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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
GEOLOGY, June 2015; v. 43; no. 6; p. 1–4; Data Repository item 2015188
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doi:10.1130/G36587.1
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Published online XX Month 2015
© 2015 Geological Society of America. For permission to copy, contact editing@geosociety.org.
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GEOLOGY
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
