1-198.
DRAMATIC HIGH SPEED VELOCITY
DEPENDENCE OF QUARTZ FRICTION
WITHOUT MELTING
1
2
Giulio Di Toro , David L. Goldsby , Terry E.
2
Tullis
1
Dipartimento di Geologia, Paleontologia e
Geofisica, Universita’ di Padova, Padova,
Italy
2
Department of Geological Sciences, Brown
University, Providence, Rhode Island, USA.
Rapid
sliding
rates
and
large
displacements are achieved during seismic
faulting (Sibson, 1989), leading to shear
heating (Kanamori and Heaton, 2000) and
possible
changes
in
deformation
mechanisms and frictional strength (Chester
and Higgs, 1992). To investigate dynamic
fault weakening mechanisms, we conducted
a series of high-speed frictional sliding
experiments in a 1-atm rotary shear
apparatus at a normal stress of 5 MPa on
quartz rocks.
Samples were slid at a velocity (v)
between 0.001 - 100 mm/s to sliding
displacements of 4.5 m. For v < 1 mm/s, the
ratio of the steady state velocity dependence
of friction (m) vs. log v is about -0.04 and the
friction coefficient m = 0.75, as has been
observed previously in low speed sliding
experiments.
For 3 < v < 100 mm/s,
however, the velocity dependence becomes
much more significant, with the ratio m / log v
= -0.2, and m = 0.2, suggesting the onset of a
new weakening mechanism.
Measurements of temperature close to
the slip surface, estimates of the average
sliding surface temperature by numerical
simulations
and
estimates
of
flash
temperature at asperity contacts all indicate
temperatures too low to induce melting of
quartz, even locally. Instead, weakening is
apparently linked to lubrication via a layer of
silica gel on the sliding surface produced by
ultracomminution in the presence of water.
Thus, in the Earth's crust, dramatic frictional
weakening might occur at conditions much
less severe than those required for melting.
The observation that pseudotachylytes
(friction melts) are rarely observed in quartz
rocks in nature (Killick and Roering, 1998)
might be explained by the dramatic reduction
of shear strength due to lubrication via silica
gel, which would drastically reduce the
amount of heat generated on a fault during
seismic slip.
xviii
REFERENCES
Chester F.M. and Higgs N.G. (1992), J.
Geoph. Res., 97, 1859-1870
Kanamori H. and Heaton T. H. (2000),
Geophysical Monograph Series, 120, 147163
Killick A. M. and Roering C. (1998),
Tectonophysics, 284, 247-259.
Sibson R.H. (1989), J. Struct. Geol., 11, 1-14
SUPERHEATED FRICTIONAL MELTS IN
ZONED PSEUDOTACHYLYTES OF THE
GOLE LARGHE – V. GENOVA FAULT
ZONE (ADAMELLO MASSIF, ITALIAN
SOUTHERN ALPS)
1
1
Giulio Di Toro , Giorgio Pennacchioni
1
Dipartimento di Geologia, Paleontologia e
Geofisica, Università di Padova, Via Giotto 1,
35137 Padova, Italy
Most pseudotachylytes (PT) are the
product of wear, comminution, and frictional
melting along a fault zone during seismic
faulting (Spray, 1995). Therefore PT may be
used potentially to constrain fault plane
processes during an earthquake (Kanamori
and Heaton, 2000) and PT volumes to
estimate the dynamic frictional shear
stresses during coseismic slip (Sibson, 1975;
Wenk et al., 2000). This latter estimate
requires the determination of the melt
temperature (T).
In
this
study
we
describe
pseudotachylytes (PT) outcropping along a
main E-W trending strike-slip fault zone (Gole
Larghe - V. Genova fault zone) that crosscuts
quartzdiorites/tonalites in the northern
Adamello massif (Italian Southern Alps).
Seismic faulting is estimated to have
occurred at 0.2 - 0.3 GPa and 200 - 250°C.
The studied PT fill fault and injection vein
associations (Sibson, 1975) inside both
"undeformed" intrusive rocks and precursor
indurated cataclasites. Fault veins include
unzoned (UFV) and zoned types (ZFV). ZFV
show two symmetric microlitic domains (MD)
towards the vein walls and a central
spherulitic domain (SD).
In ZFV the
thickness (xMD) of the MD increases linearly
with the half thickness (a) of the entire vein
according to the relation:
xMD = (0.29 ± 0.12) a
for a > 0.003 m
The MD consists of randomly oriented
An40-plagioclase microlites, with interstitial
biotite and K-feldspar, and angular lithic
clasts mainly of quartz and plagioclase.
Length, width and morphology of microlites
vary gradually across the MD. The SD is
matrix-supported and includes different types
of spherulites, embayed clasts and minor
microlites. The thermal evolution of ZFV is
modeled by a finite difference method and
together with PT microstructures are used to
constrain the initial temperature of frictional
o
melt (approx. T = 1450 C).
Melt
temperatures
are
also
estimated
xix
independently by the clast/matrix ratio
according to the geothermometer of O’Hara
(2001). This latter method gives melt T in the
o
order of 350 - 550 C.
The O’Hara geothermometer is based on the
assumption that clasts and frictional melts
are produced during one seismic event (i.e.
rupture propagation plus frictional sliding).
Therefore, we suggest that the inconsistency
between the two T estimates (clast/matrix
ratio
vs.
thermal
model
plus
PT
microstructure) is due, in the case of the Gole
Larghe – V. Genova fault zone, to the
precursor aseismic cataclastic deformation
history of the shear zone. Thus differences
in the clast/matrix ratio in PT may result from
different deformational histories of a brittle
fault and could not depend only from the T of
the frictional melt.
REFERENCES
Kanamori H. and Heaton T. H. (2000),
Geophysical Monograph Series, 120, 147163
O’Hara K. O. (2001), J. Struct. Geol., 23,
1345-1357
Sibson R. H. (1975), Geophys. J. R. astr.
Soc., 43, 775-794
Spray J. G. (1995), Geology, 23, 1119-1122
Wenk H. R. et al. (2000), Tectonophysics,
321, 253-277
POLYPHASE
NEOGENE
FAULT
KINEMATICS AT THE HANGING WALL OF
THE
NORTH
GIUDICARIE
FAULT
(WESTERN
TRENTINO,
CENTRALEASTERN ALPS)
1
2
Maria Giuditta Fellin ,Silvana Martin , Matteo
3
Massironi
1
Dipartimento di Scienze della Terra e
Geologico-Ambientali, Università di Bologna,
Bologna
2
Dipartimento di Chimica Fisica e
Matematica, Università dell’Insubria, Como
3
Dipartimento di Geologia Paleontologia e
Geofisica, Università di Padova, Padova
brittle tectonic evolution of the Austroalpine
domain at the hanging wall of the North
Giudicarie fault. New data have been
collected in this area and compared with
existing data from the Southalpine domain.
In the study area faults display an
average trend parallel to the North Giudicarie
fault and are characterised by mylonites,
ultramylonites, cohesive and uncohesive
cataclasites
and
associated
pseudotachylytes indicating a long tectonic
activity
in
a
contractional
regime.
Calculations of the stress distributions (P-T-B
axes method, numerical dynamical analysis,
direct
inversion
methods,
dihedra
calculations) have yielded stress fields which
may be attributed to different tectonic steps
of the Alpine contraction along the Giudicarie
lineament. The NNE- to NE- trending faults
(e.g., Val Clapa, Val dell'Acqua, Val Burlini
and Rumo faults) and the ENE- trending
faults (Malga Preghena and Passo Palu'
faults) at the hanging wall of the North
Giudicarie line indicate a NW- and WNWoriented compression during Neogene. The
youngest morphological lineaments trend NW
to WNW and they might be related to a
Quaternary tectonic activity. The high
frequency of faults in the study area suggest
that during Neogene in the Austroalpine
domain of Western Trentino the strain was
partitioned
between
the
strike-slip
reactivation of pre-existing mylonitic horizons
and the development of minor reverse faults
with opposite vergence. As a conclusion, at
the hanging wall of the North Giudicarie fault
brittle deformations are related to a Neogene
continuous compressive to transpressive
kinematics and although in the Austroalpine
domain and the Southern Alps deformation
styles are different, they developed in the two
domains in the same conditions of
paleostress field.
REFERENCES
Castellarin, A., Cantelli, L., Fesce, A.M.,
Mercier, J.L., Picotti, V., Pini, G.A.,
Prosser, G., Selli, L., 1992. Annales
Tectonicae 6, 62-94.
Prosser, G., 1998. Tectonics 17, 921-937.
The North Giudicarie fault is a NEtrending segment of the Periadriatic
lineament which, in the Central-Eastern Alps,
separates the Austroalpine domain from the
Southern Alps. In Western Trentino previous
structural analyses focused on the Tertiary
kinematics of the Southern Alps (Castellarin
et al., 1992; Prosser, 1998) while little
investigations have been carried out on the
xx
ASPETTI PETROLOGICI E STRUTTURALI
DI UNA ZONA DI TAGLIO DUTTILE IN
GRANITOIDI
TARDO-ERCINICI
(SILA,
CALABRIA)
Vincenzo Festa*, Alfredo Caggianelli*,
Domenico Liotta**, Giacomo Prosser***, Aldo
del Moro****
*Dipartimento Geomineralogico, Università
degli Studi di Bari
** Dipartimento di Geologia e Geofisica,
Università degli Studi di Bari
*** Dipartimento di Scienze della Terra,
Università degli Studi della Basilicata,
Potenza
**** Istituto di Geocronologia e Geochimica
Isotopica, CNR Pisa
Nei pressi di Mesoraca (Sila meridionale,
Calabria) una zona di taglio duttile interessa
sia i granitoidi di età tardo-ercinica che le
rocce
incassanti.
I
granitoidi
sono
principalmente rappresentati da granodioriti
foliate e, subordinatamente, da tonaliti foliate.
Stime effettuate da Ayuso et al. (1994) con il
40
39
metodo
Ar/ Ar sull’orneblenda indicano
un’età di raffreddamento dei granitoidi pari a
circa 293 Ma. Le rocce incassanti sono
principalmente rappresentate da paragneiss
migmatitici, che hanno registrato il picco del
metarmofismo a 300-304 Ma (Graessner et
al., 2000). L'associazione mineralogica che
definisce il picco metamorfico include granati
che sono stati successivamente coinvolti
nella deformazione per taglio.
Nel nucleo della zona di taglio le
granodioriti presentano una netta foliazione
milonitica.
Le
granodioriti
milonitiche
mostrano una chiara riduzione della grana e
microstrutture legate ad una deformazione
sviluppatasi in condizioni di temperatura da
medio ad alta.
Le foliazioni e le lineazioni di minerali
presenti nei paragneiss migmatitici, nelle
granodioniti foliate e nelle granodioriti
milonitiche
risultano
geometricamente
concordanti tra loro. Inoltre, l’orientazione
preferenziale dei cristalli euedrali di feldspato
nei granitoidi, indica che l’anisotropia della
roccia ha cominciato a svilupparsi in
presenza di fuso per rotazione dei cristalli.
Questi caratteri suggeriscono che la
deformazione per taglio sia stata attiva
durante la messa in posto dei granitoidi e che
si sia protratta in condizioni di subsolidus.
Nelle granodioriti milonitiche, gli indicatori
cinematici ( -type, -type, s/c fabric e shear
bands) e le analisi condotte sugli assi <c> del
quarzo caratterizzano coerentemente un
senso del taglio sinistro, verso W nelle attuali
coordinate geografiche. La geobarometria
indica che la deformazione per taglio si sia
sviluppata nel livello crostale intermedio (P =
445 ±138 Mpa). Dicchi pegmatitici ricchi di
muscovite attraversano in discordanza la
zona di taglio senza essere coinvolti nella
deformazione. Le stime effettuate con il
metodo Rb-Sr sulla muscovite indicano
un’età di raffreddamento pari a 265±3 Ma,
che rappresenta, inoltre, l’età minima della
fine dell'evento deformativo. Al contrario, l’età
massima della deformazione è rappresentata
da quella del picco del metamorfismo.
Pertanto, si può concludere che la zona di
taglio duttile è stata attiva tra 304 e 265 Ma
fa (Carbonifero superiore – Permiano
inferiore-superiore).
BIBLIOGRAFIA CITATA
AYUSO R.A., MESSINA A. DE VIVO B.,
RUSSO S., WOODRUFF L.G., SUTTER
J.F.
and
BELKIN
H.E.,
1994.
Geochemistry
and
argon
thermochronology of the Variscan Sila
Batholith, southern Italy: source rocks and
magma
evolution.
Contribution
to
Mineralogy and Petrology, 117, 87-109.
GRAESSNER T., SCHENK V., BROCKER
M.
and
METZGER
K.,
2000.
Geochronological contraints on the tming
of granitoid magmatism, metamorphism
and post-metamorphic cooling in the
Hercynian
crustal
cross-section
of
Calabria. Journal of Metamorhic Geology,
18, 409-421.
xxi
STRESS
DISTRIBUTION
IN
AN
ANTICLINE; A NUMERICAL APPROACH.
1
1
Alessandro Frigeri , Giorgio Minelli , Cristina
1
Pauselli
1
Università degli Studi di Perugia,
Dipartimento di Scienze della Terra, Perugia,
06100, Italia
It is general experience of geoscientists
that all the rocks are affected at all the scales
by fault and fracture systems due to the
application of stress fields different in origins
and timing.
A fractured reservoir is a kind of
anisotropic
reservoir
where
the
faults/fractures play the main role or as
storage or as barrier on the flow of the
contained fluids or gas.
In the evaluation and development of a
fractured reservoir is critical task to define the
fracture system/s location. In this project the
numerical modelling (Finite Element Method)
was successfully applied to investigate the
relationships between the state of stress and
the fracture systems distribution in the case
of the Mohr Coulomb fracture law.
The experimental procedure was: a)
construct a balanced geological cross section
of the host structure (geological model); b)
mechanically characterise each lithologic
unit; c) discretize (meshing) the model.
To the discretized model was applied a
stress field (regional stress field) and, in the
range of elastic deformation, the principal
stress components module and orientation in
each point of the mesh were calculated.
A better knowledge of these processes
will improve oil field performances, dynamic
reservoir modelling, recovery and stimulation
planning.
RELATIONSHIPS
BETWEEN
GROUNDWATER
SYSTEMS
AND
TECTONIC
STRUCTURES
IN
THE
WESTERN VENETO FOOTHILLS
(1)
(2)
R. Gambillara
, F.Quattrocchi
, M.
(3)
(1)
Massironi e S. Martin
(1) Dipartimento di Scienze CC.FF.MM.,
Università degli Studi dell’Insubria, Via Lucini
3, 22100 Como
(2) Istituto Nazionale di Geofisica e
Vulcanologia, Via di Vigna Murata, Roma
(3) Dipartimento di Paleontologia, Geofisica e
Geologia, Università di Padova, Via Riviera
Mugnai,35100 Padova
The Venetian foothills between Lake
Garda to the west and San Bonifacio to the
east, can be subdivided into four different
areas by means of chemical and physical
characters of geothermal groundwaters:
- a northern one with cold Ca or Ca-Mg
rich waters (T<18.8 °C) belonging to a very
rapid karst circulation characterized by
reduced
water-rock
interactions.
The
presumed reservoir are the Jurassic
Limestones and /or the Dolomia Principale
Formation;
- a southern sector located in the Caldiero
area, characterized by water T up to 28 °C
and sulphate-rich due to the reactions with
evaporitic rocks. These waters show a circuit
deeper and slower than the previous one,
characterized by more pervasive water-rock
interactions;
- a western area located between Lazise
and Pescantina villages characterized by T
up to 35 °C and Cl-Na rich composition due
to crystalline basement-water interactions. At
Lazise the geothermal waters show a deep
circuit, as confirmed by the attainment of
rock-water equilibrium. This water show
strong temperature and chemistry variations
depending on mixing with surface waters;
- a transition area located at the boundary
between Lazise and Caldiero zones
characterized by cold Na-K rich water due to
interactions with clay sediments.
The lineaments recognized by satellite
image interpretation (Landsat 5 TM) can be
grouped on the basis of their attitude into four
major systems, some of them related to the
geothermal source distributions:
- the NNE-SSW lineaments belongs to the
Giudicarie fault system, and they are
constituted by the Baldo, M. Pastello and
Bosco Chiesanuova faults, and other minor
structures;
Fig. 1: Geomechanic model of an NE-SW running cross
section through the M.Catria anticline. The model is based on
surface geology field data.
xxii
- the NW-SE lineaments belongs to the the
Schio-Vicenza fault system;
- the NNW-SSE lineaments are parallel to
the Castelvero fault;
- the E-W to ENE-WSW lineaments,
clearly visible in the satellite images, have
few correspondences in literature since in
Lessinean area only some faults with this
direction have been recognized (Artoni &
Rebesco, 1990; Castellarin & Cantelli, 2000;
Zampieri, 2000),.
In addition the satellite images show the
occurrence of a E-W directed paleo-valley
between Sega of Cavaion and Nogarine
(VR), interpreted as a paleo-channel of the
Adige river (Antonelli et al., 1994). This fluvial
anomaly can be controlled by E-W oriented
fault and fractures.
The contouring of chemical-physical
parameters of groundwaters, pathfinder of
222
the fault areas (for example Rn; Lombardi
et al., 1999; Mancini et al., 2000; Pizzino et
al., 2000; Quattrocchi et al., 1999, 2000), has
pointed out clear relations between chemicalphysical
anomalies
(temperature,
conductivity, major, trace elements, radon)
and the lineaments recognized by satellite
images. In particular, this integrated study
suggest a close link between ENE trending
faults and fractures of the Venetian foothill
zone and the rapid rise
of geothermal
groundwaters.
REFERENCES
ANTONELLI
R.,
CAMPAGNONI
A.,
MARCOLONGO
B.,
SURIAN
N.,ZAMBRANO R. (1994): Una ricerca
integrata tra l'alta pianura veronese e
l'anfiteatro morenico del Garda per il
riconoscimento
di
risorse
idriche
alternative e della loro vulnerabilità. Primi
risultati e prospettive di sviluppo. Quad.
Geol. Appl., N°2., pp. 57-76.
ARTONI A. & REBESCO M. (1990):
Deformazioni per thrusting nei Lessini
nord
occidentali
(VeronaItalia
settentrionale). St. Geol. Camerti, Vol.
Speciale 1990, pp. 131-137.
CASTELLARIN A. & CANTELLI L. (2000):
Neo-Alpine evolution of the Southern
eastern Alps. Journal of Geod., 30, pp.
251-274.
LOMBARDI S. (Project Leader), ANGELONE
M., BARBIERI M., BILLI A., BRUNORI C.
A., BUONGIORNO F., CIOTOLI G., DI
FILIPPO M., DOUMAZ F., DUDDRIGE G.
A., FUNICELLO R., FYTIKAS M.,
GRAINGER P., GUERRA M., MARTY B.,
MELE G., MONTONE P., ORLANDI C.,
PAPACHRISTOU M., PAVLIDIS S.,
PIZZINO
L.,
PONGETTI
F.,
QUATTROCCHI
F.,
ROMEO
G.,
RUSPANDINI T., SACCHI E., SALVI S.,
SALVINI, SCARLATO P., F., SCIACCA U.
SOULAKELLIS N., TACCETTI Q., TORO
B., URBINI G., VOLTATTORNI N.,
ZOUROS N., ZUPPI G.M. (1999):
GEOCHEMICAL SEISMIC ZONATION:
Seismic
Hazard
Zonation:
a
multidisciplinary approach using fluidgeochemistry method. Final Report.
DGXII
EC
Commission,
Brussels,
(Contract No. ENV4-CT96-0291).
MANCINI C., QUATTROCCHI F., GUADONI
C., PIZZINO L., PORFIDIA B. (2000):
222
Rn
study
throughout
different
seismotectonical
areas:
comparison
between different techniques for discrete
monitoring. Annali di Geofisica, 43 (1), pp.
31-60.
PIZZINO L., QUATTROCCHI F., SCARLATO
P., VALENSISE G. (2000): The Italian
test-site: the Gioia Tauro Plain. Part 2
Fluid Geochemistry. Second Annual
Report EC Project FAUST. Contratto
ENV4-CT97-0528.
Summary
of
contributions of the Istituto Nazionale di
Geofisica, ING-Rome, pp.62.
QUATTROCCHI F., GUERRA M., PIZZINO
L., LOMBARDI S. (1999): Radon and
Helium as pathfinders of fault system and
groundwater evolution in different Italian
areas. Il Nuovo Cimento, 22 C (3-4), pp.
309-316.
QUATTROCCHI F., PIZZINO L., GALLI G.
(2000):
New
Insights
from
fluid
geochemistry in the discrimination of fault
interactions sites and their stress-strain
pathfinders parameters. Case history from
th
Italy. Poster at the 17 Course Intern.
School Solid Earth Geophys., July, 17-23,
Erice, Italy.
ZAMPIERI D. (1995): Tertiary extension in
the southern Trento Platform, Southern
Alps, Italy. Tectonics, 14, pp. 645-657.
ZAMPIERI D. (2000): Segmentation and
linkage of the Lessini Mountains normal
faults,
Southern
Alps,
Italy.
Tectonophysicsc, 319, pp. 19-31.
PLIO-PLEISTOCENE
STRUCTURAL
EVOLUTION OF THE GELA NAPPE AND
ASSOCIATED FOLD STRUCTURES IN
CENTRAL-SOUTH SICILY
xxiii
1
1
Francesca Ghisetti , Mario Grasso , Rosanna
1
2
Maniscalco , Livio Vezzani
1
Dipartimento di Scienze Geologiche,
Università di Catania
2
Dipartimento di Scienze della Terra,
Università di Torino
Progressive shortening of the outer units
of the Sicily thrust belt during Messinianmiddle Pliocene times appears to be
ultimately controlled by the backstop of the
crustal block bounded by the Mt. KumetaAlcantara fault zone. The development of the
E-W transpressive boundary, associated with
right-lateral offset of crustal blocks, and
clockwise rotations of the sedimentary
covers, appears to have largely modified the
previous, dominant N-S to NE-SW orientation
of the whole thrust belt, as reflected in the
deformation architecture of the thin-skinned
thrust edifice and of the basins perched on
thrust-tops.
The Gela nappe is the outermost and
lowermost of a series of thin-skinned thrust
sheets in the outer domains of the Sicily
thrust belt. The deformation geometry of this
thrust sheet is marked by large scale
disharmony and detachments, soft sediments
deformation, folding, and poorly-organized
thrusting,
superimposed
on
earlier
geometries of the accretionary prism.
Though there exists a clear progression,
in both time and space, in the evolution and
deformation of the sedimentary sequences
deposited in the system of outer migrating
foredeeps during Messinian to early
Pleistocene times, there appear to be
substantial
differences
between
the
structures of the Gela nappe relative to the
structures in the more internal domains of the
Caltanissetta basin.
thinner, external parts of the sedimentary
wedge, in a position close to the outer
deformation front, and adjacent to the
impinging foreland salient of the Sciacca and
Adventure ridges.
The main conclusion inferred from the
outcrop structures is that the shortening of
the Gela nappe was acquired during
progressive
sliding
and
rotational
emplacement of the detached rock units from
north to south, consequent to an episode of
uplift and thrusting in the inner Caltanissetta
basin. The southward transport of the thrust
sheet was associated to the Plio-Pleistocene
migration of the fold belt, which is recorded
by a detailed stratigraphy. Stacking at the
western margin of the tapered wedge on top
and against salients of the underlying
carbonate platform was the cause for
displacement of the rock units in a field of
trajectories of shortening deflected from NWSE to E-W. The strong disharmony between
the geometry of the Gela thrust sheet and the
deformations in the rigid substratum appear
to be of particular relevance in an area that
represents
a
significant
target
for
hydrocarbon exploration.
The structural evolution of this segment of
the chain is suggestive of a strongly
heterogeneous history of deformation from
the innermost to the outermost thrust sheets,
and at different structural levels of the
imbricate stack, consequent to synkinematic
rotations, gravity-driven deflection of the
regional trajectories of shortening, and
multiple detachments.
In the reconstruction we propose, the
deformation geometry of the Apenninic
margin in Sicily is in fact strongly
compartmentalised between an innermost
sector, where both the thrust belt and the
covers of the deformed foreland basins bear
the imprint of NW-SE shortening, associated
with transpression along an E-W margin, and
an outermost sector, where large-scale
detachments, gravity-driven sliding, and
strong geometric discordances characterize
the deformation style of the covers relative to
the underlying, rigid substratum. The
boundary between these two domains is
transitional. Apparently, the geometric imprint
of the substratum is more effective in the
xxiv
STRATIGRAPHIC EVIDENCE FOR THE
TECTONIC EVOLUTION OF THE MOUNT
ALTESINA THRUST SYSTEM, CENTRAL
SICILY
1
also imply rather low post-Messinian net
displacements across the Sicilian thrust belt,
which fits also small amounts of CW
palaeomagnetic rotations detected from late
Miocene and Pliocene sediments.
1
Mario Grasso , Rosanna Maniscalco , Robert
2
W.H. Butler
1
Dipartimento di Scienze Geologiche,
Sezione Geologia e Geofisica, Università di
Catania,
95129Catania,
Italy,
E-mail:
[email protected]
2
School of Earth Science, The University of
Leeds, LeedsLS2 9JT, UK.
Syn-orogenic sediments, accumulated in
thrust top basins, provide clear tests of
larger-scale tectonic models. The Neogene
sediments of the central Sicilian thrust top
basins
offer
high-resolution,
readily
correlated stratigraphies that can be used to
document depocentre migration, tilt rates and
thrust activity. The example chosen for this
contribution is the Mount Altesina backthrust
system and its related basins (Corvillo and
Leonforte). The relatively long-lived history of
this structure exerted a strong impact on the
deposition. The Mount Altesina backthrust,
cored by Mesozoic carbonates to earlymiddle
Miocene
terrigenous
foredeep
deposits, is overlain by syntectonic
sediments that range from Late Tortonian to
mid-Pliocene age. These units record the
progress, nearin-situ structuring of thrust-top
basins. Topographic highs can be charted
using various markers (patch reefs, margin
evaporite facies, shore lines), as can subbasin depocentres (clays, thick evaporites).
Within both the footwall and the hangingwall
to the backthrust, middle Miocene to upper
Tortonian late orogenic sediments are
separated by major sequence boundaries
(SB1-SB2 type), which developed, during
thrust propagation, with different fashion due
to their location with respect to the evolving
structure.
All point to the sub-basins
remaining largely fixed in space. Most of the
structures show slopes that amplified with
single sense through time. Therefore these
basins have not passed up and over buried
ramps as a requirement of the "flat-on-flat"
thrust stack models. However, on regional
grounds it can be argued that the substratum
to the basins has moved as a coherent sheet
(the Gela Nappe) and the structures at
outcrop produce net thickening of this
orogenic wedge through time. These
conclusions have important implications for
the deep structure of the thrust belt, which
hosts important oil and gas reserves. They
xxv
STRUCTURAL SETTING OF THE “GRAN
SAN BERNARDO NAPPE” ALONG THE
SOUTHERN AOSTA VALLEY TRANSECT
(WESTERN ALPS, ITALY)
1
2
1
Malusà M. , Martin S. , Polino R.
C.N.R. – Istituto di Geoscienze e
Georisorse, Sezione di Torino
2
Università dell’Insubria, sede di Como, Dip.
Fisica, Chimica e Matematica
1
The present structural setting through the
southern Aosta Valley transect is the result of
a complex and polyphasic evolution:
basement units described in the literature as
belonging to the Gran San Bernardo nappe
(GSB) have followed independent evolution
during the Alpine orogenesis and now they
are separated by means of ophiolite-bearing
shear zones.
The lowest structural element of the
investigated area is the ophiolitic Rovenaud
unit, which represents the footwall of the
GSB nappe (figure 1). It is characterized by
blueschist
facies
calcschists
(RVNa),
metabasites and serpentinites (RVNb)
strongly re-equilibrated under greenschist
facies conditions.
It is tectonically overlaid by the Gran
Nomenon basement unit along the Entrelor
shear zone, that involves slices of both
basement
and
cover
rocks
(ESZ)
recrystallised under greenschist facies
conditions (Freeman et al., 1997). The
kinematics of the Enterlor shear zone is still
matter of debate: top-to-W extension (Caby,
1996), top-to-ESE back-thrusting (Butler &
Freeman, 1996) or top-to-W thrusting
(Bousquet & Schmid, 2001).
The Gran Nomenon unit (GNM) is
constituted by paragneisses with minor
metabasites (GNMa), intruded by granitoid
rocks (GNMb) that usually preserve the
original magmatic texture. They have
commonly assumed to be Permian in age,
due to the lack of pre-Alpine metamorphic
fabric, but recent SHRIMP data yield a late
Devonian age of emplacement (Bertrand et
al., 2000). The oldest metamorphic fabric of
the GNM unit, preserved in the metapelites,
is represented by a relic foliation believed to
be pre-Alpine in age on the basis of
mesostructural and metamorphic evidence.
This foliation has been strongly deformed
and transposed by the Alpine deformation
phases, which develop a new axial plane
foliation under greenschist facies conditions
in the metapelites, but it is weakly penetrative
in the metagranitoids. In places, it is
deformed by late asymmetric overturned
folds with Z-asymmetry (looking W). On the
south-eastern side, the GNM unit is
juxtaposed to the eclogitic ophiolites of the
Grivola unit (GRV) along the Belleface-Trajo
fault, a steeply-dipping kilometric-scale fault
zone marked by slices of marbles,
calcschists, serpentinites, basement rocks
and by carbonatic cataclastic breccias (BTF).
At the megascale, the GNM and the adjoining
ophiolitic units are usually described as
forming a SSE-facing anticline, the so-called
"Valsavarenche backfold". Although the
megascopic re-structuring of the GNM unit is
clearly due to E-W oriented folding phases,
the existence of a single megascopic fold is
not consistent with the observed minor folds
pattern.
Moving towards the higher and more
external GSB nappe portions exposed along
the transect, two other basement units
(Leverogne
and
Ruitor)
have
been
recognized.
The Leverogne unit (LVR) is represented
by micaschist with minor metabasites
(LVRa), intruded by granophyric rocks
(LVRb) in the middle Cambrian (Bertrand et
al., 2000). The oldest planar fabric in the LVR
unit is a tectonic foliation, developed under
blueschist facies conditions, locally preserved
in the metabasites. The main foliation,
developed
under
greenschist
facies
conditions, is associated to transposed
isoclinal folds trending from NE-SW to E-W. It
is deformed by NE-SW trending open to tight
folds, which can locally develop a penetrative
axial-plane
cleavage,
and
by
later
asymmetric EW-trending open folds with Zasymmetry (looking W). The LVR unit
overlays the GNM unit by means of the
Feleumaz shear zone (FSZ), and it is
overthrust by the Ruitor polymetamorphic
basement unit (RUI) (see Schiavo, 1997)
along the Tsaboc shear zone (TSZ). Along
these shear zones basement, cover and
ophiolitic rock elements, characterized by a
penetrative milonitic foliation developed
under greenschist facies conditions, are
preserved. Few slices of ophiolitic and
basement rocks preserve relics of an older
foliation developed under blueschist facies
conditions. As associated folds face E, these
structures are traditionally inferred to have
been associated with back-thrusting (Caby,
1968).
xxvi
1
Dipartimento di Chimica, Fisica e
Matematica, Università dell'Insubria (Como),
Italy
Fig. 1 – Geological cross-section of the study area and
structural data plotted on the equal area net (lower
emisphere): dots are poles to the main foliation, great
circles represent the attitude of the mean foliation plane
(see text for achronymous).
Therefore, the actual setting of the GSB
nappe and the “Piedmont zone” in the
southern Aosta Valley is more complicated
respect to the picture usually described in the
literature. On the basis of the new collected
data, in the part of GSB nappe exposed
along this transect it is possible to recognize
three tectono-stratigraphic units showing
peculiar
stratigraphic,
structural
and
metamorphic characters: the Gran Nomenon,
the Leverogne and the Ruitor units. In this
area, units classically ascribed to the
“Piedmont zone” form shear zones of
variable thickness which separate different
basement units.
REFERENCES
Bertrand, J. M., Pidgeon, R. T., Leterrier, J.,
Guillot, F., Gasquet, D. & Gattiglio, M.,
2000. Schweiz. Miner. Petr. Mitt. 80, 225248.
Bousquet, R. & Schmid, S., 2001. Dalla
Tetide alle Alpi – Convegno scientifico in
memoria di Giulio Elter. Volume dei
riassunti. 11-12.
Butler, R. W. H. & Freeman S. R., 1996.
Journal of Structural Geology. 18, 909923.
Caby, R., 1968. Géologie Alpine. 44, 95-110.
Caby, R., 1996. Eclogae Geol. Helv. 89, 229267.
Freeman, S. R., Inger, S., Butler, R. W. H. &
Cliff, R. A., 1997. Tectonics. 16, 57-76.
Schiavo, A., 1997. PhD Thesis, Università di
Padova, 123 pp.
THE ULTEN UNIT: AN EXAMPLE OF
STRENGHT
COMPETITION
IN
SUBDUCTION ZONE
Silvana Martin
1
The occurrence of Gt-peridotites in high T
and P felsic rock complexes is well known in
both alpine and pre-alpine orogeneses.
While the exhumation of dense and
sometimes large peridotite bodies can be
explained by invoking buoyancy effects of
surrounding crustal material, the presence of
mantle bodies within felsic (crustal) rocks
can be explained in different ways as: (i) a
tectonic emplacement of mantle slices during
plate convergence, (iii) mantle exhumation in
an ocean environment and embedding into
mélange in a subduction zone and (iiii)
driving mantle wedge slices
into a
subduction zone by convection-related
downward flow near
the slab-mantle
interface. If the orogenic Gt-peridotites do
not show any relic of a previous low T stage,
more consensus is addressed to the last
possibility. This possibility is then examined
for the Gt-peridotites of the Ulten unit.
The Ulten unit of the Austroalpine system
derives from a pre-alpine subduction
mélange (Martin and Ranalli, 2001). The rock
complex includes almost three lithological
and structural units: 1) a lower eastern slice
composed
of
predominant
stromatic
(migmatitic) gneisses, injected trondhjemitic
leucosomes,
spinel /
garnet-spinel
peridotites and rare retrogressed eclogites
(Samerberg slice); 2) a lower western
element
composed
by
predominant
micaschists, rare retrogressed eclogites,
eclogitic metagabbros and orthogneisses
(Cima Mezzana, Campana, 1994), 3) an
uppermost slice composed by predominant
retrogressed
migmatites
including
metagranitoids and Gt-peridotites (Binasia
ridge slice). These slices, separated by
ductile shear zones lately reactivated by the
alpine tectonics, preserve
diachronous
records of their eclogitization history.
Retrogressed eclogites, paragneisses and
orthogneisses from Samerberg, Cima
Mezzana and Binasia slices yield very old
Sm-Nd internal isochron and Ar-Ar amphibole
ages spanning from 399+1 to 351+1 Ma
(Martin et al., 1998; Del Moro et al., 1999).
By contrast, the stromatic gneisses and
restites
(migmatites),
the
injected
trondhjemitic leucosomes and Gt-peridotites
yield an age of about 330 Ma (U-Pb zircon
ages from pyroxenites, Gebauer and
Grunenfelder, 1978; Sm-Nd, Tumiati and
Thoni in prep.; Del Moro et al., 1999). This
suggests that migmatites and peridotites
xxvii
were involved into a final event of
crystallization
during
Carboniferous.
Considering
these
geochronological,
petrological and geochemical data, the
following scenario can be proposed: during
the Late Paleozoic lithospheric mantle went
downwards following a convection path near
a subduction zone, at the same time crustal
slices of the contiguous slab, coming from
deeper positions of the subduction zone,
moved upwards along uppermost faultingthrusts (see the model of Chemenda et al.,
1996). In the upward-moving felsic slices of
the slab, local partial melting occurred due to
a decompressional effect (Godard et al.,
1996) producing a considerable softening of
the crustal (felsic) material. Occurrence of
injected trondhjemites suggest a more
pervasive melting in the deepest crustal
portion of the slab (Del Moro et al., 1999);
this testifies for the end of the crustal
subduction and for a possible break-off of a
crustal
portion
of
the
slab
since
Carboniferous.
Peridotites of the mantle wedge were
tectonically inserted as a slice composed of
protogranular to porpyroclastic rocks within
the uppermost crustal up-thrusts. Finding of
the same geochronological ages in
peridotites and migmatites confirms that recrystallization of peridotites under Gt- stability
field and crystallization of the partially melted
gneisses (the tectonic mélange) occurred at
the same time.
The emplacement of peridotites was
favored by the contrasting behaviour of the
partially melted gneisses and peridotites due
to a "soft rheology" controlled by quartz and
able to deform ductilely in the gneisses and
to the "harder rheology" controlled by olivine
in the mantle (Ranalli, 2000). Thus, the
maximum strenght competition is expected
between the moving upward partially melted
gneiss-thrusts and the
downward going
peridotites to explain the formation of a suite
of hectometric bodies of peridotites inside the
Ulten migmatites.
REFERENCES
Campana,
R.,
1994.
Quaderni
di
Geodinamica Alpina e Quaternaria, 2, 5964.
Chemenda, A. I., Mattauer, M., Bokun, A. N.,
1996. Earth Planet. Sci. Lett. 143, 173182
Del Moro, A., Martin, S. and Prosser, G.,
1999. J. of Petrology, 40, 1803-1826.
Gebauer, D. & Grünenfelder, M., (1978). US
Geolog. Survey Open file Report 78-101,
135-137.
Godard, G., Martin, S., Prosser, G., Kienast,
J. R., Morten, L., 1996. Tectonoph. 259,
313-341.
Martin, S., Laurenzi, M., Del Moro, A.,
Susini, S. and R., Campana, 1998, 50,
86-88.
Martin S., Ranalli G., 2001. European Union
of Geosciences, EUG XI, Strasbourg.
Ranalli G., 2000. J. Geodyn. 30, 3-15.
THE SPRECHENSTEIN-VAL DI MULES
LINE: A TECTONIC LINKAGE BETWEEN
BRENNER
AND
PUSTERIA
FAULT
SYSTEMS
1
1
M.
Massironi ,
A.
Bistacchi ,
R.
2
1
3
Brandner ,G.V. Dal Piaz , B. Monopoli , A.
3
Schiavo
1
Geology Department - University of Padova
2
Institute of Geology - University of
Innsbruck
3
LTS Srl – Treviso
Tectonic relationships between the
Brennero low-angle detachment and some
major faults of the Periadriatic lineament in
the eastern Alps (North-Giudicarie, Passiria,
Pusteria, DAV) has been a matter of long
debate (eg., Ratschbacher et al., 1991;
Fügenschuh et al., 1997; Mancktelow et al.,
2001; Müller et al., 2001). Our field survey for
the Brenner Basistunnel (BBT) project
supplies new data on this topic, providing a
more detailed tectonic framework of the
southern edge of the Austroalpine-Penninic
wedge, Mules tonalitic lamella (Oligocene)
and
Southalpine
Bressanone
granite
(Permian) along the Fortezza-Val di Vizze
corridor. This area is characterised by: i)
numerous NNE-SSW Brennero-related faults,
dissecting the overturned southern limb of
the Austroalpine-Glockner nappe stack, and
ii) a major WNW-ESE dextral shear-zone,
namely the Sprechenstein-Val di Mules fault.
This previously unknown fault runs from the
Sprechenstein castle (south of Vipiteno
alluvial plain), through the Mules valley,
where it displaces the Pusteria fault, to the
Valles valley, where it is evidenced by a wide
cataclastic horizon inside the Bressanone
granite. Its NW extension beyond the Isarco
valley can not be excluded despite the
supposed continuation of the Brennero
mylonites into the Giovo deformation horizon
xxviii
(Fügenschuh et al., 1997; Viola et al., 2001),
as well as a second major fault below the
Isarco alluvial deposits which would
accommodate the dextral displacement
between the Passo Pennes (Stilves-Corno
Bianco) and Mules Permo-Triassic slices.
The brittle-ductile to brittle deformation
phases of the study area can be summarized
as follow:
D1 - The most important structures of this
phase are ductile to brittle SC' shear bands
showing a sinistral to transtensive activity
along the schistosity at the Glockner Austroalpine boundary, steeply dipping to the
north. Along the Pusteria line some fault
planes with the same kinematics can be also
attributed to D1. The age of this phase is late
Oligocene-early Miocene since several
Oligocene dikes show SC' structure with a
consistent syn-magmatic D1 kinematics
(Mancktelow et al., 2001).
D2 - This phase is characterised by
dextral strike-slip kinematic of
the
Sprechenstein-Val di Mules fault, a sinistral
transtension of the NNE-SSW system, a
dextral transpression along the Pusteria fault
and a sinistral transpression along ENEWSW structures. Paleostress inversion show
an horizontal and E-W oriented σ3. In low
strain domains σ1 is horizontal and N-S to
NNW-SSE striking,
locally σ1 and σ2
inversion is
recorded
along faults
alternatively
with dominant normal and
strike-slip kinematics.
Late
dextral
movement
of
the
Sprechenstein-Val di Mules fault is recorded
by
consistent
kinematic
indicators
(slickensides, T and R-type joints) and by
displacement of (from S to N) the Pusteria
fault, Mules tonalitic lamella, Mules / StilvesCorno Bianco Permo-Eotriassic cover slices
and Austroalpine-Glockner nappe contact.
The total dextral offset across the Mules /
Stilves-Corno
Bianco
slices
and
Austroalpine-Glockner nappe contact is 4.5
Km.
This kinematics is compatible with
continuing N-S plate contraction, late
evolution of the Brennero low-angle
detachment and vertical to eastward lateral
extrusion of the Penninic units. After the
Oligocene
magmatic
pulse,
the
Sprechenstein-Val di Mules fault was the
most important tectonic feature in the study
area becoming the tectonic linkage between
Brennero and Pusteria systems.
Fügenschuh, B., Seward, D. & Mancktelow,
N. (1998): Exhumation in a convergent
orogen: the western Tauern window. Terra Nova, 9, 213-217.
Mancktelow, N., Stöckli, D.F., Grollimund, B.,
Müller, W., Fügenschuh, B., Viola, G.,
Seward, D. & Villa I.M. (2001): The DAV
and Periadriatic fault system in the
Eastern Alps south of the Tauern window.
- Intern. J. Earth Sci., 90, 3, 593-622.
Müller, W., Prosser, G., Mancktelow, N.,
Villa, I.M., Kelley, S.P., Viola, G. & Oberli,
F. (2001): Geochronological constraints
on the evolution of the Periadriatic Fault
System (Alps). - Intern. J. Earth Sci., 90,
3, 623-653.
Ratschbacher, L., Frisch, W., Linzer, H.G.,
Merle, O. (1991): Lateral extrusion in the
eastern Alps. Part 2. Structural analysis. Tectonics, 10, 257-271.
Viola, G., Mancktelow, N., Seward D., (2001).
Late Oligocene-Neogene evolution of
Europa-Adria collision: new structural and
geochronological evidence from the
Giudicarie fault system (Italian Eastern
Alps). Tectonics, 20, 6, 999-1020.
REFERENCES
xxix
EVOLUTION
OF
“BRITTLE-DUCTILE”
SHEAR
ZONE
ARCHITECTURE,
DISPLACEMENT AND SHEAR STRAIN
DURING NORMAL FAULT NUCLEATION
IN LIMESTONE: STRUCTURAL AND
FLUID INCLUSION ANALYSIS
a
b
Stefano Mazzoli , Daniela Di Bucci , Chiara
c
Invernizzi
a
Facoltà di Scienze Ambientali, Università di
Urbino, Campus Scientifico Sogesta, 61029
Urbino (PU); E-mail: [email protected]
b
Servizio Sismico Nazionale, Via Curtatone
n.
3,
00185
Roma;
E-mail:
[email protected]
c
Dipartimento di Scienze della Terra,
Università di Camerino, Via Gentile III da
Varano, 62032 Camerino (MC); E-mail:
mailto:[email protected]
[email protected]
The process of fault initiation by the
coalescence of en échelon arrays of tensile
cracks has been discussed by several
workers and is now well established.
However, not many studies have tried to
quantitatively describe this process in terms
of displacement and shear strain that control
the early stages of fault nucleation. In this
study, a large number of conjugate “brittleductile” extensional shear zones have been
analysed. These structures are exposed in
well bedded micritic limestones deformed at
very low-grade conditions in the area of
Monte Cugnone (Marsico Nuovo, Basilicata).
Three different types of structures can be
distinguished: (i) “brittle-ductile” shear zones,
characterised by arrays of en échelon,
calcite-filled tension gashes; (ii) shear zones
showing
incipient
development
of
discontinuous shear-parallel fractures cutting
through the vein array; and (iii) faulted shear
zones, in which a continuous, discrete normal
fault breaks through – and clearly developed
from – an original en échelon vein array.
Environmental (P-T) conditions existing
during the development of the analysed
shear zones were mostly controlled by the
tectonic burial resulting from the previous
contractional episodes. Fluid inclusions from
vein calcite sampled from the different sets of
structures (i) to (iii) above indicate that
environmental conditions remained constant
during the different stages of shear zone
evolution and normal fault development.
Maximum homogenisation temperatures from
primary fluid inclusions are all consistently in
the range of 130°-140°C.
Our results suggest that displacement and
shear strain exert a major control on fault
initiation by the coalescence of en échelon
vein arrays in the studied limestones. Critical
values of displacement (D ≈ 9 cm) and shear
strain (g ≈ 1.2) can be determined for the
onset of normal fault nucleation from “brittleductile” shear zones in these rocks. The
process of fault initiation mostly takes place
for displacement values of 9 < D < 17 cm and
shear strain in the range of 1.2 < g < 2.7.
Shear zones characterised by D < 9 cm and g
< 1.2 consist, with very few exceptions, of
classic
“brittle-ductile”
features.
The
deformation processes responsible for the
formation of these structures are very likely to
be of dominant viscous type (i.e.
intracrystalline
deformation,
pressure
solution-crystallisation),
obviously
accompanied by brittle fracturing (as shown
by the coeval formation of tensile cracks).
Viscous deformation processes, of both
linear (solution mass transfer) and/or power-
xxx
law type (dislocation glide, dislocation creep)
are likely to become progressively less
important as discontinuous shear-parallel
fractures, characterising the incipient stages
of fault nucleation, start to form. Eventually,
once through-going faults are well developed,
deformation becomes of essentially frictional
type. In this context, during incipient fault
nucleation (9 < D < 17 cm and 1.2 < g < 2.7),
a transition from a dominant viscous to a
frictional behaviour occurs.
Displacement data from the analysed
shear zone population indicate that this
parameter is self-similar for more than one
order of magnitude. Similar power-law
relationships are well established for normal
fault populations, which tend to be self-similar
over a range of scales. Therefore, it seems
that, in this respect, “brittle-ductile” shear
zones dominated by viscous deformation
processes (accompanied by brittle fracturing)
display a behaviour of the type generally
shown by faults in the frictional regime.
FAULT ZONES ARCHITECTURE: HOW TO
SEPARATE FAULT SEGMENTS INTO
HIERARCHIC
ORDERS?
FIELD
EXAMPLES
1
2
Fabrizio Piana , Paolo Perello
1
CNR Istituto di Geoscienze e Georisorse,
sezione di Torino
2
SEA Geoconsulting, via Gioberti 78, Torino
Fault zones consist of linked fault
segments whose geometric relations are
clearly understood only if a large part of each
studied fault zone is exposed.
Unfortunately, this case is rare on the
field, where the most common cases are
those in which fault segments crop out as
individual shears, separated by unexposed
areas or fault-free rock domains. Genetic
relations among faults cannot be easily
reconstructed, so that faults are commonly
separated into groups (systems) simply on
the basis of their azimuth distribution or
kinematic characters. Consequently, regional
kinematic interpretations based only on
inferred kinematics of distinct "fault systems"
often fail to explain some (or many)
controversial data sets whose weight
appeared as statistically relevant in many
studied cases.
Subparallel sets of faults and joints often
have different kinematic significance in the
same area, due to the progressive evolution
of brittle discontinuous fault zones. Fault sets
which in the first stages of the evolution
played a certain kinematic role (for example
Andersonian conjugate faults) could be
reactivated as secondary structures (for
example Riedel shears of a larger shear
zone) in subsequent kinematic stages, when
the fault system evolved toward a more
complex and hierarchical organisation. In
such cases, local kinematic data often
indicate a dual kinematic and strain history,
whose understanding could be problematic.
The recognisance of "homogeneous
sectors" (domains within which the geometric
and kinematic features of faults and/or
fractures are relatively constant) is a useful
tool to reconstruct the hierarchy relations
among segments of fault zones, in order to
separate local strain from bulk (or regional)
strain. This approach can be used when
"paleostress" analyses of data sets dispersed
over a wide area are unsuccessful, due to
strong strain partitioning and local stress
variations. Moreover, the subdivision of rock
domains into "homogeneously faulted
sectors" could allow a better understanding
of joint distribution, especially in rocks
affected by low amount of strain.
The structural interpretation of each
"homogenous sector" cannot disregard in any
case the recognisance of structural
associations widely distributed over the entire
investigated area. On the contrary, the
homogeneous structural sectors should be
defined
according
to
a
preliminary
interpretation of structural associations.
Moreover, it is here pointed out that the
concept of homogeneous structural sector is
also useful for the application of geological
data (for example in the frame of works
involving large rock volumes - i.e. tunnels,
dams etc...) since it allows geologists to
furnish statically weighted data for modelling
and engineering, and to refer data to
"correctly" delimited rock volumes. In this
way, fracture models should be grounded, for
each fault zone, on "rules" directly derived
from the hierarchic relations among the fault
segments
REFERENCES
Forlati F. Piana F., 1998. In: Barla G. Ed.
"MIR 98" Patron, Bologna, 105-110.
Hobbs B.E. (1993) - In: J.A.Hudson Ed.,
Comprehensive Rock Engineering, 1,
Pergamon, Oxford.
Mandl G., 1987. Discontinous fault zones J.
Struct. Geol. 9, 105-110.
xxxi
Perello et al., 2002 Geodinamica acta in
press.
Stone
D.
(1984)
Int.J.Rock.Mech.,
Riv.Sci.&Geomech., Abstr., 21, 183-194.
PARTICLE SIZE DISTRIBUTIONS AND
THE EVOLUTION OF CATACLASIS IN
CARBONATE FAULT ROCKS
1
1
1
F. Storti , A. Billi , F. Salvini
Dipartimento di Scienze Geologiche,
Università “Roma Tre”, Roma, Italy,
[email protected]
1
Fragmentation within fault zones has
been modeled as a scale invariant, fractal
process, as well as the particle size
distribution of cataclastic rocks in fault cores.
The fractal dimension D of the particle size
distribution in cataclastic rocks is a measure
of their sorting and depends on the
fragmentation process, the stress conditions
during faulting and the rock type. D values for
fault gouges from crystalline basement rocks
have been shown to average around 2.6,
possibly varying with the distance from the
master slip surface. Experimental work on
simulated fault gouges confirmed their
tendency to develop D values of about 2.6
supporting
self-similar
cataclasis.
Displacement localization in experimental
shear bands has been shown to cause an
increase of D, that may reach values close to
4. Such high (apparent) D values are
unexpected in self similar cataclasis since D
= 2.58 describes a geometry that minimizes
the fracture probability of particles having
similar dimensions. A proposed explanation
for D values higher than about 2.6 is the
preferential fragmentation of large particles,
that decreases the upper fractal limit of the
fractal particle-size distribution. An additional
possibility to explain the development of D
values higher than 2.6 is the increase of the
number of small particles by grinding during
the overall particle size reduction process.
We studied cataclastic rocks developed
along outcrop-scale strike-slip, extensional,
and reverse faults in massive carbonates of
the Apennines, Italy. We selected well
exposed fault zones having few meters of
displacement and a single gouge-bearing slip
zone in the fault core. Particle size analysis
were performed in the 2 mm-0.125 mm range
and gave D values ranging from about 1.8 to
3.9. The higher D values pertain to the gouge
layers, and the lower values to the embryonic
fault cores. To explain such a large variability
of D and its relations with the structural
architecture and fragmentation processes in
fault cores, we propose an evolutionary
model which involves a relation between
particle size and fault slip, the transition from
dominant fragmentation to dominant surface
xxxii
grinding with increasing displacement, and
the variation of particle strength with size.
DEFORMATION IN THE ULTRAMAFIC
ROCKS FROM A SUBDUCTION ZONE.
CASE
STUDIED:
THE
HOCHWART
PERIDOTITES (ULTEN ZONE)
1
2
S.Tumiati , S. Martin , P. Nimis
1
Università di Padova
2
Università dell'Insubria, Como
1
A hectometric concordant body and minor
lenses of garnet- bearing ultramafic rocks are
present within stromatic (high grade)
gneisses in the Hochwart area, Ultental
(Austroalpine domain, South Tyrol). They
were first reported and drawn by Andreatta
(1935) in the n° 10 “Bolzano” 1:100.000
sheet of the geologic map of Italy. The
largest body is characterized by the
occurrence of centimetric (5–8 cm) pyropic
garnets in a fine-grained strained olivine
matrix. Garnet-peridotites and gneisses form
a tectonic mélange, typical of crustal
subduction zones, which can be related to
the Variscan (Hercynian) orogeny on the
basis of geochronological data (U–Pb on
zircons: 332–336 Ma, Gebauer and
Grünenfelder, 1979; K–Ar on phlogopite: 300
(
manular, (ii) coarse- to fine-grained
porphyroclastic, (iii) fine-grained equigranular
and (iv) coarse recrystallized.
(i) Rare coarse-grained (centimetric)
protogranular lherzolites show garnet as
coronas around Cr-spinel and as exsolutions
in orthopyroxene and clinopyroxene. The
paragenesis includes minor secondary
amphibole after garnet and clinopyroxene
and interstitial pyrrotine and pentlandite.
Pyroxenes exhibit kink bands and curved
crystal planes underlined by garnet
exsolution lamellae which demonstrate that
plastic deformation (Nicolas and Poirier,
1976) occurred in the stability field of garnet.
Olivine only shows undulose extinction.
Garnet forms thick coronas around relic Crspinel, and are partially transformed into Cpxfree undeformed amph + spl + opx
kelyphites. These protogranular lherzolites
were largely preserved from the influence of
fluids and deformation. The most likely
setting is in a lithospheric mantle
environment.
(ii) The porphyroclastic texture is
widespread in the Hochwart peridotite and
shows transitions to protogranular and
mosaic-equigranular types. In this type of
rock, composed of ol + opx + grt + spl +
amph
(
cpx,
undulated
millimetric
porphyroclasts tend to recrystallise into
smaller grains (0.5 mm max), which are
xxxiii
embedded in a middle- to fine-grained (( 0.2
mm) ol + opx + amph flow-textured matrix.
(iii) The fine-grained mosaic-equigranular
texture is the most common in the Hochwart
area and in general in the whole Ulten area
(Obata and Morten, 1987; Susini and Martin,
1996). The fine-grained rocks exhibit a
marked tectonic foliation (calculated mean
vector 107.6/83.4), concordant with the
country-rock schistosity, underlined by
transposed bands with different modal
contents of pyroxene, amphibole and garnet.
The deformation is ductile (Ranalli and
Murphy, 1987), with solid-state flow
structures typical of wet mantle rocks. In fact,
addition of H2O produces a weakening effect
and a viscosity reduction of a factor as much
as 100 in "wet" olivine-rich peridotites (Hirth
and Kohlstedt, 1996). These peridotites seem
to have been generated from elastoplastically deformation of protogranular
lherzolites, as proposed by Obata and
Morten (1987). In this rocks, garnet is both
pre- (porphyroclasts) and syn- (centimetric
porphyroblasts) kinematic and is surrounded
by a well developed post-kinematic spl +
amph + opx kelyphite. Large opx
porphyroclasts are also commonly preserved.
This fine-grained type is markedly affected
by metasomatic enrichment in LREE, K and
Sr and by a modal increase of pargasiticedenitic amphibole (up to 25% modal,
Rampone and Morten, 2001) in textural
equilibrium with garnet (Obata and Morten,
1987). Presence of margarite, phlogopite,
tremolite, actinolite, dolomite, zircon (Susini
and Martin, 1996) and allanite (Tumiati in
prep.) confirms infiltration of crust-derived
H2O,
CO2,
LILE-rich
fluids
during
deformation–recrystallization. This stage
presumably occurred when the peridotites
were already in the crustal mélange.
(iv) One meter from the contact with the
gneisses, the fine-grained peridotite shows
an increase in grain size (up to centimetric),
which is associated with disappearance of
garnet and increase of amphibole content.
Near the contact the assemblage also
includes tremolite, chlorite and serpentine,
probably due to metasomatic exchange with
the surrounding rocks. These coarse-grained
recrystallized (protogranular-like) garnet-free
lherzolites include a poorly developed matrix
composed by olivine (<1mm) and amphibole.
The large lobate recrystallized olivines (>1
cm ) are pervaded by fluid inclusion trails and
include ilmenite rods and palisades, like the
olivines from Alpe Arami, interpreted by
Risold et al. (2001) as related to breakdown
of Ti-bearing humite layers in olivine. Opx is
always kinked and shows spl + rutile (?)
exsolution, but no grt exsolutions. Cpx is
almost completely replaced by amphibole
growing from the borders of the crystal.
Amphibole is also present as single
recrystallized grains and in rounded kelyphitic
aggregates presumably pseudomorphic after
garnet. Large amoeboid millimetric Cr-rich
spinel is scattered throughout the rock.
Locally, pockets of fine-grained dunite with a
typical mosaic texture, including small spinel
grains and spl + amph clusters are found. In
one of these pockets, a centimetric crystal of
Cr-rich allanite was discovered, indicating
interaction with crustal fluid. The coarsegrained recrystallized peridotites can be
interpreted as due to static recrystallization of
the fine-grained peridotites in a fluid-rich
environment.
This study demonstrates that the
deformation style of peridotites in a
subducted slab can vary from brittle to ductile
as a function of fluid availability. It is believed
that only a combination of structural,
geochemical,
geochronological
and
petrological studies can provide the
necessary constraints to unravel the
metamorphic history and geodynamic
significance of crustal rocks in the crystalline
domain.
REFERENCES
Andreatta C., 1935. Mem. Mus. Stor. Nat.
Venezia Tridentina. 3 (2), 87-245.
Gebauer D., Grünenfelder M., 1979. Proc.
Int. Ophiolite Symposium, Cyprus. 215218.
Hirth G., Kohlstedt D.L., 1996. Earth Planet.
Sci. Lett. 144, 93-108.
Nimis P., Morten L., 2000. J. Geodynamics.
30, 93-115.
Obata M., Morten L., 1987. J. Petrology. 28,
599-623.
Rampone E., Morten L., 2001. J. Petrology.
42, 207-219.
Risold A.C., Trommsdorff V., Grobéty B.,
2001. Contrib. Mineral. Petrol. 140, 619628.
Ranalli
G.,
Murphy
D.C.,
1987.
Tectonophysics 132, 281-295
Nicolas A., Poirier J.P., 1976. Crystalline
Plasticity and Solid State Flow in
Metamorphic Rocks, John Wiley & Sohns,
London
Rost F., Brenneis P., 1978. Tschermak's
Mineral. Petrogr. Mitt. 25, 257-286.
Susini S., Martin S., 1996. Atti Ticinensi di
Scienze della Terra. 4, 47-63.
xxxiv
Thöni M., 1999. Schweiz. Mineral. Petrogr.
Mitt. 79, 209-230.
MYLONITIZATION PROCESSES IN THE
RED RIVER SHEAR ZONE, NORTHERN
VIETNAM:
MICROSTRUCTURAL
EVIDENCE
AND
TECTONIC
IMPLICATIONS
1
2
G. Viola , R. Anczkiewicz
Department of Geological Sciences,
University of Cape Town, 7701 Rondebosch,
South Africa
2
Institute of Geological Sciences, Polish
Academy of Sciences, ul. Senacka 1,
Krakow, Poland
1
The Red River Shear Zone (RRSZ) has
been described as a left-lateral fault
absorbing significant amount of postcollisional
India-Asia
convergence
by
accommodating lateral extrusion of the
Indochina block. The most valuable
information on the RRSZ comes from studies
of the metamorphic massifs exposed within
the shear zone itself. Geochronological and
PT data obtained for the Ailao Shan (Yunnan,
S China) and the Day Nui Con Voi (DNCV, N
Vietnam) massifs indicate a period of rapid
cooling (100° C/Ma), which was attributed to
uplift and exhumation in a transtensional
setting. Based on field
work
and
microstructural studies we offer the first
evidence of a transtensional phase and
suggest a twofold evolution for the DNCV in
N Vietnam.
Early deformation of DNCV is related to
left-lateral NW-SE oriented shearing, which
resulted in a complex large-scale antiformal
structure with axis parallel to the
subhorizontal stretching lineation. The
foliation attitude is very steep to subvertical in
the limbs of the antiform (bound by the two
discrete faults, the SW dipping Song Hong
fault in the southwest and by the NE dipping
Song Chay fault in the northeast) and tends
to flatten out in the middle part of the antiform
to very gentle dip angles. There is a clear
strain gradient with the strongest mylonitic
fabric concentrated on the limbs of the
antiform. Most of the stromatic gneisses,
migmatites and leucogranites typical of the
DNCV massif are located in the core of the
structure. The stromatic gneisses and
migmatites are essentially of metapelitic
composition and show a well-developed
metamorphic
foliation.
The
regional
schistosity is defined by red biotite, elongated
sillimanite crystals and layers of dynamically
recrystallised quartz and plagioclase. This
first phase of shearing took place under high
temperature conditions, as shown by
amphibolite facies minerals, stable in shear
bands and clasts pressure shadows and by
quartz fabric measurements. Garnet is
abundant in the rocks and is often flattened in
the schistosity planes with measured aspect
ratios up to 5:1. Sillimanite is present in
spectacular, long euhedral crystals always
parallel to the greatest finite stretching
direction. It is also found in the pressure
shadows of the garnet clasts and sometimes
aligned on shear band planes (S/C’
structures, always sinistral) pointing to T
shearing conditions well in the sillimanite
stability field. Shear bands offsetting biotite
and sillimanite, numerous sigma and delta
type clasts, quartz oblique fabric and quartz
crystal preferred orientation confirm the
sinistral kinematics of the RRSZ. Quartz
mylonites display optical microstructures that
are characterized by a strong mylonitic
foliation defined by thin mica crystals aligned
parallel to the regional schistosity observed in
the field and by lenses of strongly sheared
clasts of recrystallized plagioclase, mostly by
subgrain rotation recrystallization. The
plagioclase clasts are elongated parallel to
the X direction of the strain ellipsoid and
locally display asymmetric pressure shadows
marked by white mica crystals. Quartz is
dynamically
recrystallised
by
bulging
mechanisms and grain boundary migration
processes. The grain boundaries are very
irregular and the crystals display very
complex geometries and shapes. However,
locally a weak shape fabric is recognizable
The latter is mostly due to slightly elongate
grains and asymmetric grain-boundary
bulges and it defines a weakly developed
oblique
foliation
SB.
Intracrystalline
deformation is recognizable in deformation
bands and in widespread undulose
extinction. Based on these microstructural
features, as well as on field geological and
petrographic indications, a deformation
temperature in the amphibolite facies of at
least 500-550 °C is inferred. Support in this
direction comes from the crystal preferred
orientation patterns (CPO) measured at the
texture goniometer for several specimens.
The bulk texture generally shows a c-axis
cluster with (sub)maxima parallel to the
intermediate strain axis Y with a marked
tendency to spread along the YZ plane.
Along the primitive circle there are the a-axis
maxima, suggesting a “single crystal”
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orientation (strong preferred orientation). In
some cases there is an important C-axis
cluster along the primitive circle, pointing to a
probable transitional character of the fabric
between a crossed girdle distribution and a
single, sharp maximum on the Y axis. The
patterns obtained for the quartz samples are
attributed to quartz dislocation creep on the
prism planes in the <a> direction, which is
mainly active under amphibolite-facies
conditions. An upper temperature limit is
provided by the absence in the measured
sample of prism <c> slip (reported instead by
Leloup and Kienast, 1994, for a sample in the
Ailao Shan complex), which limits the
temperature of deformation to below the
granitic solidus. A lower temperature limit is
provided by the pervasive dynamic
recrystallization of feldspar in the studied
mylonites, which indicates temperatures
higher than 500-550 °C, as noted above.
A second phase of deformation is
associated with extensional movement
accommodated by the two faults bounding
the DNCV massif. This is mostly inferred
from asymmetric NW-SE trending folds with
gently to moderately NE or SW dipping axial
planes. The fold vergence is in accordance
with the dip direction of the two faults (i.e. SE
for the Song Hong and NE for the Song Chay
fault), which, together with stretching
lineation oriented at high angle to fold axes,
indicate normal sense of shear. Kinematic
indicators are consistent with the sense of
shear deduced from fold vergence. We
interpret these folds as resulting from vertical
shortening associated with rapid uplift of the
DNCV during northeast southwest oriented
extension. Structures and microstructures of
tectonites representative of this second
phase cover a large variety of environmental
conditions. Field evidence shows the onset of
normal faulting along steep fault planes still
under high temperature conditions. Massive
amphibolites are deformed ductilely and
folded
consistently
with
the
normal
displacement and show folds with axes
perpendicular to the stretching direction.
Quartz-feldspar mylonites show evidence of
feldspar dynamic recrystallization and quartz
grain boundary migration processes, pointing
again to temperatures in excess of 500-550
°C. However, the same structures continued
acting along a retrograde path and extensive
evidence of the same deformation phase is
found under greenschist facies conditions
down to pure brittle structures. Several thin
sections reveal remarkable examples of
brittle cataclastic flow, dramatic grain size
reduction by mechanical action and
abundance of hydraulic fractures. It all points
to a very rapid final exhumation phase and/or
high fluid pressure activity. These features
are mostly concentrated on the steep limbs of
the antiformal structure and express the local
accommodation of the rapid exhumation
process of the DNCV.
CENOZOIC KINEMATIC EVOLUTION OF A
MAJOR STRIKE-SLIP FAULT: THE SCHIOVICENZA FAULT, NORTH ITALY
Dario Zampieri, Matteo Massironi, Vincenzo
Sparacino
Dipartimento di Geologia, Paleontologia e
Geofisica dell'Università di Padova
The Schio-Vicenza fault is a steep shear
zone at high angle (NW-SE) to the eastern
Southalpine chain trend (ENE-WSW). This
100 km-long structure is a very prominent
major fault, which separates the EuganeiBerici-Lessini wedge from the Veneto-Friuli
plain. Despite its essential role for several
geodynamic reconstructions macro- and
mesostructural data are contradictory and
suggest opposing Neogene strike-slip
kinematics with along-strike variable throws.
Remote sensing (Landsat ETM7 and
ASTER) and photoaerial interpretations
coupled with new field investigations have
highlighted the post-Jurassic kinematic
evolution of the fault and related structures.
The
Schio-Vicenza
fault
can
be
subdivided into two distinct sectors. The
southern part extends from the southeastern
margin of the Euganei hills to Schio
juxtaposing Quaternary alluvial deposits and
Mesozoic rocks and showing a more than 1
km total throw. All the strike-slip evidences
suggest dextral movements: (i)
the
Montegalda hills, located between the Berici
and Euganei hills, are carved on Oligocene
shallow water rocks offset from the eastern
margin of the Berici Oligocene carbonate
platform, indicating a minimum dextral
displacement of 7 km; (ii) the fracture pattern
derived from remote sensing analyses,
literature and new field investigations is
consistent
with
dextral
shear;
(iii)
paleomagnetic analyses of Eocene rocks
show a 40° clockwise rotation of the Priabona
station with respect to nearby regions; this
can be explained by a simple shear block
rotation (domino style) occurred between two
dextral NW-trending bounding faults.
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In contrast, the mountainous northern
sector of the Schio-Vicenza fault shows only
strike-slip
movements,
which
locally
reactivate inherited structures. These are Ntrending
Mesozoic
synsedimentary
extensional faults, already reactivated by
sinistral transtension during the Paleogene
extensional
phase,
as
proved
by
synkinematic basaltic dykes injected along
the faults. The Neogene sinistral strike-slip
activity of the N-trending faults is clearly
shown by the development of restraining and
releasing stepovers between different fault
segments. Restraining stepovers deform
inherited extensional structures. While
sinistral movements along N-trending faults
are consistent with both dextral and sinistral
movements of the Schio-Vicenza fault, other
large-scale structures are consistent only
with the dextral one (Paleogene Borcola pullapart) or the sinistral one (Neogene pop-up
of the Mt. Cornetto of Folgaria). Although the
change from dextral to sinistral kinematics
has been previously suggested, these
geological structures have never been
recognized previously.
The understanding of the complex
geodynamic evolution of the Schio-Vicenza
fault needs integration of several constraints,
which are the Europe-Africa movements, the
development of the eastern Southalpine
foredeep, the interplay with the Apennine
chain, the kinematic linkage with other
regional faults.
xxxvii