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Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as "earthquake fossils." Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The direction and sense of seismic slip cannot always be determined due to the generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas, arises in this case from the asymmetric preferred orientation of paramagnetic grains during viscous shear of the friction melt. This kinematic information, coupled with fault plane orientation and generation vein thickness, provides new and critical insight for the earthquake focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a valuable kinematic criterion for these fault rocks, but also could expand our knowledge of prehistoric seismic events.
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GEOLOGY
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Volume 43
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Number 6
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www.gsapubs.org 1
Focal mechanism of prehistoric earthquakes deduced from
pseudotachylyte fabric
Eric C. Ferré1, John W. Geissman2, Alain Chauvet3, Alain Vauchez3, and Matthew S. Zechmeister4
1Department of Geology, Southern Illinois University, 1259 Lincoln Drive, Carbondale, Illinois 62901-4324, USA
2Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080-3021, USA
3Géosciences, UMR (Unité Mixte de Recherche) 5342 CNRS and Université de Montpellier 2, 34095 Montpellier, France
4Shell Exploration and Production Company, 150 North Dairy Ashford, Houston, Texas 77079, USA
ABSTRACT
Fault pseudotachylytes form by frictional melting during seismic slip and therefore are
widely interpreted as “earthquake fossils.” Rapid movement along a rupture surface typically
forms a pseudotachylyte generation vein, the thickness of which increases with earthquake
magnitude. The direction and sense of seismic slip cannot always be determined due to the
generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the
orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense
of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas,
arises in this case from the asymmetric preferred orientation of paramagnetic grains during
viscous shear of the friction melt. This kinematic information, coupled with fault plane ori-
entation and generation vein thickness, provides new and critical insight for the earthquake
focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a
valuable kinematic criterion for these fault rocks, but also could expand our knowledge of
prehistoric seismic events.
INTRODUCTION
Fault pseudotachylytes, widely regarded as
“earthquake fossils,” provide critical insight into
the physics of seismic rupture (e.g., Austrheim
and Boundy, 1994; Di Toro et al., 2005a; Lin,
2008). These fault rocks form during relatively
large magnitude earthquakes by sudden heat-
ing along the slip surface due to fast friction,
immediately followed by melting and rapid
cooling (e.g., Magloughlin, 1992; Di Toro et al.,
2005a; Abercrombie et al., 2006; Andersen and
Austrheim, 2006; Kirkpatrick et al., 2009; Lin,
2011; Spray, 2010).
When three-dimensional exposures are avail-
able, the direction and sense of seismic slip that
produced the friction melt can be determined
using offset geologic markers (e.g., Grocott,
1981; Di Toro et al., 2005b). Pseudotachylyte
vein geometry and terminology were defined
by Ferré et al. (2005) and Lin (2011). If injec-
tion veins form as tensile cracks at the tip of the
rupture, their orientation may indicate the sense
of slip (Griffith et al., 2009). However, the com-
plex geometry of frictional melt veins and the
lack of suitable exposures make kinematic anal-
ysis of pseudotachylyte veins generally difficult.
Here we show that the magnetic fabric of
pseudotachylytes provides a rapid and reliable
solution to this kinematic analysis by estimat-
ing the direction and sense of seismic slip. The
anisotropy of magnetic susceptibility (AMS) of
fault pseudotachylytes arises from coseismic
viscous flow of the frictional melt, which there-
fore tracks the direction of slip. Our new fabric
results from the Val Gilba pseudotachylytes in
the western Italian Alps demonstrate that the
sense of slip can be deduced from the obliquity
of the magnetic foliation with respect to the fric-
tional shear plane.
GEOLOGIC SETTING AND AGE OF THE
PSEUDOTACHYLYTES
The Val Gilba pseudotachylytes are hosted by
the mylonitic ultrahigh-pressure (UHP) gneisses
of the Dora Maira massif of the western Ital-
ian Alps (Fig. 1A) (Henry, 1990; Henry et al.,
1993; Avigad et al., 2003; Cosca et al., 2005;
Zechmeister et al., 2007). The UHP coesite- and
phengite-bearing gneisses recorded pressures
as high as 4.2 GPa between ca. 50 and 30 Ma
(Gebauer et al., 1997; Chopin and Schertl, 1999;
Hermann, 2003). Their rapid, noncoaxial exhu-
mation resulted in formation of an ~1-km-thick
UHP mylonitic gneiss unit (Henry, 1990). The
estimated average rate of ascent of ~3 mm/yr is
high, but typical of UHP massifs (Ernst et al.,
1997). In contrast, the Sanfront-Pinerolo foot-
wall units and the high-pressure hanging-wall
units recorded lower peak pressures of ~0.75 and
~2.4 GPa, respectively (Avigad et al., 2003; Mes-
siga et al., 1999). In situ ultraviolet laser ablation
40Ar/39Ar thermochronologic data on the pseudo-
tachylytes imply formation ca. 20.1 ± 0.5 Ma,
during the late stages of exhumation, when the
mylonitic gneisses were at <3 km depth (Cosca
et al., 2005; Zechmeister et al., 2007).
Structural and microstructural studies (e.g.,
Zechmeister et al., 2007) show that the UHP
gneisses are granitic to granodioritic in composi-
tion and have a strong and consistent mylonitic
foliation. This foliation is formed by quartz rib-
bons with a prominent lattice-preferred orienta-
tion, stretched alkali feldspars, and clusters of
fine-grained biotite-epidote-albite-quartz form-
ing pressure shadows around phengite clasts
(Zechmeister et al., 2007). Phengite-chlorite–
rich zones typically form ultramylonitic bands
parallel to the mylonitic foliation and are cut by
cataclasite bands and pseudotachylyte veins. The
pseudotachylyte veins are best exposed in stone
quarries where the mylonitic foliation generally
dips gently to the west (144°, 15°W; Figs. 1B and
1C), with an east-west–trending stretching linea-
tion (276°, plunge 12°; Fig. 1D). S-C structures
and quartz lattice-preferred orientation indicate a
top-to-west sense of shear in the mylonitic host
rock. The pseudotachylytes form (1) extensive
generation veins parallel to the mylonitic folia-
tion (and exposed over tens of meters), (2) asym-
metric injection veins injected along P-shear
fractures, and (3) vein doublets connected by a
pseudotachylyte network. At least 20 subparallel
pseudotachylyte veins, each separated by a few
meters, are exposed in the quarries.
The pseudotachylyte generation veins, 10–25
mm thick, display a compositional flow band-
ing microstructure (Fig. 2A) and are commonly
surrounded by a 2–25-mm-thick cataclasite-
ultracataclasite zone. The pseudotachylyte veins
contain rounded quartz and/or plagioclase clasts,
evenly spaced 10–20-mm-diameter spherules of
quartz and alkali feldspar, 2–5-mm-long feld-
spar microcrystallites, and a matrix that is either
cryptocrystalline and phengite rich or axiolitic.
The clast/matrix ratio in the pseudotachylyte
varies considerably along and across veins.
The microcrystallites generally display a strong
shape-preferred orientation (SPO) quantified
using the Intercept method (Launeau and Robin,
1996; Figs. 2B and 2C). Quartz-filled, 50-mm-
long elliptical amygdules that formed as a result
of melt degassing, as well as the spherulitic or
axiolitic microstructures commonly observed in
the Val Gilba pseudotachylytes (Zechmeister et
al., 2007), attest to their melt origin.
SEISMIC SLIP DISPLACEMENT
At Val Gilba, the mylonitic host rock of the
pseudotachylytes does not have offset markers
to quantify seismic slip. In this case, seismic dis-
placement can be crudely approximated using
Sibson’s (1975) empirical equation: d = 436∙a2,
where d is displacement, and a is vein thickness,
both measured in centimeters. The true thick-
ness of the generation veins ranges from 0.7 to
2.8 cm with an average of 1.4 cm. These values
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© 2015 Geological Society of America. For permission to copy, contact editing@geosociety.org.
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yield displacements between 2.1 and 34.2 m
with an average of 8.5 m. These results may be
underestimates because the volume of melt gen-
erated is not always accurately represented by
the thickness of the vein due to the fact that fault
lubrication reduces friction and frictional melts
are injected into fractures and voids (Spray,
2005). In Figure 2B, for example, vein thickness
varies considerably along the vein and some of
the melt may have been lost from the genera-
tion zone by injection into fractures of the host
rock. A displacement of 8.5 m for a normal fault
would correspond to an earthquake magnitude
of ~7.4 (Wells and Coppersmith, 1994).
DIRECTION AND SENSE OF THE
SEISMIC SLIP EVENT
The AMS, a well-established method (Tar-
ling and Hrouda, 1993), has been successfully
used to determine the viscous flow direction in
materials ranging from basaltic lava to plaster of
Paris (e.g., Ernst and Baragar, 1992; Abelson et
al., 2001; Cañón-Tapia and Castro, 2004). With
the exception of Scott and Spray (1999), no pre-
vious AMS study of pseudotachylytes addressed
the issue of slip direction or slip sense.
The Val Gilba pseudotachylytes formed by
preferential melting of ferromagnesian phases, as
shown by their higher Fe2O3 content (3.2 ± 0.8
wt%) compared to the mylonitic gneiss host rock
(2.3 ± 0.7 wt%) (Ferré et al., 2012). The AMS
of 1 cm3 specimens from the host rock (n = 30)
and the pseudotachylyte (n = 15) was measured
using a Kappabridge KLY-4S instrument (Figs.
1C and 3A). Magnetic hysteresis measurements
on a vibrating sample magnetometer show that
both host rock and pseudotachylyte samples are
dominantly paramagnetic (see the GSA Data
Repository1). These pseudotachylytes have
bulk magnetic susceptibilities (Km) comparable
1
GSA Data Repository item 2015188, anisotropy
of magnetic susceptibility and magnetic hysteresis
data (Table DR1 and Fig. DR1), is available online at
www.geosociety.org/pubs/ft2015.htm, or on request
from editing@geosociety.org or Documents Secre-
tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
to those of paramagnetic gneisses (Ferré and
Améglio, 2000; Kruckenberg et al., 2010), with
relatively low values (Km = 203 ± 17 × 10-6, SI)
dominated by the contributions of Fe-rich pheng-
ite and chlorite (Ferré et al., 2012). In the pseudo-
tachylytes, the uniformity of the corrected degree
of magnetic anisotropy (P = 1.08 ± 0.1) shows
that the anisotropy is controlled by a uniformly
distributed mineral phase (see the Data Reposi-
tory). The symmetry of the magnetic fabric is
A
B
CD
µ
Figure 1. A: Geologic
cross section of the Dora
Maira massif, western Ital-
ian Alps (modified after
Henry et al., 1993; Avigad
et al., 2003). The pseudo-
tachylytes examined in
this study are exposed
near the Val Gilba local-
ity. (For additional geo-
logic details about this
locality, see Zechmeister
et al., 2007.) HP—high
pressure; UHP—ultrahigh
pressure. B: Typical out-
crop of the Val Gilba pseu-
dotachylytes exposed in
a quarry wall. The pseu-
dotachylyte generation
vein is parallel to the host
mylonitic gneiss foliation
and shows asymmetric
injection veins indicative
of a top-to-west coseis-
mic sense of shear. Pseu-
dotachylyte generation
veins tend to form dou-
blets interconnected by a
complex network of veins
and reservoirs. C: Macro-
scopic view of the sample
used for anisotropy of
magnetic susceptibility
(AMS) analysis. The sam-
ple shows prominently
layered and foliated my-
lonitic gneiss. The 1 cm3
AMS cubes were cut
parallel to mylonitic folia-
tion and pseudotachylyte
margins. D: Macroscopic
structures measured in
the field in the sampling
quarry and neighboring areas. The mylonitic foliation is very consistent in attitude, and the
mylonitic stretching lineation shows a slight girdle distribution. The mean foliation strike is
144° with a dip of 15°W. The mean lineation trends 276° with a plunge of 12°.
Figure 2. A: Photomicrograph in polarized
light of generation vein (horizontal), show-
ing flow banding, and a branching injec-
tion vein (vertical, up), cutting a preexisting
generation vein. The injection vein (on left)
displays clear flow structures attesting to
viscous flow of friction melt from the gen-
eration into the fracture during seismic slip.
The host mylonitic gneiss displays cataclas-
tically deformed ultramylonite fabric against
the pseudotachylyte margins. UHP—ultra-
high pressure. B: Flow structures in the
generation veins exhibit a strong preferred
orientation of phengite and other micas.
Qz—quartz; Pl—plagioclase. C: Viscous flow
fabric is documented at a smaller scale by
the systematic alignment of microcrystallites
of various sizes. Shape-preferred orientation
was quantified using the Intercept method
(Launeau and Robin, 1996). Aspect ratio is
1.416; the angle between the long axis and
pseudotachylyte plane is 43°.
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overall oblate (T = 0.4 ± 0.2), indicating the con-
trol exerted by magnetically oblate markers.
AMS data for the host gneiss are very consis-
tent (Fig. 3A), and have a subhorizontal magnetic
foliation (173°, 9°W) and a subhorizontal mag-
netic lineation (234°, plunge 9°). These magnetic
structures are indistinguishable from the macro-
scopic mylonitic structures measured across the
Val Gilba quarries (Fig. 1D). The slight girdle
distribution of magnetic lineations (Fig. 3A)
mimics the macroscopic lineations (Fig. 1D). In
the pseudotachylyte, the well-defined magnetic
foliation (174°, 22°E) and the magnetic lineation
(055°, 20°) are both oblique with respect to the
subhorizontal vein margins (173°, 9°W).
The magnetic fabric of the pseudotachylyte
most likely arises from the preferred orienta-
tion of phengite. This fabric developed as the
melt cooled rapidly and recorded viscous flow
parallel to the seismic slip direction. Injection
veins may locally modify the flow pattern of
the melt, as shown by the sheath-fold like struc-
tures documented elsewhere by Berlenbach and
Roering (1992). The specimen we analyzed was
collected away from injection veins. In addition,
in the Val Gilba pseudotachylytes, the width of
the generation veins is ~1 cm and the length of
seismic displacement is a few meters. These fig-
ures demonstrate that seismic deformation was
primarily simple shear with a very high shear
strain. Under conditions where the kinematic
vorticity, Wk, is close to 1.0, the melt stretching
direction parallels the host-rock shear direction
(i.e., seismic slip direction). The presence of an
elongated amygdule (long axis 068°, 17°) in
the pseudotachylyte (Zechmeister et al., 2007)
further supports our interpretation that the fric-
tional melt did not quench instantaneously, but
continued to flow viscously during the termi-
nal stages of seismic slip. Coseismic viscous
flow imparts a strong SPO on microcrystallites,
whereas postseismic devitrification results in
radial, nonoriented spherulitic microstructures.
The microcrystallite SPO (Fig. 2C), broadly
parallel with the magnetic fabric long axis (K1),
forms an angle of ~31° with the generation vein
margin. We interpret this obliquity as resulting
from viscous noncoaxial shear in the frictional
melt (Fig. 3B). This obliquity, observed both in
microcrystallites and magnetic fabric, is con-
sistent with the macroscopic top-to-west sense
of shear given by asymmetric injection veins.
The direction of seismic slip inferred from mag-
netic fabric supports a normal sense of move-
ment along the slip plane, in agreement with the
regional top-to-west kinematics.
PALEOSEISMOLOGIC IMPLICATIONS
The magnetic fabric data from the Val Gilba
pseudotachylytes provide four key parameters
for the earthquakes that formed the pseudotachy-
lyte veins (Fig. 3): (1) the slip plane, represented
by the parallel margins of the vein; (2) the slip
direction, indicated by the magnetic lineation of
the pseudotachylyte; (3) the slip sense, deduced
from the obliquity of the magnetic foliation with
respect to the vein margins; and (4) the approxi-
mate displacement, estimated from vein thick-
ness. These results can be integrated with historic
seismicity data in the Dora Maira massif area that
show continued exhumation of crystalline mas-
sifs accommodated along normal faults (Giglia
et al., 1996; Sue et al., 1999). The United States
Geological Survey catalog (earthquake.usgs.gov/
earthquakes/search/) includes a list of 189 earth-
quakes for the Val Gilba area (Mw > 2.0) between
A.D. 1900 and 2014 statistically showing rupture
at ~10 km depth along normal faults.
Our approach gives the complete focal mech-
anism solution, corresponding to a normal fault
in this case, from a single ca. 20 Ma pseudo-
tachylyte vein (Fig. 3A). This new paleoseis-
mologic method, combined with isotopic age
dating of fault pseudotachylytes, could provide
invaluable kinematic information on many other
seismically active regions where pseudotachy-
lytes formed.
ACKNOWLEDGMENTS
We thank Giulio Di Toro, John Spray, and an anon-
ymous reviewer for their insightful comments. We
also gratefully acknowledge support from National
Science Foundation grants EAR-0228818 and EAR-
0521558 (Ferré) and EAR-0228849 (Geissman).
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Manuscript received 7 January 2015
Revised manuscript received 30 March 2015
Manuscript accepted 2 April 2015
Printed in USA
... At greater depths, pseudotachylytes develop in response to fault slip. Ferré et al., 2015Ferré et al., , 2016 proposed that the magnetic fabric of pseudotachylytes provides insight into the focal mechanism of ancient earthquakes. In the pseudotachylytes, the glassy material, which includes neoformed ferromagnetic mineral, is generally related to an earthquake event, formed by frictional melting during coseismic slip. ...
... A magnetic foliation is identified at~30°oblique to the main plane of pseudotachylyte. According to Ferré et al. (2015), this obliquity yields the sense of coseismic viscous shear, and the magnetic lineation indicates the slip direction. This finding provides a new tool to assess the focal mechanism of ancient earthquakes. ...
... In addition to these permutations, it should be noted that some K1 axes, oriented approximately at declination =~35°and inclination =~45°, are present in both the surface and deep gouges (Figures 3c and 3d). In pseudotachylyte, Ferré et al. (2015) suggested that magnetic lineation can be used as a seismic slip marker. In fault gouge, Marcén et al. (2018) observed that the magnetic lineation is either parallel or perpendicular to the S-C plane intersection; S refers here to observed foliation (schistosity) and C to the shear plane. ...
Article
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Plain Language Summary Fault gouges often provide important and valuable information that can help to understand the fault mechanisms and to assess the seismic hazard, but to get access to the shape preferred orientation of micrometric minerals is generally a complex, time consuming and expensive procedure. In this study, we show that the anisotropy of magnetic susceptibility, which is a fast and nondestructive technique, can be a proxy of the structural fabric of historical gouges. This technique has already been previously applied to gouge zones, but this is the first time that it is used in the gouge from one of the best documented historical large seismic events: the 1999 Chi‐Chi earthquake (Mw 7.6) along the Chelungpu fault (Taiwan). Samples within and around this fault were obtained from fresh surface outcrop and 1‐km‐deep borehole (Hole B, Taiwan Chelungpu‐fault Drilling Project). The 1‐km‐deep gouge hosts the 3‐mm‐thick principal slip zone of the Chi‐Chi earthquake. The results show that the reconstructed focal mechanism based on the magnetic fabric geometry matches closely the focal mechanism solutions for the Chi‐Chi earthquake estimated by several previous studies.
... The anisotropy of magnetic susceptibility (AMS) has long been used to determine the flow characteristics of igneous rocks (e.g., Tarling and Hrouda, 1993). The mini-AMS method (e.g., Ferré et al., 2015;2016), followed early investigations using small size cubes (Gattacceca et al., 2003) and provides an unprecedented mean to test the consistency of flow fabrics in pseudotachylytes formed from frictional melts, at the scale of a few millimeters. This method offers insight into the flow fabric scalar and directional consistency and helps deciphering laminar vs turbulent flow, as proposed in some fault pseudotachylytes (e.g., Otsuki et al., 2003). ...
... Idealized geometry and predicted fabrics of pseudotachylytes in a generation vein and an injection vein. The flow fabric of the generation vein is predicted to be oblique with respect to vein margins following the model of Ferré et al. (2015). Fabrics near and at the junction of injection veins with generation veins are predicted to be divergent and therefore not investigated here. ...
... Here the mean viscous flow direction, calculated from the AMS data is N185 ± 21 • . In addition, the obliquity of the magnetic foliation plane (K 1 -K 2 plane) shows a top-to-south sense of transport, as shown elsewhere in other pseudotachylytes (Ferré et al., 2015). This sense of transport and the mean viscous flow direction (N185 • ) are in good agreement with the azimuth of the elongated vesicles (N200 • ). ...
Article
The pseudotachylyte of the Markagunt slide in Utah is the best example of gravity slide frictional melt. A key exposure near Panguitch provides opportunities to investigate the kinematics of frictional melts. Here we analyze the anisotropy of magnetic susceptibility (AMS) in eight oriented samples and show that magnetite dominates the magnetic susceptibility (97.7% in generation veins and 87.6% in injection veins) and most likely also the AMS. These rocks have magnetic susceptibility one order of magnitude greater (∼46,106 • 10⁻⁶ [SI]) than their host rock (∼1850 • 10⁻⁶ [SI]), which shows that magnetite must have formed during the gravity slide. The AMS of these rocks records the laminar flow kinematics of the frictional melt, with a North-South direction and a top-to-South sense of displacement consistent with regional observations. The dominantly planar symmetry of the AMS points to a flow regime dominated by pure shear, towards the end of the gravity slide movement. Our results, obtained on a landslide pseudotachylyte where the emplacement kinematics was already known, fundamentally validate the use and interpretation of AMS in other pseudotachylytes including those formed along seismogenic faults.
... Fault rocks, however, have been the subject of rock magnetic studies only recently, with a focus on faults from seismically active zones (e.g., Almqvist et al., 2020;Cai et al., 2019;Chou et al., 2012aChou et al., , 2012bChou et al., , 2014aChou et al., , 2014bFerré et al., 2005Ferré et al., , 2012Ferré et al., , 2014aFerré et al., , 2014bFerré et al., , 2015Ferré et al., , 2016Ferré et al., , 2017Fukuchi, 2003;Fukuchi et al., 2005Fukuchi et al., , 2007Fukuchi et al., , 2009Liu et al., 2014Liu et al., , 2016Mishima et al., 2006Mishima et al., , 2009Nakamura et al., 2002;Pei et al., 2014aPei et al., , 2014bTanikawa et al., 2007Tanikawa et al., , 2008Yang et al., 2012aYang et al., , 2012bYang et al., , 2013aYang et al., , 2013bYang et al., , 2016aYang et al., , 2016bYang et al., , 2018Yang et al., , 2019Zhang et al., 2017Zhang et al., , 2018. By providing a trove of information on physical and chemical processes associated with faulting in seismically active zones, these pioneer works have laid the foundation of "fault magnetism", an emerging sub-discipline in the geosciences. ...
... "Mini-cores" of 12 mm diameter × 11 mm long, cubic mini-AMS samples with 1 cm edges, or even down to a size of 3.5 mm on-a-side (hence ~250 times volumetrically smaller than standard-size samples), often work fine. AMS analysis of pseudotachylyte fabric on such 3.5 mm cubes enabled determining the focal mechanisms of ancient earthquakes (Ferré et al., 2015(Ferré et al., , 2016. Despite sample size issues for very small samples where a cautious and careful interpretation is appropriate, AMS studies on small samples are nonetheless a promising new avenue to unveil detailed geological features Ferré et al., 2015Ferré et al., , 2016Zhu et al., 2017). ...
... AMS analysis of pseudotachylyte fabric on such 3.5 mm cubes enabled determining the focal mechanisms of ancient earthquakes (Ferré et al., 2015(Ferré et al., , 2016. Despite sample size issues for very small samples where a cautious and careful interpretation is appropriate, AMS studies on small samples are nonetheless a promising new avenue to unveil detailed geological features Ferré et al., 2015Ferré et al., , 2016Zhu et al., 2017). Berndt et al. (2016) demonstrated that samples with a thermoremanent magnetic moment larger than 10 -11 Am 2 irrespective of their sizes (ranging from the centimeter scale to tens of nm scale) contain enough magnetic particles to be accurate magnetic recorders. ...
Article
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As iron‐bearing minerals – ferrimagnetic minerals in particular – are sensitive to stress, temperature and presence of fluids in fault zones, their magnetic properties provide valuable insights into physical and chemical processes affecting fault rocks. Here, we review the advances made in magnetic studies of fault rocks in the past three decades. We provide a synthesis of the mechanisms that account for the magnetic changes in fault rocks and insights gained from magnetic research. We also integrate non‐magnetic approaches in the evaluation of the magnetic properties of fault rocks. Magnetic analysis unveils microscopic processes operating in the fault zones such as frictional heating, energy dissipation, and fluid percolation that are otherwise difficult to constrain. This makes magnetic properties suited as a “strain indicator”, a “geothermometer”, and a “fluid tracer” in fault zones. However, a full understanding of faulting‐induced magnetic changes has not been accomplished yet. Future research should focus on detailed magnetic property analysis of fault zones including magnetic microscanning and magnetic fabric analysis. To calibrate the observations on natural fault zones, laboratory experiments should be carried out that enable to extract the exact physicochemical conditions that led to a certain magnetic signature. Potential avenues could include (1) magnetic investigations on natural and synthetic fault rocks after friction experiments; (2) laboratory simulation of fault fluid percolation; (3) paleomagnetic analysis of post‐kinematic remanence components associated with faulting processes; and (4) synergy of interdisciplinary approaches in mineral‐magnetic studies. This would help to place our understanding of the microphysics of faulting on a much stronger footing.
... of magnetic susceptibility of pseudotachylytes for deducing the focal mechanism of ancient earthquakes (Ferré et al., 2015). In an attempt to find the kinematics of a ductile-tobrittle shear zone in the Köli Nappe Complex, we adopted this method to the pseudotachylytes in the Köli Nappe Complex. ...
... The rapid quenching leads to a remanent magnetization, which is acquired coseismically but sometimes contains post-seismic superimposed magnetizations, and hence impacts interpretation of paleomagnetism (Ferré et al., 2014;Fukuchi, 2003). Anisotropy of magnetic susceptibility of pseudotachylytes may also record information about the viscous flow of the friction melt (Ferré et al., 2015;Scott and Spray, 1999). Comparison of fault plane geometry and orientation with the magnetic fabric and petrofabric has been used to deduce earthquake kinematics and focal mechanism (Ferré et al., 2015). ...
... Anisotropy of magnetic susceptibility of pseudotachylytes may also record information about the viscous flow of the friction melt (Ferré et al., 2015;Scott and Spray, 1999). Comparison of fault plane geometry and orientation with the magnetic fabric and petrofabric has been used to deduce earthquake kinematics and focal mechanism (Ferré et al., 2015). The magnetic fabric of a rock is defined by its anisotropy of magnetic susceptibility (AMS), which in turn reflects the sum of individual magnetic responses of the rock-forming minerals (Borradaile, 1987). ...
Article
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Fault kinematics can provide information on the relationship and assembly of tectonic units in an orogen. Magnetic fabric studies of faults where pseudotachylytes form have recently been used to determine direction and sense of seismic slip in prehistoric earthquakes. Here we apply this methodology to study magnetic fabrics of pseudotachylytes in field structures of the Köli Nappe Complex (central Swedish Caledonides), with the aim to determine fault kinematics and decipher the role of seismic faulting in the assembly of the Caledonian nappe pile. Because the pseudotachylyte veins are thin, we focused on small (ca. 0.2 to 0.03 cm3) samples for measuring the anisotropy of magnetic susceptibility. The small sample size challenges conventional use of magnetic anisotropy and results acquired from such small specimens demand cautious interpretation. Importantly, we find that magnetic fabric results show inverse proportionality among specimen size, degree of magnetic anisotropy and mean magnetic susceptibility, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. In general, however, it is shown that the principal axes of magnetic susceptibility correspond to the orientation of foliation and lineation, where the maximum susceptibility (k1) is parallel to the mineral lineation, and the minimum susceptibility (k3) is dominantly oriented normal to schistosity. Furthermore, the studied pseudotachylytes develop distinct magnetic properties. Pristine pseudotachylytes preserve a signal of ferrimagnetic magnetite that likely formed during faulting. In contrast, portions of the pseudotachylytes have altered, with a tendency of magnetite to break down to form chlorite. Despite magnetite breakdown, the altered pseudotachylyte mean magnetic susceptibility is nearly twice that of altered pseudotachylyte, likely originating from the Fe-rich chlorite, as implied by temperature-dependent susceptibility measurements and thin-section observations. Analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust, but these data yield no kinematic information on the direction and sense of seismic slip. Additionally, the combined structural field and magnetic fabric data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Although the objective of finding kinematic indicators for the faulting was not fully achieved, we believe that the results from this study may help guide future studies of magnetic anisotropy with small specimens (<1 cm3), as well as in the interpretation of magnetic properties of pseudotachylytes.
... The stereoplots from the Dora-Maira basement rocks (Fig. 7a, b) show a weakly dispersed flat-lying foliation that overprints earlier high-P parageneses and formed during top-to-SW shearing occurring under retrograde greenschist-facies conditions, associated with the exhumation of the Dora-Maira units along low-angle extensional faults (Avigad et al., 2003;Henry et al., 1993). Cataclasites and pseudotachylites are locally hosted in the top-to-the SW/W mylonitic foliation (Dana, 2020;Ferré et al., 2015;Henry et al., 1993;Zechmeister et al., 2007). According to Rubatto and Hermann (2001), greenschist-facies conditions in the Dora-Maira units prevailed during the 33-30 Ma time interval. ...
Article
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Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and-thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the "Prepiemonte units" known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dron-ero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T RSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
... The stereoplots from the Dora-Maira basement rocks (Fig. 7a, b) show a weakly dispersed flat-lying foliation that overprints earlier high-P parageneses and formed during top-to-SW shearing occurring under retrograde greenschist-facies conditions, associated with the exhumation of the Dora-Maira units along low-angle extensional faults (Avigad et al., 2003;Henry et al., 1993). Cataclasites and pseudotachylites are locally hosted in the top-to-the SW/W mylonitic foliation (Dana, 2020;Ferré et al., 2015;Henry et al., 1993;Zechmeister et al., 2007). According to Rubatto and Hermann (2001), greenschist-facies conditions in the Dora-Maira units prevailed during the 33-30 Ma time interval. ...
Article
Full-text available
Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T RSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
Article
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We present a study on the Dongguashan quartz diorite stock which is the largest Early Cretaceous intrusion associated with skarn-porphyry polymetallic ore deposit in the Tongling Ore Cluster, Middle-Lower Yangtze Metallogenic Belt in East China. The Dongguashan quartz diorite show massive texture without obvious foliation, and intrusive contact was locally observed inside of the stock. The anisotropy of magnetic susceptibility (AMS) results of the stock show two distinct groups. The Group I (G-I) is dominated by NE-SW-striking magnetic foliation with variably oriented magnetic lineation. The Group II (G-II) that intruded into G-I is characterized by steep NW-SE-striking magnetic foliation and lineation, which are parallel to the vein-like orebody developed in the stock. The 3D geometrical modelling of the stock displays a triangular shape in plan-view with an eastward bulge and irregular stock boundary in the eastern side and the contact surface is steeper in the west than that in the east, denoting that the stock was constructed with an eastward magma accretion trend. Furthermore, the Dongguashan quartz diorite has a wide range of composition and geochronological data, suggesting a multiple magma pulses emplacement model. Accordingly, we propose that the Dongguashan stock was constructed by at least two stages magma pulses. The earlier stage magma pulses intruded along the NE-SW-striking pre-emplacement structures in the country rock and partly intruded into the lithological and mechanical discontinuous interfaces which yielded the stratabound skarn orebody. The eastward magma accretion produced a highly deformed and longer heated country rock on the eastern side of the stock, favoring the orebody development along the eastern stock-country rock contact interface. The late-stage magma pulse and the parallelism among the trend of magnetic foliation of G-II, extensional structure and vein-like orebody suggest that the vein-type orebodies may controlled by both magma emplacement and regional tectonics.
Article
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Devastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear-heating event. The well-preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti-poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10⁻⁶ [SI] volume, with P′ ranging from 1.05 to 2.5) to reconstruct the co-seismic deformation parameters, that is, fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included mostly normal mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite-hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution.
Article
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Pseudotachylytes originate from the solidification of frictional melt, which transiently forms and lubricates the fault plane during an earthquake. Here, we observe how the pseudotachylyte thickness a scales with the relative displacement D both at the laboratory and field scales, for measured slip varying from microns to meters, over 6 orders of magnitude. Considering all the data jointly, a bend appears in the scaling relationship when slip and thickness reach ∼1 mm and 100 µm, respectively, i.e., MW > 1. This bend can be attributed to the melt thickness reaching a steady‐state value due to melting dynamics under shear heating, as is suggested by the solution of a Stefan problem with a migrating boundary. Each increment of fault is heating up due to fast shearing near the rupture tip and starting cooling by thermal diffusion upon rupture. The building and sustainability of a connected melt layer depend on this energy balance. For plurimillimetric thicknesses (a > 1 mm), melt thickness growth reflects in first approximation the rate of shear heating which appears to decay in D−1/2 to D⁻¹, likely due to melt lubrication controlled by melt + solid suspension viscosity and mobility. The pseudotachylyte thickness scales with moment M0 and magnitude MW; therefore, thickness alone may be used to estimate magnitude on fossil faults in the field in the absence of displacement markers within a reasonable error margin.
Thesis
In this thesis the relationships between mylonite, cataclasite and pseudotachylyte, outcropping in the in the Brossasco-Isasca Unit localized in the south-western portion of the Dora Maira Massif (Western Alps), have been analysed. This work is based on a multi-scale structural geology approach starting with the realisation of a geological-structural map of the area at the scale of 1:5000. Emphasis is given on the observations, and overprinting relationships, of the different types of sheared rock, at the mesoscale, followed by their microstructural/microtectonics characterization on eight thin sections of selected samples. The late structural evolution of the area, from the greenschist facies metamorphic re-equilibration associated to non-coaxial ductile shearing, up to the latest brittle structures, formed at very shallow conditions, was reconstructed. Different types of mylonitic rocks, developed under general flow conditions with a top-to-the SW/W sense of shear during greenschist facies metamorphism have been recognized and mapped. Cataclasite and pseudotachylyte, hosted in the mylonitic gneiss, nucleated often (but not always) on structural discontinuities precursors (the mylonitic foliation). Interesting overprinting relationships between mylonite, cataclasite, pseudotachylyte veins and foliated/mylonitic pseudotachylyte are described and linked to different frictional-viscous cycles. The microstructural study allowed to infer useful information on the kinematics of the fault rocks, on their overprinting features and on the possible temperature range of mylonite and pseudotachylyte formation. Finally, in this area is possible to recognize microstructures that are tentatively correlated with a complete seismic cycle from viscous flow up to coseismic loading and postseismic relaxation.
Article
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We investigate the petrographic, geochemical and magnetic properties of fault pseudotachylytes formed by frictional melting in granitic rocks from Southern California, the Italian Alps and Kyushyu, Japan. The main magnetic remanence carriers are mixtures of grain sizes of fine grained magnetite. These ferrimagnetic grains record a stable, multicomponent magnetization that consists of one or more of the following: coseismic thermal remanent magnetization, coseismic lightning isothermal remanent magnetization and post-seismic chemical remanent magnetization. Fault pseudotachylytes from the three localities display contrasting magnetic properties, which suggests that oxygen fugacity and host rock composition ultimately control the magnetic assemblage.
Article
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The Dora-Maira ultra-high-pressure (UHP) unit, exposed in the internal Western Alps, is a ∼1-km-thick continental slice sandwiched between lower-pressure rock units. The contact at the base of the UHP unit is a major contractional shear zone that thrust the UHP unit and the overlying continental and oceanic units onto the Sanfront-Pinerolo unit. The latter, metamorphosed under blueschist-greenschist conditions, is the lowest structural unit exposed in the Western Alps. Our analysis indicates that all rock units of the Dora-Maira massif were sheared and juxtaposed together within a top-to-the-SW, large-scale, medium-temperature/low-pressure ductile shear zone in the middle crust. Thrust faulting was accompanied and followed by vertical thinning that progressively reduced the thickness of the orogenic wedge and brought the ultra-high-pressure (UHP) unit into the brittle crust. Contractional tectonic contacts in the southern Dora-Maira were concealed by late extensional structures. The majority of the extensional structures post- dated the emplacement of the UHP unit over the Sanfront-Pinerolo unit, implying that extension lagged behind much of the exhumation of the UHP unit. Extensional shear zones in the Dora-Maira and elsewhere in the Western Alps are parallel to the strike of the orogen; their overall effect was to widen the orogenic belt in the (north)east-(south)west direction. The sense of extensional movement was toward the frontal part of the orogen so that rather than defining tectonic wedges formed during extrusion, the extensional structures represent lateral spreading of the mid- to upper crust of the internal Western Alps toward the foreland. Rapid compression of the Dora-Maira was coeval with the transition from marine flysch deposits to molasse sedimentation in the foreland of the Western Alps. The switchover from burial to exhumation and the lateral spreading of the upper crust in the internal Western Alps were coupled by readjustment of the entire orogenic wedge.
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Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow. Field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation. In the Naxos dome (Greece), we test the anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites. Measurements of 155 migmatite samples yield dominantly low values (<300 × 10-6 [SI]) of bulk magnetic susceptibility (Km) consistent with biotite being the dominant carrier of the AMS. Higher values of Km, thermomagnetic, hysteresis, and microstructural data, however, suggest a ferromagnetic contribution from magnetite in a subset of samples (N = 15). Using electron backscatter diffraction (EBSD) analysis, we establish the correspondence of the biotite subfabric with the AMS and structural fabric of the Naxos migmatites. EBSD data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow. Over a range of spatial scales, migmatitic foliation and magnetic foliation are well correlated. The magnetic lineation recovered by AMS displays a coherent organization despite the heterogeneous structure and composition of the Naxos migmatites. These data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow. Furthermore, our study demonstrates the utility of the AMS method for studying the dynamics of partially molten orogenic crust.
Thesis
Dans le massif Dora-Maira méridional, la cartographie réalisée au 1/25.000e entre val Pô et val Varaita révèle l'existence d'une unité à coésite, d'environ 10 X 5 X 0.5 km, où la coésite est présente dans des roches variées. Cette unité fait partie d'un ensemble de trois unités de socle polymétamorphique, dont la plus haute présente quelques éléments de couverture permo-carbonifère à triasique. Cet ensemble est intercalé entre : - une unité métasédimentaire graphitique d'âge Carbonifère présumé (unité de Sanfront-Pinerolo), et - un ensemble supérieur incluant des écailles de socle polymétamorphique (unité de Dronero-Sampeyre p.p.), une écaille de "Schistes lustrés" ophiolitifères et des éléments de couverture d'âge Permo-Carbonifère à Crétacé probable (unité de Dronero-Sampeyre p.p.).Le métamorphisme typomorphe de très haute et haute pression est caractérisé par une grande variété de degrés, avec d'énormes sauts de métamorphisme entre certaines des unités. De bas en haut, les unités sont superposées dans l'ordre suivant : unité "schiste bleu" (SB : 10-14 kbar, 550°C), unité de très haute pression à coésite et éclogites à disthène (THP : 30 kbar, 750°C), unité de haute pression éclogitique (HP : 15-20 kbar, 550°C), unités "schiste bleu". Un refroidissement tardif, en décompression, à partir d'une dizaine de kbar et s'équilibrant en climat "schiste vert", est commun à toutes les unités.Dans toute la pile de nappes, la structuration principale est une foliation mylonitique régionale, portant une linéation ENE-WSW avec sens de cisaillement associé vers l'WSW ; cet épisode de déformation ductile cisaillante est contemporain des recristallisations "schistes verts". Quelques reliques structurales de haute pression sont caractérisées par des structures planaires plutôt symétriques et des linéations dispersées dans le quadrant NE.Les données géochronologiques (U-Pb, Rb-Sr, 39Ar-40Ar) indiquent un âge éo-alpin (100 Ma environ) pour les métamorphismes de THP et HP. La rétromorphose et les structures régionales, contemporaines de la juxtaposition des unités, sont datées d'environ 40 Ma.Le scénario résultant indique : - une "subduction" profonde (jusqu'à 100 km environ) de matériel continental crustal européen au Crétacé moyen, - un empilement et un charriage vers l'ouest, à faible profondeur, d'unités continentales et océaniques variées pendant l'Eocène. Pour expliquer la remontée des unités les plus profondes, on invoque la reprise du plan de subduction éoalpin, à pendage sud, par un plan de subduction à pendage est, vers 80-60 Ma.
Article
Dark, aphanitic material occurs as a thin (<3 mm) fault vein along the fault plane on which the principal slip took place during the 2008 M-w 7.9 Wenchuan, China, earthquake. It also forms injection veins in a >5-cm-wide coseismic shear zone within the Longmen Shan thrust belt. Powder X-ray diffraction data and microstructural analysis indicate that the veins contain little or no glassy material, and are composed of fine-grained fragments set in an ultrafine-grained matrix, all sourced from the country rock of interbedded siltstone and mudstone. On the basis of X-ray diffraction and mesostructural and microstructural features, it is concluded that these veins are ultracataclastic veins generated by coseismic comminution with little melting, accompanied by rapid fluidization in the coseismic shear zone during the 2008 Wenchuan earthquake. The findings support earlier suggestions that (1) ultracataclastic veins that resemble pseudotachylyte veins in appearance can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting; and (2) ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as "earthquake fossils" in much the same way as pseudotachylyte veins.
Article
The crystalline nappes of the Dora-Maira massif, Western Alps, essentially made of continental material from the upper crust, show petrological relics of an ultra-high-pressure (UHP) to high-pressure (HP. 'cold' eclogite) Eoalpine metamorphism. They also display relics of UHP-HP structures, preserved in boudins and/or within large UHP porphyroclasts, in a retrograde, greenschist-facies regional deformation fabric. The greenschist-facies overprint has the character of a shallow-dipping mylonitic foliation (Sin), bearing a penetrative stretching lineation (Lm) which roughly parallels the axes of coeval, isoclinal folds. Shear sense markers indicate a W-verging overthrust mechanism. The UHP and HP relic structures are of variable nature. The coexistence of equant and inequant, either symmetric or asymmetric fabrics, indicates that the deformation at UHP-HP conditions was strongly hetero-geneous and partitioned. This is also supported by the local preservation of Hereynian, magmatic fabrics. The UHP and HP deformation involved, at least locally, rotational components, although less intensive than during the latter retrograde stage. The regional structural evolution is envisaged as follows: (i) the Eoalpine subducted crust was subdivided into lenticular bodies surrounded by UHP-HP shear zones. The main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the UHP-HP structural relics; conflicting models are possible depending on the interpretation of the early sense of movement (normal vs reverse) along the faults that limit the lens-shaped units; and (ii) the late, heterogeneous, regional greenschist deformation can be attributed to the Eocene collapse of the Alpine orogenic wedge.
Article
Paleo-earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of Alpine Corsica. Pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the Schistes Lustrés of Cape Corse. Ultramafic pseudotachylyte fault-and injection veins are found within well-preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses. Numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified. Veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures. The newly formed minerals are zoned olivine (Fo 93–89), clino-and ortho-pyroxene with compositions indicative of high crystallization temperatures (1300–1400 8C), zoned Cr-spinel, and a glassy to micro-vesicular hydrous matrix showing that frictional melts contained up to 4% water. Frictional heating on co-seismic faults raised the temperature from ambient blueschist facies conditions (450 8C and 1–1.5 GPa) to more than 1700 8C, which is required for ~75% disequilibrium melting of spinel peridotite at 1.5 GPa. The characteristic fault-vein thicknesses observed are 1 to 3 cm, but several fault-veins are thicker than 10 cm. Co-seismic displacement of 1 m, a stress of 300 MPa, and seismic efficiency of 5%, may melt ca 60 kg peridotite pr. m 2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte. The ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the Alpine subduction zone. If the interpretations of typical displacements of approximately 1 m are correct, the most common pseudotachylyte fault-veins are related to magnitude ca. 7 or larger subduction earthquakes. D 2005 Elsevier B.V. All rights reserved.
Article
Magnetic fabric analysis and petrography have been carried out on ∼1100 samples from the South Range Breccia Belt (SRBB), a 45-km-long arc of Huronian Supergroup breccia fragments and crystallized friction melt (pseudotachylyte), sub-concentric to the 1.85 Ga Sudbury Impact Structure. Optical petrography and analytical SEM analyses show the pseudotachylyte to consist of fine-grained quartz, biotite and ilmenite, with minor plagioclase, apatite, zircon, pyrite and pyrrhotite. Alternating field and thermal demagnetization spectra indicate that the principal magnetic carrier in the matrix is pyrrhotite, while clasts can contain either pyrrhotite or magnetite. Mean magnetic susceptibilities in the matrix are generally on the order of 10−4 SI. Two AMS (anisotropy of magnetic susceptibility) fabrics have been determined in the matrix. The most common is an oblate, sub-vertical foliation, sub-parallel to the local strike of the SRBB, with anisotropies (P) ranging from 1.08 to 1.20. The second AMS fabric is a weakly to strongly prolate sub-vertical lineation with generally higher P values of 1.2 to 1.55. Both fabrics are coaxial to the petrofabric in the matrix. Huronian clasts exhibit scattered, heterogeneous AMS fabrics, distinct from those found in the SRBB matrix. The strongly prolate (T = −0.73) and anisotropic (P = 1.55) AMS fabric found directly over the giant Frood-Stobie NiCuPGE deposit, hosted within the SRBB, suggests that sulfide droplets were injected and/or squeezed into the matrix above the deposit. Samples containing pyrrhotite from the edges of the deposit do not exhibit this magnetic signature. Similar prolate, moderately anisotropic lineations are also found further to the east of the Frood-Stobie Mine, and appear to be associated with a magnetotelluric conductor at depth. It is suggested that these lineations are due to injection of material from depth into the SRBB during failure of the inner rim of the Sudbury Impact Structure during the crater modification stage. The oblate magnetic fabric found within the SRBB is consistent with frictional comminution and melting of the wall rock lithologies during collapse of the transient cavity and subsequent slumping of the crater walls. However, subsequent deformation of the Sudbury Structure by late Penokean folding and thrusting to the NW could produce a similar oblate fabric. Tight clustering of the maximum principal axes of the magnetic susceptibility could be interpreted as being consistent with tectonometamorphic transposition of the matrix petrofabric, as well as generation by slumping and friction melt flow during failure of the hanging wall. Further work is needed on undeformed impact structures of similar size to Sudbury in order to constrain the petro- and magnetic fabrics produced during rim collapse. Additional work in the westerly trending SRBB along the edge of the Creighton Pluton would help to determine if the dominant NE-SW-trending matrix fabric is regional in origin, or is controlled by the current orientation of portions of the SRBB.