A seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite-facies rocks of Calabria (Southern Italy)

Abstract

Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting.

Full text

ORIGINAL PAPER
A seismogenic zone in the deep crust indicated
by pseudotachylytes and ultramylonites in granulite-facies
rocks of Calabria (Southern Italy)
U. Altenberger
•
G. Prosser
•
A. Grande
•
C. Gu
¨
nter
•
A. Langone
Received: 10 September 2012 / Accepted: 5 June 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Pseudotachylyte veins frequently associated
with mylonites and ultramylonites occur within migmatitic
paragneisses, metamonzodiorites, as well as felsic and
mafic granulites at the base of the section of the Hercynian
lower crust exposed in Calabria (Southern Italy). The
crustal section is tectonically superposed on lower grade
units. Ultramylonites and pseudotachylytes are particularly
well developed in migmatitic paragneisses, whereas sparse
fault-related pseudotachylytes and thin mylonite/ultramyl-
onite bands occur in granulite-facies rocks. The presence of
sillimanite and clinopyroxene in ultramylonites and myl-
onites indicates that relatively high-temperature conditions
preceded the formation of pseudotachylytes. We have
analysed pseudotachylytes from different rock types to
ascertain their deep crustal origin and to better understand
the relationships between brittle and ductile processes
during deformation of the deeper crust. Different protoliths
were selected to test how lithology controls pseudotachy-
lyte composition and textures. In migmatites and felsic
granulites, euhedral or cauliflower-shaped garnets directly
crystallized from pseudotachylyte melts of near andesitic
composition. This indicates that pseudotachylytes origi-
nated at deep crustal conditions ([0.75 GPa). In mafic
protoliths, quenched needle-to-feather-shaped high-alu-
mina orthopyroxene occurs in contact with newly crystal-
lized plagioclase. The pyroxene crystallizes in garnet-free
and garnet-bearing veins. The simultaneous growth of
orthopyroxene and plagioclase as well as almandine, sug-
gests lower crustal origin, with pressures in excess of
0.85 GPa. The existence of melts of different composition
in the same vein indicates the stepwise, non-equilibrium
conditions of frictional melting. Melt formed and intruded
into pre-existing anisotropies. In mafic granulites, brittle
faulting is localized in a previously formed thin high-
temperature mylonite bands. migmatitic gneisses are
deformed into ultramylonite domains characterized by s-c
fabric. Small grain size and fluids lowered the effective
stress on the c planes favouring a seismic event and the
consequent melt generation. Microstructures and ductile
deformation of pseudotachylytes suggest continuous duc-
tile flow punctuated by episodes of high-strain rate, leading
to seismic events and melting.
Keywords Pseudotachylyte  Calabria  Lower crust 
Palaeo-seismicity
Introduction
Fault-related pseudotachylytes record ancient hypocentres,
usually located at upper crustal depths (e.g. Hetzel et al.
1996; Sibson and Toy 2006; Lin 2008a). However,
Communicated by G.Moore.
U. Altenberger (&)  C. Gu
¨
nter
Institute of Earth and Environmental Sciences,
University of Potsdam, Karl-Liebknecht-Str. 24,
14476 Potsdam-Golm, Germany
e-mail: uwe@geo.uni-potsdam.de
G. Prosser
Dipartimento di Scienze, Universita
´
della Basilicata,
Via dell’Ateneo Lucano, 10, 85100 Potenza, Italy
e-mail: giacomo.prosser@unibas.it
A. Grande
Istituto di Geoscienze e Georisorse-CNR, Via Moruzzi 1,
56124 Pisa, Italy
A. Langone
Istituto di Geoscienze e Georisorse-CNR, Via Ferrata 1,
27100 Pavia, Italy
123
Contrib Mineral Petrol
DOI 10.1007/s00410-013-0904-3

evidence derived from both present seismicity and fossil
earthquake sources indicates that the lower continental
crust may also be a site where seismic events can be
generated (Clarke and Norman 1993; Austrheim and Bo-
undy 1994; Jackson 2002; Lin 2008b; Altenberger et al.
2011). Generally, pseudotachylytes formed below
12-15 km, that is, below the classical seismogenic brittle
upper part of the crust, are intimately associated with
mylonites and ultramylonites (Passchier 1982; White 1996;
Pennacchioni and Cesare 1997; Pittarello et al. 2010).
Particularly, mutual overprinting relationships between
pseudotachylytes and ultramylonites in upper and middle
crust indicate that a long-lasting creep regime was inter-
rupted by episodic rapid slip events (Handy and Brun
2004). This association of fault rocks, apparently con-
nected to strongly different strain rates and rheology, has
been interpreted to represent a number of processes such
as: (i) alternating brittle faulting and ductile shearing along
a fault developed across the brittle-ductile transition
(Takagi et al. 2000); (ii) downward propagation along
mylonites of a fault nucleated at higher crustal levels
(Sibson 1980; Lin et al. 2003, 2005 Lin 2008b; Moecher
and Steltenpohl 2009; Pittarello et al. 2012); (iii) stress
concentration at stronger lensoid-shaped bodies (Sibson
1980); (iv) stress concentrations around or within the more
competent layers (Fagereng 2010); (v) plastic instabilities
during ductile flow leading to seismic events (Hobbs et al.
1986). The last process is more likely activated during
high-strength crystal-plastic deformation in a fluid-defi-
cient environment (White 1996). The concurrent formation
of mylonites and pseudotachylytes has been confirmed also
by recent experiments carried out at seismic slip rates on
halite gouge (Kim et al. 2010). Summarizing, high stress
and strain rates, at low fluid pressures, may lead to con-
ditions for instabilities during high-strength flow (White
1996). This process may produce rapid sliding and fric-
tional heating in mylonites or ultramylonites, if deforma-
tion rate suddenly increases. Alternatively, pre-existing
anisotropies, such as thin mylonite bands, may localize the
propagation of a fault into a region where intermittent
brittle deformation occurs.
The presence of pseudotachylytes in high-grade rocks is
not a proof that they formed at lower crustal depths, since
seismic events can generate during exhumation of origi-
nally deep units (Techmer et al. 1992; Pittarello et al.
2012). Seismic instabilities are thought to occur when
exhumation rate slows down after a faster exhumation
episode (Zechmeister et al. 2007). However, a deep origin
for pseudotachylytes can be proven by mineral composi-
tion of newly crystallized microlites (Austrheim et al.
1996; Austrheim and Andersen 2004; Altenberger et al.
2011) or from the recrystallization products developed
during a subsequent mylonitic overprint (White 1996).
In this paper, we will describe pseudotachylytes devel-
oped in a kilometre-thick zone of migmatitic paragneisses,
felsic, intermediate and mafic granulites exposed in Cala-
bria (Southern Italy). One sample of a pseudotachylyte
within a felsic protolith has already been described in detail
by Altenberger et al. (2011) and its lower crustal origin has
been assessed from the occurrence of garnets with cauli-
flower and euhedral shapes directly crystallized from the
pseudotachylyte melt. Starting from this early finding, we
present new analyses of pseudotachylytes from a wide area
of the Calabria high-grade basement (Fig. 1), where dif-
ferent rock types can be recognized and give some evi-
dence for localization mechanisms. This significantly
expands the earlier observations carried out by Altenberger
et al. (2011), since the present work reports microstructures
and mineral assemblages of pseudotachylytes from mark-
edly different protoliths, improving our knowledge on
faulting in the deep crust. Our analyses have been carried
out on eight samples collected in felsic and in mafic
granulites as well as in migmatitic paragneisses and met-
amonzodiorites (Table 1). We performed detailed micro-
structural and chemical analyses with optical and SEM
microscopy as well as by X-ray maps and electron
microprobe. Pressure and temperature operating during
pseudotachylyte formation have been estimated from
microstructural evidence and from the occurrence of
peculiar minerals. This information has been used to doc-
ument the textures of deep crustal pseudotachylytes, the
relative timing between ductile and brittle deformation and
the evidence of strain localization along faults at deeper
crustal levels.
Geological setting
The Calabria Terrane represents an exotic block, mostly
consisting of Palaeozoic basement, which strongly differs
from the Southern Apennines and the Sicilian Maghre-
bides, where sedimentary rocks of Mesozoic to Cenozoic
age are widespread (Bonardi et al. 2001; Fig. 1). Fault-
related pseudotachylytes mainly occur in high-grade rocks
and mylonites at the hanging wall of a main tectonic
contact of Alpine age, separating nappes of Hercynian
basement rocks. The main metamorphism in the Calabria
basement occurred at 299–304 Ma, during the late stages
of the Hercynian orogeny (Graessner et al. 2000; Langone
et al. 2010). The present tectonic setting of the late
Hercynian high-grade rocks is related to the evolution of
the Alpine-Apenninic orogen that mainly took place in the
Cretaceous (?) – late Tertiary time interval. During the
early evolutionary stages, the Calabria basement underwent
internal shortening and was thrusted above ophiolitic units
belonging to the Tethyan oceanic domain (Rossetti et al.
Contrib Mineral Petrol
123

2001). Afterwards, starting from the Oligocene time, the
Calabria basement was affected by extensional tectonics
and rapid exhumation (Thomson 1994; Grande et al. 2009),
connected to a general tilting towards the SE. This allowed
the exposure of the deeper part of the Calabria basement.
Finally, from the late Miocene onwards, the Calabria ter-
rane drifted to the SE, reaching the present position in the
Apennine chain in connection with the opening of the
Tyrrhenian Sea (Rosenbaum and Lister 2004).
In the Serre Massif, a nearly complete section of the late
Hercynian continental crust, showing an overall thickness
of 20–25 km (Schenk 1990), makes up the upper nappe of
the Alpine tectonic edifice (Fig. 1). The base mostly con-
sists of felsic and mafic granulites, grading upwards to
migmatitic metapelites, whereas in the intermediate and
upper parts thick sheets of granitoids covered by medium-
to low-grade phyllites and schists are present. In the upper
nappe, the late Hercynian fabric is generally well
Iacurso
ma
erts
a
lli
P
Girifalco
Curinga
y
e
l
laV
an
i
r
r
uT
S. Pietro
a Maida
Curinga-Girifalco Line
Bosco
Vosina
Turrina
quarry
CALAB0a
CALAB01
CALAB02
CM73
Calab03
Calab2009
Calab04
DR5
Inferred tectonic contact
Normal fault
Tectonic contact with undefined kinematics
Main thrust contact
Felsic granulites, metagabbros, mafic
granulites and peridotites (Upper Nappe)
Pliocene-pleistocene clays, sandstones
and conglomerates
Schists and orthogneisses (Intermediate
Nappe)
Phyllites (Lower Nappe)
N
1 km
Granitoids
Paleozoic
metamorphic rocks
Mesozoic to
Quaternary rocks
Sila
Tyrrhenian
sea
Ionian sea
0
20
40
km
39°
17°
39°
16°
38°
S
e
r
r
e
Aspromonte
17°
Curinga
N
16° E 18’
16° E 20’
16° E 22’
16° E 24’
38° N 49’
30”
38° N 50’
38° N 50’
30”
38° N 51’
38° N 51’
30”
Fig. 1 a Schematic geology of the Calabria Terrane; inset shows the
location of the Calabria Terrane in the Apennine-Maghrebian chain.
b Simplified geological map of the Curinga-Girifalco area. Modified
following Paglionico and Piccarreta (1976), Langone et al. (2006) and
Altenberger et al. (2011)
Table 1 Sample description (opaques: iron sulphides and other not clearly identifiable opaque phases)
Host rock Pseudotachylyte
Sample Rock type Composition Microlites
Calab 2009 Felsic granulite Kfs (20), Pl (20), Qtz (30), Grt (10), Sil (10), Bt (10), Rt Kfs, Pl, Qtz, Grt, Sil, Bt, opaques
Calab 0a Migmatitic paragneiss,
layered
Kfs (30), Pl (10), Qtz (20), Bt (10), Grt (10), Rt Pl, Bt, Sil, Qtz Grt, Rt, opaques
Calab 01 Migmatitic paragneiss,
layered
Light layers: Kfs (25), Qtz (70), Bt (5); dark layers: Bt (35),
Sil (30) Grt (10), Pl (15), Qtz (10), Rt
Pl, Bt, Sil, Qtz, Grt, Rt, opaques
Calab 02 Migmatitic paragneiss,
layered
Light layers: Kfs (20), Qtz (75), Bt (5); dark layers: Bt (35),
Sil (35) Grt (10), Pl (10), Qtz (10), Rt
Pl, Bt, Sil, Qtz, Grt, Rt, opaques
Calab 03 Metamonzodiorite Pl (antiperthitic, 40), Opx (30), Kfs (perthitic, 30), opaques Opx, Pl, Kfs, Rt, opaques
Calab 04 Mafic granulite Pl (50), Cpx (20), Hbl (brown, 20), Opx (10), opaques Cpx, Opx, Pl, Grt, Rt (?), opaques
CM73 Mafic granulite Pl (50), Cpx (20), Hbl (brown,15), Opx (15), opaques Cpx, Pl, Grt, Rt (?), Ilm, opaques
DR 5 Felsic granulite Kfs (25), Pl (20), Qtz (30), Grt (10), Bt (10), Opx (5), opaques Kfs, Pl, Qtz (?), Grt, Bt, opaques
Mineral abbreviations according to Kretz (1983). Sample Calab 2009 following Altenberger et al. (2011). Numbers in brackets are roughly
estimated volume % of the major constituents
Contrib Mineral Petrol
123

preserved. Altenberger and Kruhl (2000) document the
presence of a high-temperature shear zone (about 800 °C/
700 MPa) within granulite-facies metapelites, connected to
top-to-the-NW shearing during the Hercynian orogeny.
The upper nappe is located above an intermediate and a
lower nappe consisting of medium- to low-grade Hercynian
basement rocks, respectively. Nappe stacking has been
referred to the Alpine orogeny (Amodio-Morelli et al.
1976; Langone et al. 2006). The main tectonic contact
separating the upper and the intermediate nappe has been
identified as the Curinga-Girifalco line (Schenk 1980;
Fig. 1). It represents the tectonic contact between high-
grade hanging wall rocks and medium-grade footwall
rocks, mostly consisting of orthogneisses and schists. In the
eastern part of the area (Girifalco area; Fig. 1), normal
faulting of Oligocene-early Miocene age (15–30 Ma)
reworked the Curinga-Girifalco line under brittle condi-
tions (Thomson 1998; Langone et al. 2006). However, an
older deformation history, probably related to Alpine
thrusting, can be recognized both in the footwall and in the
hanging wall rocks (Spiegel 2003; Langone et al. 2006). In
the footwall block, thick layers of mylonites are mainly
derived from granitoid orthogneisses. The only age deter-
mination of these mylonites provided by Rb–Sr method on
biotites suggests an age of about 43 ± 1 Ma (Schenk
1980). This determination is in agreement with other
Alpine Ar/Ar ages obtained in different portions of the
Calabria basement (Heymes et al. 2010). Thermobaro-
metric estimates indicate that these mylonites formed at
relatively high pressures (0.75–0.9 GPa), within the epi-
dote–amphibolite facies (Langone et al. 2006). Within the
hanging wall, deformation is partitioned into a 100–150-m
thick belt of mylonites, ultramylonites and pseudotachy-
lytes in the migmatitic paragneisses. This is particularly
evident in the Bosco Vosina outcrop (Fig. 1), where
pseudotachylytes form some centimetre-thick veins nearly
parallel or at low angle with the mylonitic foliation. Here,
the generation surfaces are difficult to identify, since they
are almost parallel to the foliation of the mylonites/ultra-
mylonites. Locally, some injection veins or pull-apart
structures cutting the foliation of the host ultramylonite can
be observed. No data on the age of deformation have been
obtained from these rocks up to now; however, they are
generally referred to as the mylonites of the Curinga-Gi-
rifalco line (Spiegel 2003).
Mylonites in both the hanging wall and the footwall
show SE- to SSE-plunging stretching lineations on a
shallow-dipping foliation. This is also evident in the Bosco
Vosina outcrop (Fig. 2a), where kinematic indicators in
mylonites and ultramylonites are mostly consistent with
SE-directed shearing (Langone et al. 2006). This shear
sense, probably related to Alpine thrusting, now shows an
apparent normal movement after Oligocene tilting of the
whole Calabria basement (Thomson 1994).
In the western part of the area (Curinga area; Fig. 1), the
Curinga-Girifalco line is interrupted by a younger
N-trending fault. Here, the high-grade basement consists of
felsic granulites containing bodies of granulite-facies
metagabbros, fine-grained spinel peridotites and granulites
with a monzodioritic or dioritic composition. Layering of
high-grade rocks dips mostly to the S or SW with variable
dip angles. In the main outcrop located in the Turrina
quarry (Fig. 1), granulite-facies metagabbros form a huge
body within felsic granulites.
In the Curinga area, pseudotachylyte veins and patches
developed in decimetre-thick ultramylonite bands
(Fig. 2a), occurring mostly within felsic granulites. Pseu-
dotachylytes may form either foliation parallel millimetre-
thick veins (Fig. 2b), or millimetre- to centimetre-thick
injection veins crosscutting the mylonitic foliation. Orien-
tations of foliation and lineation in ultramylonite bands can
be compared with those measured in the mylonites of the
Girifalco area (Fig. 2b). Lineations, occurring on SSE
dipping foliation planes, mostly plunge towards N140°-
N160°.
In mafic and felsic granulite-facies rocks, pseudotachy-
lyte-bearing faults can also be observed. Fault planes dip
mostly towards the S-SE with variable inclinations
(Fig. 2c). Lineations have been rarely observed on fault
planes; normally, they plunge towards the S or SE. Kine-
matics has been detected in rare cases from the presence of
about 2 cm wide and 1-cm thick pull-apart structures; they
mostly point to top to the S-SE-directed faulting, that
appear extensional in the present-day attitude. Fault planes
are nearly parallel or intersect at low angles to the layering
of the granulites that dip towards the S and SW (Fig. 2d).
Analytical methods
Concentrations of major and some minor elements in
minerals were analysed with wavelength dispersive spec-
trometry using a Jeol JXA 8200 electron microprobe.
Acceleration voltage of 15 kV and a beam current of
20 nA for quantitative analyses and 35 nA for element
maps have been used. Backscattered electron images and
areal bulk composition analyses of the pseudotachylyte
matrix were obtained with an SEM JEOL JSM-6510
combined with an Oxford Instruments Incax-act analytical
EDX system in Potsdam and with a Philips XL-30 ESEM
in Potenza. Mineral detection like sillimanite in the fine-
grained pseudotachylyte matrix and in ultramylonites has
been confirmed by means of X-ray diffraction (XRD) with
a Siemens D 5005 X-ray diffractometer.
Contrib Mineral Petrol
123

Petrography of the protoliths of the pseudotachylytes
Lower crustal protoliths of the studied fault rocks show
significant variations in composition and mineral assem-
blage, as summarized in Table 1. We have selected
different protoliths in order to investigate how lithology
controls the pseudotachylyte composition and textures.
In the eastern part of the area (Girifalco, Figs. 1, 3a,b,
Table 2), pseudotachylytes and ultramylonites formed
mostly within migmatitic paragneisses, showing a late
N=59
N=63
N=27
N=29
N
N
NN
A
B
C
D
Fig. 2 Equal area projection
(lower hemisphere) of structural
data obtained in the Curinga-
Girifalco area. In diagrams A, B
and D, poles to the foliation
planes have been represented
with filled circles and related
contour lines (1 % area contour;
contour intervals = 2.0/1 %
area). a Poles to the foliation
planes and stretching lineations
(filled triangles)of
ultramylonites from the eastern
part of the area (Girifalco).
b Poles to the foliation planes
and stretching lineations (filled
triangles) of ultramylonites
from the western part of the area
(Curinga). c Fault planes (great
circle) and related slickenside
striations (filled circles). d Poles
to the high-temperature layering
of the granulites from the
Curinga area
Fig. 3 a Pseudotachylyte patches and veins (Girifalco) c clast. b Contact of pseudotachylyte and ultramylonite, hand specimen (Girifalco).
Foliation is indicated by white lines
Contrib Mineral Petrol
123

Hercynian mineral assemblage consisting of quartz ?
garnet ? sillimanite ? biotite ± K-feldspar and ± pla-
gioclase, developed under upper amphibolite- to granulite-
facies conditions. A relict banded fabric formed by alter-
nating quartz-feldspar-rich and biotite-sillimanite-rich
layers is preserved in the less deformed portions (Table 1).
The ultramylonites reveal s-c fabrics in the classical sense
of Berthe
´
et al. (1979). They are formed preferentially in
the sillimanite—and biotite—rich layers, but occur also in
quartz-rich layers as thin bands of dynamically recrystal-
lized grains. The pseudotachylytes preferentially formed
parallel or subparallel to the c planes and intruded into the s
planes (Fig. 4a).
In the western part of the area (Curinga, Fig. 1,
Table 2), pseudotachylytes formed within different types
of granulite-facies rocks of mafic, intermediate and felsic
composition. Among them, metagabbros are medium
to coarse-grained rocks consisting of plagioclase ? ortho-
pyroxene ? clinopyroxene ± amphibole ± garnet. They
show alternating pyroxene or amphibole-rich and plagio-
clase-rich levels, interpreted as reflecting a primary mag-
matic layering (Moresi et al. 1979). Towards the contact
with the felsic granulites, mafic rocks show garnet por-
phyroblasts almost replaced by spectacular radial sim-
plectites, composed of orthopyroxene ? amphibole ?
plagioclase (Acquafredda et al. 2008). Locally, this rock
type grades to intermediate- to fine-grained granulites with
a monzodioritic composition, consisting of plagioclase ?
K-feldspar ? orthopyroxene ± clinopyroxene ± amphibole.
In the mafic granulites from Curinga outcrop (Fig. 1),
small-scale ductile shear zones show grain-size reduction
by dynamic recrystallization of clinopyroxene and plagio-
clase, pointing out to minimum temperatures of about
600 °C (Kruhl and Huntemann 1991; Passchier and Trouw
1996). In the studied sample, a ductile shear zone is
crosscut at low angle by a pseudotachylyte vein.
Felsic granulites make up most of the high-grade base-
ment. The most common rock type consists of quartz ?
K-feldspar ? plagioclase ? garnet ? sillimanite ± biotite
with accessory zircon and rutile. They show a banded
fabric, outlined by alternating garnet-sillimanite-rich and
feldspar-rich layers. More rarely, orthopyroxene-bearing
and sillimanite-free granulites also occur. Shear zones
mostly consist of ultramylonites characterized by syntec-
tonic sillimanite and dynamic recrystallization of quartz,
biotite and feldspar.
According to Schenk (1984), maximum pressures
recorded by the mafic granulites are about 0.75 GPa, at
temperatures of 800 °C. More recent analyses carried out
by Acquafredda et al. (2008) on mafic granulites from the
Turrina quarry resulted in significant higher P–T conditions
of 1.1 GPa and 900 °C, respectively. The presence of
corona textures around garnet allowed to reconstruct the
retrograde path that shows decompression and cooling up
to pressure conditions of 0.7–0.8 GPa and temperatures of
750–650 ° C.
Microstructures of pseudotachylytes
Rock fragments in pseudotachylyte veins
The pseudotachylyte veins contain clasts of the host rock in
different size, quantities and composition. Monomineralic
as well as polymineralic clasts occur. The size of the clasts
is mostly smaller than 1 cm and correlates positively with
the thickness of the pseudotachylyte and often with the
distance to the pseudotachylyte margin (Fig. 4b). In one
case, a folded or fold-shaped rock fragment occurs
(Fig. 4c). This clast shows internal s-c fabric, probably
preserved from the ultramylonitic country rock and con-
tains stretched quartz grains in the outer part.
In another sample, a quartz-dominated clast is dis-
membered by a pseudotachylyte vein network arranged
into an s-c-like fabric (Fig. 4d, e). Within the pseudot-
achylyte vein, a disrupted quartz sigma clast, enclosed
within a biotite-rich matrix, occurs (Fig. 4f). These
important microstructures will be used to interpret the
relative timing and the formation mechanisms of pseudot-
achylytes in the Calabria lower crust. Few veins have small
zones at the margin to the country rock in which cataclastic
deformation prevails.
In felsic granulites and migmatitic paragneisses, the
pseudotachylytes show a dominance of quartz and silli-
manite clasts as well as rock fragments composed of
quartz, sillimanite and plagioclase (Fig. 5a, b). Monomi-
neralic plagioclase has a limited grain size between 450
and 500 lm. Grains smaller than 450 lm are very rare or
missing. K-feldspar rarely survived frictional melting.
Clasts of biotite, which is a prominent phase in the country
rocks, and opaques are not commonly preserved.
Table 2 Geographical coordinates of the studied samples
Sample no. Coordinates
Calab 2009 38°49
0
22
00
16°18
0
33
00
Calab 0a 38°49
0
11
00
16°24
0
09
00
Calab 01 38°49
0
10
00
16°24
0
09
00
Calab 02 38°49
0
09
00
16°24
0
08
00
Calab 03 38°49
0
22
00
16°17
0
51
00
Calab 04 38°50
0
34
00
16°19
0
02
00
CM73 38°50
0
34
00
16°19
0
02
00
Dr5 38°49
0
21
00
16°17
0
43
00
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123

Quartz fragments are rounded, elongate to lens shaped
and range in sizes from a few lm to 500 lm, rarely up to
1,000 lm. Few grains show euhedral to subhedral shapes,
due to a probable later overgrowth in the pseudotachylyte
melt (Fig. 4a). The minerals often show evidence of
internal deformation like subgrains and dynamically re-
crystallized grains, comparable to those of the country rock
nearby. Two generations of dynamically recrystallized
grains of significantly different grain size are preserved in
clasts and in the wall rock adjacent to the pseudotachylyte
Fig. 4 a s-c fabrics, shown by quartz, biotite and garnet with thin
pseudotachylyte (white line, pseudotachylyte proven by SEM anal-
ysis) parallel to the c plane. Sample Calab 02, migmatitic paragneiss.
b Chilled margin at the border of a pseudotachylyte vein at the border
with an ultramylonite. Size of clasts increases discontinuously
towards the centre of the pseudotachylyte vein, to the left of the
figure. c Folded rock clast of ultramylonite from migmatitic
paragneisses with internal s-c fabric (thick arrow) enclosed in a
pseudotachylyte; at the clast margin and in the matrix lens-shaped
quartz clasts also occur (thin arrows). d s-c-like structure in a quartz-
bearing rock clast in pseudotachylyte matrix. Arrow points to fine-
grained recrystallized quartz. Migmatitic paragneisses. e Backscat-
tered image (BSE) of the s-c-like structure of c. Square refers to
Fig. 4f). Disrupted sigma clast of quartz in pseudotachylyte matrix
(see box in e). Spaces are filled by biotite (light, arrow). At the top
tree-like garnet, microlites are clearly visible. Photomicrographs with
crossed nicols: a–d; backscattered SEM image (BSE): e–f
Contrib Mineral Petrol
123

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123

(Fig. 5c). The very fine grains of the second generation,
often affecting the first generation grains, are up to
5–10 lm in size. Some larger quartz grains show fractures
and open cracks filled by pseudotachylyte containing
rounded quartz fragments embedded in a matrix enriched
in biotite or in biotite-plagioclase (Fig. 4d,e,f).
Monomineralic sillimanite clasts reach sizes of up to
200 lm, although clasts with a size smaller than 5 lm are
more frequent, as shown by SEM analyses. Some crystals
show embayed grain boundaries, which are typical for
clast-melt reaction (Fig. 5a,b).
Scarce monomineralic garnet clasts occur in the felsic
pseudotachylytes. The presence of subhedral and fractured
monazite suggests that this mineral underwent only brittle
deformation without melting during pseudotachylyte
formation.
In pseudotachylytes formed in mafic wall rocks, clasts
are dominated by plagioclase, orthopyroxene and minor
hornblende. Plagioclase shows rough bimodal grain-size
distributions with clasts smaller than 40 lm and clasts
around 400 lm. In mafic granulites, where ductile shear
zone is crosscut at low angle by a pseudotachylyte vein,
clasts of plagioclase and clinopyroxene show dynamic re-
crystallized grains of the same size as those occurring in
the shear zone (Fig. 5e,f). On the other hand, rare clasts
composed of recrystallized hornblende do not have an
equivalent in the wall rock of the pseudotachylyte.
Summarizing, embayed grain boundaries in clasts of
sillimanite and quartz and the lack of biotite clasts indicate
that these minerals were involved in melting processes.
Matrix and newly crystallized phases
This section provides the petrological and textural evidence
for a melt origin of the pseudotachylytes. Detailed
descriptions of newly formed phases are necessary for the
interpretation of the veins in terms of pressure (i.e. for-
mation depth), temperature and fluid composition during
melt generation and cooling.
Most samples are fully crystallized with no evidence of
preserved glass, even though the grain size is often at the
limit of high resolution of the used SEM (ca. 0.5 lm). As
pointed out by Altenberger et al. (2011), optically isotropic
spheres, composed of fine-grained Ca–Na-rich and K–Mg–
Fe-rich phases, may occur in the chilled margin of injection
veins in felsic rocks of the study area. There, biotite-rich
zones often transect these spheres. All pseudotachylytes
show chilled margins, where the mean grain size of the
microlites is reduced to up to one-tenth of the size in the
vein centre (Fig. 4b). Matrix and newly crystallized phases
in pseudotachylytes formed in different protoliths show
significant variations in composition and fabric. The
pseudotachylyte veins often show zoning by fabric and
composition. Some pseudotachylytes show obvious com-
positional layering and folds formed by flow processes
(Fig. 6a).
All minerals forming the granulite-facies country rocks
are also present as newly crystallized phases in the pseu-
dotachylyte, except monazite, zircon and hornblende. In
one domain of the mafic pseudotachylyte, irregular cloud-
like mineral aggregates formed by clinopyroxene and pla-
gioclase (± opaques) can be observed (Fig. 6b.). They do
not represent pseudomorphs since they are in direct contact
with typically rounded clasts, mainly consisting of
hornblende.
Orthopyroxene
In a pseudotachylyte in mafic host rocks, new orthopy-
roxene was generated (Fig. 6c,d). Orthopyroxene is up to
20 or 50 lm long and 3 lm thick. Growth often starts near
orthopyroxene clasts and show needle- or feather-shaped
grains radiating around the clast. However, in most cases,
there is no direct contact to the clast. More fine-grained
orthopyroxene shows skeletal growth (Fig. 6d). Domains
of clasts with surrounding needle- and feather-shaped
orthopyroxene alternate with domains of skeletal growth
with subordinate single orthopyroxene needles embedded
in a rim of plagioclase. The shapes of new orthopyroxene
are typical for rapid cooling. These features are very sim-
ilar to the clinopyroxenes described by Austrheim and
Andersen (2004) in subduction-related pseudotachylytes
from Corsica.
Sillimanite
Newly generated sillimanite, in pseudotachylyte of felsic
host rocks, forms small needles with length between 3 and
Fig. 5 Images A to D have been obtained in pseudotachylytes in
migmatitic paragneisses. a Element maps of a pseudotachylyte with
anhedral and subhedral sillimanite (Sil) and quartz (Qtz) reflecting
mineral growth in the melt by straight grain boundaries (arrows).
Quartz shows also small corrosion (?) embayment. b Element map of
Si shows broken sillimanite, partially with embayed grain boundaries
(arrow) indicating corrosion by the melt. The BSE image below
shows biotite (Bt) between the sillimanite clasts (light). c Quartz-rich
layer in the host rock with two generations of recrystallized quartz.
Black arrows indicate very fine-grained recrystallized grains. The
distance to the pseudotachylyte is ca. 0.25 mm (direction indicated by
white arrow). Sample Calab 03, migmatitic paragneisses. d BSE
image and element maps of Ca and Ti of cauliflower garnet, showing
straight grain boundaries on the right (arrow) and clear Ca zoning. Ti-
rich inclusions (rutile) are preferentially concentrated in the inner
parts of the grain. e Dynamic recrystallization of clinopyroxene (Cpx)
and plagioclase in the wall rock at the margin to a pseudotachylyte
(dark) from a mafic protolith. f Clast with dynamically recrystallized
plagioclase and clinopyroxene in the pseudotachylyte of Fig. 4e. a–
d Electron microprobe element maps and BSE images; c,e,f photo-
micrographs with crossed nicols
b
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123

5 lm often containing tiny inclusions of probable quartz
(Fig. 7a). Sillimanite as the Al
2
SiO
5
phase has been proven
by XRD. Some significant nearly euhedral larger grains are
probably due to the overgrowth of sillimanite clasts
(Fig. 4a). Sillimanite needles show no obvious preferred
shape orientation. In some samples, they occur as tiny
euhedral inclusions in newly formed garnet.
Garnet
Garnet is present in all pseudotachylytes from felsic
protoliths and in some pseudotachylytes from mafic pro-
toliths. It reaches sizes of up to 50 lm. Most grains show
cauliflower, fern- or tree-like and euhedral shapes (Fig.
4f). Some garnets show the clear development of crys-
tallographically controlled surfaces in parts of the cauli-
flower-shaped grains (Fig. 5D). This phenomenon is
primarily present in the central parts of the veins, whereas
in injection veins or ‘‘melt’’-filled intra-crystalline cracks,
garnet shows simple cauliflower- or fern-like shapes
(Fig. 4f). The grain size of garnet increases from chilled
margins to the centre of the veins. This indicates that
garnet grew directly from a melt, as described earlier by
Altenberger et al. (2011). In some garnets, inclusions of
different compositions can be observed. The cores show
either no inclusions or probable tiny biotites or rutiles
(Fig. 7b), whereas the volume-dominating rim encloses
quartz, sulphide droplets or sillimanite clasts (Fig. 7b).
This probably indicates either an older core, originated
from a clast, surrounded by garnet crystallization from the
melt or a cauliflower-shaped garnet with a younger
metamorphic overgrowth.
In addition, in some samples, garnet cores have slightly
lower Ca concentrations compared to the cauliflower-
shaped outer rims. The Ca concentration of the cores lies
between host rock garnet composition and rim (Fig. 5d,
Table 4). This is another, albeit weak, indication that clasts
act as a nuclei.
Fig. 6 a Element map of Mg shows flow fold in a pseudotachylyte.
Dark layers plagioclase rich; light clinopyroxene-rich layers. This
shows a heterogeneous melt that was being mingled by stretching and
folding. Mafic wall rock (sample CM73). b Light domain composed
of very fine-grained clinopyroxene-dominated clinopyroxene-plagio-
clase intergrowth in a matrix of plagioclase-dominated
clinopyroxene-plagioclase intergrowth (CM73, see Table 4). c Euhe-
dral and skeletal orthopyroxene microlites, partially nucleating at
orthopyroxene clast. Orthopyroxene is embedded in plagioclase
(dark) (sample) C73). d Skeletal growth of orthopyroxene microlites
(sample Calab 04, mafic granulite). a–d BSE images
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123

Quartz
Although quartz is the dominant clast in pseudotachylytes of
quartz-bearing host rocks, there is also evidence of the
growth of new quartz. In some samples, quartz clearly shows
the generation of new faces at clasts that are in part fractured
or melted (Fig. 5a). In other samples, quartz nucleates on
sillimanite clasts, or forms tiny euhedral needles.
Fig. 7 a Growth of new sillimanite laths with tiny plagioclase
inclusions and biotite (light) around quartz (dark). Sample Calab 0a,
migmatitic paragneiss. b Cauliflower-like garnet, with inclusion-poor
core. Rim with plagioclase and iron-sulphide (light) inclusions.
Sample Calab 0a, migmatitic paragneiss. c Clinopyroxene-plagioclase
matrix (neoliths) with rounded hornblende clast (Hbl); pseudotachy-
lyte in a mafic wall rock. Sample Calab 04, mafic granulite. d Oblique
foliations indicated by the preferred orientation of biotite in a
pseudotachylyte. Note biotite in the pressure shadow of plagioclase
clast (arrow). Sample DR 5, felsic granulite. e Growth of skeletal
ilmenite (light) near to large ilmenite grain. Margin of pseudotachy-
lyte. Sample CM73, mafic granulite. f Oblique foliations (//white
line). Sample C73. a–e BSE images; f photomicrograph
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123

Hornblende
Although in some mafic samples the wall rocks either
contain significant amounts of hornblende or hornblende
could be present as small clasts in the pseudotachylyte
(Fig. 7c), there is a lack of amphibole microlites.
Clinopyroxene
Newly crystallized clinopyroxene microlites are restricted
to the mafic pseudotachylytes. They occur as anhedral to
lath-shaped intergrowth with plagioclase in the matrix of
the pseudotachylyte (Fig. 7c). The grain size is smaller
than 8 lm. In addition, clinopyroxene is an essential
component of the irregular cloud-like plagioclase-clin-
opyroxene domains (Fig. 6b).
Feldspars
Plagioclase often forms the fine-grained matrix of the
pseudotachylytes, together with biotite or clinopyroxene. It
occurs as an anhedral to rarely euhedral crystal in the mafic
pseudotachylytes and as subhedral to euhedral laths in the
felsic types. Grain size is always below 10 lm. It is
noteworthy that newly formed orthopyroxene is always
rimmed or embedded in newly formed plagioclase.
K-feldspar occurs as fine-grained intergrowth with pla-
gioclase and biotite in pseudotachylytes from meta-mon-
zodioritic rocks. Grain size of the anhedral crystals is
mostly below 5 lm.
Biotite
Newly formed biotite is present in all pseudotachylytes
from felsic protoliths. Often it crystallized in pressure
shadows of quartz clasts. In addition, biotite fills the space
between broken and dismembered parts of clasts (Figs. 4f,
5b). In the case of the monzodioritic sample, biotite is part
of the intimate intergrowth in the matrix. In one sample,
the preferred orientation of biotite microlites defines a
strong fabric oblique to the vein margin (Fig. 7d) indicat-
ing plastic creep after pseudotachylyte generation.
Opaques
In some mafic rocks, new fern-shaped ilmenite crystallized
only very near and along the margin of pseudotachylytes
(Fig. 7e). Iron-sulphide droplets occur frequently in the
matrix or as inclusions in newly formed minerals (Fig. 7b).
In addition, anhedral rutile or magnetite is often included
in newly formed garnet (Fig. 5d).
Table 3 Composition of different domains in the pseudotachylyte matrix
Sample Calab 04 Calab 03 Calab 02 Calab 01
Rock type Mafic granulite Metamonzodiorite Migmatitic paragneiss Migmatitic paragneiss
Domain Protolith Pst; Matrix,
homogeneous
Cloudy, light
domain
Dark
domain
Protolith;
ultramylonitic
Matrix,
centre
Chilled
margin
Protolith;
ultramylonitic
Pst;
matrix
Protolith;
ultramylonitic
Pst;
matrix
Pst; matrix around
Grt microlite
wt %
SiO
2
53.80 53.02 53.14 53.6 56.96 58.76 58.83 75.96 62.82 73.36 62.12 62.82
TiO
2
0.26 1.45 1 1.71 1.67 1.93 1.94 0.89 1.41 0.17 1.32 1.19
Al
2
O
3
19.50 16.68 16.07 17.47 17.59 15.24 14.33 13.16 21.03 14.45 20.6 21.33
FeO 4.42 7.65 8.32 7.53 8.34 9.7 8.96 4.46 8.18 4.24 7.19 7.04
MgO 8.98 8.7 8.84 7.01 5.14 6.19 6.89 2.07 2.74 2.03 2.66 2.26
MnO u.d.l. 0.33 0.2 0.42 u.d.l. u.d.l. u.d.l. u.d.l. u.d.l. u.d.l. u.d.l. u.d.l.
CaO 9.91 10.53 10.61 10.6 4.72 3.2 3.62 0.97 0.2 1.01 0.64 0.74
NaO 3.49 1.55 1.58 1.97 4.79 3.37 3.54 1.2 0.8 2.38 2.1 2.13
K
2
O 0.20 0.09 0.11 0.15 0.9€ 1.61 1.88 1.3 3.07 2.62 3.59 3.03
pst pseudotachylyte, u.d.l. under detection limit
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123

Apatite
A band of multiple apatite grains is formed at a clast
margin in the pseudotachylyte of the mafic granulite sam-
ple. Apatite grains contain inclusions of tiny clinopyroxene
and plagioclase, proving the direct crystallization from the
pseudotachylyte melt.
Mineral and bulk chemistry of pseudotachylytes
Pseudotachylytes in quartz-bearing wall rocks have a
matrix composition, in the range of an andesite, that is,
significantly more basic than the protoliths (sample Calab
01 and 02, Table 3). This shift is due to the relative
enrichment of quartz in the clast fraction, pointing to dis-
equilibrium melting. In contrast, pseudotachylytes in mafic
and monzodioritic protoliths do not show a significant
difference in SiO
2
concentration with respect to the host
rocks, due to the absence of melt-resistant phases like
quartz. However, in these samples, a decrease in Al
2
O
3
and
a slight increase in Fe
2
O
3
and MgO point to a preferential
melting of hornblende (sample Calab 03) and/or orthopy-
roxene (sample 04, Table 3).
The analyses of the cloudy domains in the mafic pseu-
dotachylyte show a composition enriched in MgO and FeO
and depleted in Na
2
O in comparison to the surrounding
matrix (Calab 04, Table 3, Fig. 6b). This could be explained
by the higher amount of clinopyroxene and lower concen-
tration in plagioclase. However, these different domains are
probable indicators of two different portions of frictional
melts that in some places remained unmixed. Some biotite-
rich matrix layers indicate melts of near biotite composition,
reflecting previous non-equilibrium melting.
Table 4 Composition of garnet
Garnet
Calab 2009* Calab 0a Calab
01
Calab
02
Calab 04 Calab Dr5
Host rock Pseudotachylyte Host
rock
Pseudotachylyte Pst Pst Central
vein,
Chilled
margin
Pst Pst
Core Rim Core Rim Core Core Rim Core Core Core Core Core Rim
wt %
SiO
2
37.86 38.08 37.60 37.46 38.49 36.93 36.96 37.18 37.14 39.43 38.91 37.69 37.80
TiO
2
0.03 0.09 0.06 0.03 u.d.l. 0.32 0.58 0.09 0.58 0.05 0.09 0.04 0.23
Al
2
O
3
22.39 21.96 22.12 22.08 21.17 21.35 21.67 21.59 21.92 22.15 22.07 21.97 21.48
FeO 31.70 31.61 34.11 33.81 33.82 34.71 33.57 35.13 33.41 25.44 25.17 30.71 31.48
MgO 6.76 6.95 4.82 5.10 2.83 2.19 2.04 1.94 2.22 5.73 5.64 4.82 5.01
MnO 0.87 0.76 1.12 1.08 2.13 2.79 2.75 2.27 2.41 1.50 1.95 1.12 1.08
CaO 1.62 1.59 1.29 1.51 1.13 2.05 2.65 1.84 2.41 7.15 6.74 3.31 2.47
Sum 101.24 101.05 101.13 101.08 99.57 100.34 100.28 100.12 100.09 100.12 100.12 99.65 99.55
Cations
Si 5.85 5.91 5.91 5.88 6.16 5.95 5.94 5.99 5.95 6.03 6.01 6.06 6.10
Al 4.08 4.02 4.10 4.09 3.99 4.05 4.10 4.10 4.14 3.99 4.02 4.16 4.09
Fe
2?
3.91 3.94 4.42 4.29 4.53 4.68 4.51 4.74 4.47 3.25 3.25 4.13 4.25
Fe
3?
0.23 0.17 0.07 0.15 \0.1 \0.1 \0.1 \0.1 \0.1 \0.1 \0.1 \0.1 \0.1
Mn 0.11 0.10 0.15 0.14 0.29 0.38 0.37 0.31 0.33 0.19 0.26 0.66 0.68
Mg 1.56 1.61 1.13 1.20 0.68 0.53 0.49 0.46 0.53 1.31 1.30 0.27 0.26
Ca 0.27 0.27 0.22 0.25 0.19 0.35 0.46 0.32 0.41 1.17 1.12 0.57 0.43
Fe
2?/
Mg 2.51 2.45 3.90 3.59 6.70 8.88 9.24 10.19 8.44 2.49 2.50 15.33 16.29
mol %
Almandine 65.91 66.61 74.67 72.93 79.63 78.76 77.39 77.39 77.89 54.90 54.91 73.42 75.59
Pyrope 26.26 27.21 19.12 20.32 11.88 8.86 8.37 8.37 9.22 22.04 21.94 4.79 4.64
Grossular 0.00 0.27 1.89 0.49 5.08 6.41 6.41 6.41 5.69 19.73 18.85 10.02 7.45
Spessartine 1.92 1.69 2.52 2.43 3.41 5.97 7.83 7.83 7.04 3.28 4.31 11.66 12.18
Andradite 5.95 4.21 1.80 3.83 0.06 0.06 0.06 0.16 0.14 0.04 0.04 0.10 0.10
pst pseudotachylyte, u.d.l. under detection limit
* Following Altenberger et al. (2011)
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123

Newly formed garnets are FeO and MgO rich (Table 4).
In the pseudotachylytes, garnets are richer in FeO com-
pared to the wall rocks (Calab 2009, Calab 02, Calab 01,
Calab 04, Dr5). In the migmatites and felsic granulites, the
almandine concentration reaches up to 80 mol %, whereas
in the mafic sample almandine reaches only 55 mol %.
These differences are probably influenced by the host rock
composition. Garnet shows in some parts an outer, some-
times euhedral, rim, generally characterized by a higher Ca
content (Table 4, Fig. 5d).
The newly formed orthopyroxenes show high-alumina
concentrations with up to 8 wt % Al
2
O
3
(Table 5). Their
composition is significantly different compared to the low-
alumina orthopyroxenes of the granulite-facies wall rocks
and clasts.
It seems worth mentioning that biotite shows always
lower TiO
2
concentration in the pseudotachylyte compared
to the wall rocks (Table 6). Generally, TiO
2
is a rough
temperature-sensitive indicator. The temperature-sensitive
anorthite (CaO) concentration of plagioclase is higher in
the pseudotachylytes than in the host rock, pointing to
higher temperatures (Table 6).
Relative timing and PT conditions of frictional
melting during deep crustal faulting
PT conditions of frictional melting during deep crustal
faulting
There are several attempts to define the temperature and
pressure during melt generation. Frictional melting is
described as non-eutectic (e.g. Spray 1992). Therefore, the
melt composition, as displayed by the matrix of the
Table 5 Composition of pyroxenes
Orthopyroxene Clinopyroxene
Calab 03 Calab 04
Host rock Clast in pst Microlites pst Microlites pst
Core* Core Core Rim Core Core Core* Core* Core*
wt %
SiO
2
54.17 53.63 47.27 47.74 48.66 48.84 54.73 55.70 56.00
TiO
2
0.09 0.72 0.72 0.92 0.69 u.d.l. 0.13 u.d.l.
Al
2
O
3
1.58 1.94 7.76 7.84 6.76 5.40 4.73 1.24 2.16
FeO 20.92 19.87 25.79 26.05 24.88 26.29 6.01 5.99 5.23
MnO 0.36 0.53 0.54 0.53 0.60 u.d.l. u.d.l. 0.18
MgO 23.33 23.97 17.51 17.69 16.36 17.44 13.54 15.20 14.87
CaO 0.75 0.33 0.34 0.93 0.56 20.99 21.32 21.02
Na
2
O 0.11 0.02 0.02 0.40 0.11 u.d.l. 0.42 0.41
Cr
2
O
3
u.d.l. 0.01 0.01 u.d.l. u.d.l. u.d.l. u.d.l. u.d.l.
Sum 100.72 99.95 100.95 99.44 99.92 100.00 100.00 100.00
Cations
Si 1.95 1.95 1.81 1.80 1.84 1.86 1.98 2.03 2.03
Ti 0.01 0.00 0.03 0.03 0.03 0.02 0.00 0.00 0.00
Al 0.04 0.08 0.35 0.35 0.30 0.24 0.01 0.05 0.09
Fe
3?
0.03 0.02 0.06 0.12 0.07 0.03 0.01 0.01 0.01
Fe
2?
0.52 0.59 0.69 0.62 0.72 0.80 0.17 0.17 0.15
Mg 1.21 1.30 0.92 0.91 0.92 0.99 0.73 0.82 0.80
Ca 0.03 0.05 0.05 0.04 0.02 0.81 0.83 0.82
Mn 0.01 0.02 0.02 0.02 0.02 0.00 0.00 0.01
Na 0.01 0.06 0.06 0.03 0.01 0.00 0.03 0.03
Sum 3.98 3.98 3.98 3.96 3.96 3.99 3.91 3.95 3.94
Mean pressure (GPa)** 1.2 1.2 1.0 0.9
Minimum pressure (GPa) 1.0 1.0 0.8 0.7
Maximum pressure (GPa) 1.5 1.5 1.3 1.1
pst pseudotachylyte, u.d.l. under detection limit
* Analysed with EDX, ** estimated following Longhi et al. (1993,1999)
Contrib Mineral Petrol
123

pseudotachylyte, differs from the protolith in a way not
computable by equilibrium thermodynamics. A possible
albeit rough procedure for temperature estimation is based
on the composition of consumed or newly formed minerals
(Spray 1992; Austrheim et al. 1996). Very fast non-equi-
librium melting means that individual minerals do not
interact with one another to melt in eutectic reactions but
they melt individually in the order of melting point or
breakdown temperature. The studied Calabria pseudot-
achylytes indicate the complete or partial melting of bio-
tite, orthopyroxene, quartz, garnet, amphibole, feldspars,
sillimanite and opaques.
The melting point, or ‘‘breaking point’’ in particular, for
(OH)-bearing minerals is significantly lower in frictional
melts than in static melting (Spray 1992), caused by
‘‘disequilibrium flash melting’’ of those minerals. Based on
the studies of Spray (1992), the biotite breakdown in
pseudotachylytes is at about 650 °C. The probable highest
temperature for the presented samples is the melting of b-
quartz. Although the melting point of b-quartz is 1,750 °C,
it is well known that rapid melting reduces the melting
point up to about 1,515 °C (Petzold and Hinz 1976). High-
pressure experiments show that fluid activity can reduce
melting point to about 1,100 ° C at pressures of 0.8 GPa
(Kennedy et al. 1962). In addition, the generation of sul-
phide droplets, although rarely occurring, indicates low
O-fugacity and immiscibility of the Fe-sulphide melt with
the silicate melt, suggesting minimum temperatures of ca.
1,200 °C (Magloughlin 2005). The roughly estimated
melting points for clinopyroxene and orthopyroxene under
friction condition are around 1,400 °C (Spray 1992).
Newly formed garnet occurs in nearly all analysed
samples. The grain-size increase in garnet from chilled
margins to the centre of the veins proves the rapid crys-
tallization from a melt (e.g. Austrheim and Boundy 1994,
Altenberger et al. 2011). In some cases, garnet core dis-
plays a possible relict nucleus derived from a clast. In
addition, the generation of sulphide droplets that are typical
for pseudotachylyte melts (Magloughlin 2005) is only
present in the cauliflower-shaped parts of the garnet and is
lacking in the cores. The presence of some straight grain
boundaries in some cauliflower garnets together with their
Table 6 Composition of feldspars and biotite
Plagioclase Alkali feldspar Biotite
Calab 04 Calab 02 Calab 03 Calab 04 Calab 03 Calab 2009** Calab 02
Host rock Pst Pst Pst Pst Pst Pst Host rock Pst Ultramylonite Pst
Core* Core Core* Core Core Core Core* Core Core Core** Core
wt % wt %
SiO
2
56.74 55.26 56.70 62.97 60.96 64.74 65.91 SiO
2
37.71 38.56 37.41 37.29
Al
2
O
3
27.65 26.98 26.34 23.51 23.47 18.74 19.03 TiO
2
4.67 2.05 3.20 2.88
FeO 0.52 0.53 0.50 0.88 0.46 Al
2
O
3
17.59 20.34 19.73 18.75
CaO 10.16 11.16 12.94 5.52 5.13 1.15 1.09 FeO 13.06 15.80 18.20 18.01
Na
2
O 5.45 5.04 3.45 8.00 8.41 1.88 2.19 MgO 14.81 9.96 10.82 12.44
K
2
O 0.17 0.04 0.71 11.66 11.31 MnO 0.00 1.92 u.d.l. u.d.l.
Sum 100.00 99.14 100.00 100.00 99.18 99.05 99.99 Na
2
O 0.08 u.d.l. u.d.l. u.d.l.
Cations K
2
O 9.85 8.86 8.40 7.31
Si 2.54 2.49 2.81 2.74 2.97 2.99 Sum 97.77 97.49 97.77 96.68
Al 1.46 1.44 1.24 1.22 1.01 1.02 Cations
Fe
2?
0.02 0.02 0.03 0.02 Si 2.67 2.76 2.77 2.74
Ca 0.49 0.54 0.26 0.24 0.06 0.05 Ti 0.25 0.11 0.18 0.16
Na 0.47 0.44 0.69 0.72 0.17 0.19 Al 1.47 1.72 1.72 1.63
K 0.01 u.d.l. 0.04 0.68 0.65 Fe
?2
0.41 0.64 1.13 1.11
Mn 0.12 0.00
mol % Mg 1.56 1.06 1.20 1.36
An 50.74 54.50 67.25 27.61 23.69 6.24 5.89 Na 0.01
Ab 49.26 44.52 32.49 72.39 72.25 18.45 21.40 K 0.89 0.81 0.79 0.69
Or 0.00 0.98 0.26 0.00 4.06 75.31 72.71 Fe
3?
0.36 0.30 0.00 0.18
pst pseudotachylyte, u.d.l. under detection limit
* Analysed with EDX, ** following Altenberger et al. 2011
Contrib Mineral Petrol
123

zoning pattern may suggest a later solid state growth
(Fig. 8). The studied garnets are rich in almandine com-
ponent (FeO). Experimental studies (Green and Ringwood
1968; Eggler and Burnham 1973; Allen et al. 1975; Conrad
et al. 1988; Green 1992) as well as analyses of natural
systems (Harangi et al. 2001; Barnes and Allen 2006)
established a lower pressure stability of almandine-rich
garnet in melts of andesitic to dacitic composition. Esti-
mated pressures only weakly correlate with temperature.
These works show that igneous garnets are formed under
minimum conditions of about 0.7–0.8 GPa, assuming
moderate H
2
O activity. The pseudotachylyte matrix, rep-
resenting the former melt, shows an andesitic composition
in most of the garnet-bearing pseudotachylytes. Therefore,
we conclude that the experiments can constrain the pres-
sure of garnet crystallization. In addition, crystallization of
garnets with cauliflower habit has been hitherto described
only in pseudotachylytes formed at greater depths (Austr-
heim and Boundy 1994; Lund and Austrheim 2003; Pitta-
rello et al. 2012).
To further constrain this interpretation, we compared
pseudotachylytes from Calabria with a pseudotachylyte
found in Archean lower crustal gneiss exposed in the
Mather Peninsula, Antarctica (Altenberger, in prep.). The
pseudotachylyte formed in a very similar sillimanite-garnet
protolith in association with brittle structures. This indi-
cates that pseudotachylytes from Antarctica formed when
the gneisses were already exhumed at upper crustal levels.
In addition, no further metamorphic overprint occurs. The
pseudotachylyte vein contains biotite and plagioclase
microlites as well as sulphide droplets. However, in con-
trast with the deep-seated Calabria pseudotachylytes, gar-
net occurs as angular clasts only (Fig. 8a).
According to a very simplified sketch (Fig. 8), the
probable evolution of garnets from analysed Calabrian
pseudotachylytes starts with clasts, comparable to those of
the Antarctica (Fig. 8a), forming the nuclei of the cauli-
flower garnets (Fig. 8b), and ends with the later meta-
morphic overgrowths (Fig. 8c). In the pseudotachylytes
from Antarctica (Fig. 8a), the early fragmentation stage is
observed, whereas in Calabria the complete evolution can
be recognized.
It is well known that the solubility of aluminium in
orthopyroxene increases with increasing pressure. Pyrox-
enes with high aluminium concentration are known from
Proterozoic anorthosites formed near the base of the lower
crust (HAOM; high-alumina orthopyroxene megacrysts,
for example, Emslie 1975; Longhi et al. 1993). Experi-
mental work of Fram and Longhi (1992) and Longhi et al.
(1993, 1999) displays that the alumina content of ortho-
pyroxenes in equilibrium with plagioclase in magmatic
systems is a function of pressure and it has been empiri-
cally calibrated as a geobarometer. The crystallization of
high-alumina orthopyroxenes in the pseudotachylytes of
the metamonzodiorites and mafic granulites (samples Ca-
lab 03 and Calab 04, Table 1) may indicate high pressures,
assuming crystallization from a melt and equilibrium
conditions. The Al-rich orthopyroxenes in the present study
are embedded in plagioclase and formed in garnet-free as
well as in garnet-bearing veins. However, in the latter case,
orthopyroxene is not in contact with garnet. Using the
composition of orthopyroxene for microlites crystallized in
pseudotachylytes from meta-monzodioritic and mafic
hosts, a mean pressure of 1.08 ± 0.23 GPa can be
obtained, pointing to a mean of about 35 km and a mini-
mum crystallization depth of about 31 km (calculated with
a mean crustal density: q = 2.85 g/cm
3
, Table 5). The
presence of almandine-rich garnets in the same pseudot-
achylytes further supports the interpretation of a deep
crustal growth of high-Al orthopyroxene.
Fig. 8 Simplified sketch of the probable evolution of the garnets.
a Cataclastic destruction of garnet of the protolith forming fragments.
b Cauliflower garnets are formed by rapid crystallization from the
melt around clast nuclei. c Later high-grade metamorphic overgrowth
forming euhedral crystals. All images are BSE images. Image A is
made from a pseudotachylyte in a garnet-sillimanite gneiss of the
Mather Peninsula, Antarctica. a–c BSE images
Contrib Mineral Petrol
123

However, the problem of disequilibrium phenomenon
during rapid cooling makes these depth estimates some-
what uncertain. In addition, Xue and Morse (1994) describe
Al-rich orthopyroxene (up to 5.4 wt %) from an anorthosite
in Labrador to be rather the product of rapid crystallization
than of the depth of origin. It should be noted also that
Toyoshima (1990) describes upper crustal pseudotachy-
lytes containing orthopyroxene microlites with high-alu-
mina content. Therefore, pressure estimates based on the
aluminium content in orthopyroxene, although consistent
with direct garnet crystallization from the pseudotachylyte
melt, should be taken with some caution. However, the
coexistence of Al-rich orthopyroxene and plagioclase
indicates that plagioclase crystallization, as the main alu-
minium-consuming phase, was not delayed during rapid
crystallization, as described by Longhi et al. (1993). In
addition, the Al-poor orthopyroxenes described by Lund
and Austrheim (2003) in high-pressure pseudotachylytes
from mafic protholith are formed by metamorphic reactions
after olivine clasts and within the matrix.
According to the nearly anhydrous mineral assemblage
of the pseudotachylyte formed within a mafic protolith, we
could indicate that seismic events led to dehydration in a
water-deficient environment. The presence of newly
formed biotite in the felsic pseudotachylytes may be
derived from the water extracted by the melting of the
biotite in the host rock.
Relative timing of deep crustal pseudotachylyte
generation
The studied pseudotachylytes are associated with ultra-
mylonites or at least to thin mylonite zones (Fig. 4a). The
relationship between ductile deformation and pseudot-
achylyte generation is complex, since in our samples there
is evidence that frictional melt formed between two phases
of plastic creep. The second phase of ductile deformation is
indicated by the preferred orientation of biotite microlites
and clasts, discordant to the vein margin in a pseudot-
achylyte (Fig. 7d,f).
The wall rock of the mafic pseudotachylytes is charac-
terized by mylonites defined by grain-size reduction via
dynamically recrystallization of plagioclase and clinopy-
roxene (Fig. 5e,f). In addition, in the pseudotachylyte,
fragments of clasts very similar to these deformed rocks are
present. This suggests that localized dynamic recrystalli-
zation must have happened before the seismic event and
formation of the frictional melt.
Another strong argument is the s-c-like deformation
pattern in a quartz-rich rock clast (Fig. 4d). In pseudot-
achylyte-filled open space along a c plane, a sigma clast of
plastically deformed quartz occurs (Fig. 3c,e). This sigma
clast is disrupted in a fine-grained biotite-rich matrix
representing former melt. These structures show clearly the
pre-seismic ductile deformation of the rock and the sub-
sequent brittle disruption, connected with a seismic event.
In addition, fine-grained recrystallized quartz grains occur
directly in contact with the ‘‘melt-filled s-c planes’’
(Fig. 4d).
In other samples, we can document sigmoidal fabrics or
s-c-like ultramylonites with very thin pseudotachylytes
preferentially located along or with small angle to the c
planes (Fig. 4a). However, the sigmoidal shape of quartz
derives from a ductile process and therefore the bending is
not due to a seismic event. The seismic-induced displace-
ment uses the previously formed c planes, whereas the melt
injection occurs also along s planes. The same samples
show also some deformed lens-shaped quartz grains in the
pseudotachylyte; they resemble in size and shape the
deformed quartz grains in the zones of ductile deformation
near the pseudotachylyte.
The very fine recrystallized quartz grains (approxi-
mately 5–10 lm, second generation; recrystallization by
bulging) in clasts within the pseudotachylytes and in the
wall rocks nearby point to high differential stress near the
pseudotachylyte veins. Using classical palaeo-piezometers
(Twiss 1977; Christie and Ord 1980), this would indicate a
high differential stress between 0.2 and 0.3 GPa. The
experimental and empirical work of Stipp and Tullis (2003)
and Stipp et al. (2010), based on different recrystallization
mechanisms indicates that this method gives reliable
results for the observed range of grain size.
In the case of the mafic sample (Calab 04 and CM73,
Table
1), the pseudotachylyte follows the thin zone of
dynamic recrystallization of plagioclase and clinopyroxene
and therefore a high-temperature ductile shear zone
(Fig. 5e,f). Here, the formation of the pseudotachylyte can
be interpreted either as following an inherited fabric
anisotropy or as the continuous evolution of the shear zone-
forming process. In contrast, other pseudotachylytes show
unambiguous evidence of oblique foliations, pointing to
ductile deformation after pseudotachylyte formation
(Fig. 7d,f).
Discussion and conclusions
The analysis of pseudotachylytes in different lithologies of
the Calabria high-grade metamorphic rocks reveals new
evidence on the depth of the fossil palaeo-seismic zone and
indications on the presence of fossil deep-seated seismic
events.
The matrix of the pseudotachylytes indicates melts of
different compositions: some biotite-rich matrix layers or
‘‘pressure shadows’’ indicate melts of near biotite compo-
sition (Fig. 4f, 6a). Others with cloudy or drop-like
Contrib Mineral Petrol
123

domains show different evidence of unmixing (drops of
mingling) of frictional melts composition (Fig. 6b). The
existence of melts of different composition in the same
vein indicates the stepwise, non-equilibrium conditions of
frictional melting, as supposed by Spray (1992).
Our results provide new arguments for the formation
depth of the pseudotachylytes. Microstructures, together
with occurrence and composition of new minerals, indicate
that the studied pseudotachylytes formed at deeper crustal
levels (C0.85 GPa). This is also consistent with the pres-
sure estimates of 0.75-0.9 GPa previously obtained from
the mylonites located at the footwall of the Curinga-Gi-
rifalco line (Langone et al. 2006). Even higher pressure
estimates of about 1.1 GPa have been obtained also in
garnet-bearing metagabbros by Acquafredda et al. (2008).
They interpreted this pressure as reflecting peak conditions
for the granulite-facies rocks in the Calabrian lower crust.
The minimum formation depth of the studied pseudot-
achylytes is constrained by the growth of almandine garnet
in pseudotachylytes with andesitic (matrix) composition
([0.7–0.8 GPa) and the formation of Al-bearing orthopy-
roxene (C0.85 GPa). This suggests melt generation at a
minimum depth of about 31 km. Assuming a nearby hyp-
ocentre, these values are similar to earthquake foci in
active orogens like the Andes. Beneath the present back arc
basin (Western Sierra Pampeanas, Argentina), continental
crust above a flat-lying subduction slab is highly seismi-
cally active (Alvarado et al. 2005). Although hypocentres
along a nearly flat subduction are common, most of the
recent earthquakes have their foci at about 20 km of depth,
that is, in the middle to lower crust (Alvarado et al. 2005).
These earthquakes are due to reverse and thrust faults,
related to the interaction with the flat subduction. Closer to
the Andean mountain range, few earthquakes, resulting
from displacements at older terrane boundaries, happened
at around 26 km depth (Alvarado et al. 2005).
Summarizing, it can be concluded that pseudotachylyte
formation is timely framed between episodes of high-
temperature plastic deformation. This suggests intermittent
brittle deformation within an overall ductile environment.
However, some differences arise when different protoliths
are considered.
In mafic granulites, strong localization of deformation is
observed (Fig. 5e,f), since pseudotachylyte-bearing faults
occur only in millimetre-thick ductile shear zones defined by
dynamically recrystallized plagioclase and clinopyroxene.
These were able to drive fault nucleation and propagation.
Ductile to brittle processes took place in a water-deficient
environment, as shown by the absence of alteration in the
wall rock and by the formation and presence of anhydrous
microlites in the pseudotachylyte matrix. This feature is
particularly significant since an anhydrous assemblage has
been observed also when the wall rocks contain hydrous
phases, such as amphibole. In these cases, frictional heating
led to dehydration of the seismic plane.
A contrasting behaviour is observed in biotite rich
migmatitic paragneisses, where deformation is partitioned
in a thick horizon of pseudotachylytes, mylonites and ult-
ramylonites. In these rocks, pseudotachylyte veins are
almost parallel or at a very low angle to the foliation planes
of the ultramylonites or the c planes of s-c fabrics. In
addition, melt intruded into the s planes. Failure propaga-
tion along pre-existing anisotropies may have been assisted
by the presence of fluids (the so-called Terzaghi effect;
Terzaghi 1943; Hubbert and Ruby 1959), as described for
eclogite-facies pseudotachylytes by Lund and Austrheim
(2003). Fluids may originate from the breakdown of biotite
or may have been supplied externally. In addition, the
studied pseudotachylytes are preferentially located along
pre-existing fine-grained bands formed by ductile defor-
mation and dynamic recrystallization. Strain localization
on sites of smaller grain size is a common mechanism
(Altenberger 1997).
Interestingly, in s–c structures pervaded by pseudot-
achylyte melt, indicators of deformation occurring under
increasing strain rates can be documented. This is nicely
illustrated by the fragmentation of a stretched sigma clast
made up of recrystallized quartz within a foliated pseu-
dotachylyte (Fig. 4f). A continuous ductile flow, with
intermittent high-strain rate episodes, leading to seismic
events and related melting, is the most likely process able
to explain the observed microstructures.
Acknowledgments We would like to thank B. Fabian for her tire-
less and instructive help in graphics, C. Fischer for excellent thin
section preparation, A. Laurita for the assistance at the SEM in Pot-
enza and R. Oberha
¨
nsli for the easy access to the institute’s labs; H.
Austrheim, V. Toy and G. Pennacchioni for their constructive criti-
cism and discussions. We would like to express our gratitude to R.
Weinberg and anonymous referees for their very detailed, construc-
tive and fruitful reviews. The OSXStereonet software by N. Cardozo
has been used for plotting structural data. Funding by PRIN project
(year 2007) is acknowledged.
References
Acquafredda P, Fornelli A, Piccarreta G, Pascazio A (2008) Multi-
stage dehydration–decompression in the metagabbros from the
lower crustal rocks of the Serre (southern Calabria Italy). Geol
Mag 145:397–411
Allen JC, Boettcher AL, Marland G (1975) Amphiboles in andesite
and basalt: I Stability as a function of P-T-f
o2
. Am Mineral
60:1069–1085
Altenberger U (1997) Strain localization mechanisms in deep seated
layered rocks. Geol Rdsch 86:56–68
Altenberger U, Kruhl JH (2000) The long life of a thin and dry high-
temperature shear zone in the Hercynian lower crust of Calabria
(S.Italy). Period Mineral 69:125–142
Altenberger U, Prosser G, Ruggiero M, Gu
¨
nther C (2011) Garnet-
bearing pseudotachylytes of Calabria-a petrological link to lower
Contrib Mineral Petrol
123

crustal hypocenters. In: Fagereng A, Toy VG, Rowland JV (eds)
Geology of the earthquake source: a volume in honour of rick
Sibson. Geol Soc London, Special Publications 369: 153–168
Alvarado P, Beck S, Zandt G, Araujo M, Triep E (2005) Crustal
deformation in the south-central Andes backarc terranes as
viewed from regional broad-band seismic waveform modeling.
Geophys J 163:580–598
Amodio-Morelli L, Bonardi G, Colonna V, Dietrich D, Giunta G,
Ippolito F, Liguori V, Lorenzoni S, Paglionico A, Perrone V,
Piccarreta G, Russo M, Scandone P, Zanettin-Lorenzoni E,
Zuppetta A (1976) L’arco Calabro-Peloritano nell’orogene
Appenninico-Maghrebide. Mem Socı
`
Geol Ital 17:1–60
Austrheim H, Andersen TB (2004) Pseudotachylytes from Corsica:
fossil earthquakes from a subduction complex. Terra Nova
16:193–197
Austrheim H, Boundy TM (1994) Pseudotachylytes generated during
seismic faulting and eclogitization of deep crust. Sci 265:82–83
Austrheim H, Erambert M, Engvik AK (1996) Garnets recording deep
crustal earthquakes. Earth Planet Sci Lett 139:223–238
Barnes CG, Allen CM (2006) Depth of origin of late Middle Jurassic
garnet andesite southern Klamath Mountains California. Geol
Soc Am Spec Papers 410:269–286
Berthe
´
D, Choukroune R, Jegouzo R (1979) Orthogneiss, mylonite
and non coaxial deformation of granites: the example of the
South American shear zone. J Struct Geol 1:31–42
Bonardi G, Cavazza W, Perrone V, Rossi S (2001) Calabria-Peloritani
terrane and northern Ionian sea. In: Martini IP, Vai GB (eds)
Anatomy of an Orogen. The apennines and adjacent mediterranean
basins. Kluwer Academic Publishers, Dordrecht, pp 287–306
Christie JM, Ord A (1980) Flow stress from microstructures of
mylonites: example and current assessment. J Geophys Res
85:6253–6262
Clarke GL, Norman AR (1993) Generation of pseudotachylite under
granulite facies conditions and its preservation during cooling.
J Metam Geol 11:319–335
Conrad WK, Nicholls IA, Wall VJ (1988) Water-saturated and
–undersaturated melting of metaluminous and peraluminous
crustal compositions at 10 kb: evidence for the origin of silicic
magmas in the Taupo volcanic Zone New Zealand and other
occurrences. J Petrol 29:765–803
Eggler DH, Burnham CW (1973) Crystallization and fractionation
trends in the system andesite-H20-CO2-02 at pressures to 10 kb.
Geol Soc Am Bull 84:2517–2532
Emslie RF (1975) Pyroxene megacrysts from anorthositic rocks: new
clues to the sources and evolution of the parent magmas. Can
Mineral 13:138–145
Fagereng A (2010) Melange rheology and seismic style. Geology
38:751–754
Fram M, Longhi J (1992) Phase equilibria of dikes associated with
proterozoic anorthosite complexes. Am Mineral 77:605–616
Graessner T, Schenk V, Bro
¨
cker M, Mezger K (2000) Geochrono-
logical constraints on the timing of granitoid magmatism
metamorphism and post metamorphic cooling in the Hercynian
crustal cross-section of Calabria. J Metam Geol 18:409–421
Grande A, Di Vincenzo GF, Prosser G, Caggianelli A (2009) Direct
evidence of Middle Oligocene extension in the Calabria–
Peloritani terrane from co-seismic faulting: the pseudotachy-
lyte-bearing shear zones of Palmi (southern Calabria Italy). Terra
Nova 21:293–303
Green TH (1992) Experimental phase equilibrium studies of garnet-
bearing I-type volcanics and high-level intrusives from North-
land New Zealand. Trans R Soc Edinb Earth Sci 83:429–438
Green TH, Ringwood AE (1968) Origin of the garnet phenocrysts in
calc549 alkaline rocks. Contrib Mineral Petrol 65:59–67
Handy MR, Brun JP (2004) Seismicity structure and strength of the
continental lithosphere. Earth Planet Sci Lett 223:427–441
Harangi SZ, Downes H, Ko
´
sa L, Szabo
´
CS, Thirlwall MF, Mason
PRD, Mattey D (2001) Almandine garnet in calc-alkaline
volcanic rocks of the Northern Pannonian Basin (Eastern-Central
Europe): geochemistry petrogenesis and geodynamic implica-
tions. J Petrol 42:1813–1843
Hetzel R, Altenberger U, Strecker MR (1996) Structural and chemical
evolution of pseudotachylytes during seismic events. Mineral
Petrol 58:33–50
Heymes T, Monie
`
T, Arnaud N, Pecher A, Bouillin JP, Compagnoni
R (2010) Alpine tectonics in the Calabrian-Peloritan belt
(southern Italy): new 40Ar/39Ar data in the Aspromonte Massif
area. Lithos 114:451–472
Hobbs BE, Ord A, Tyssier C (1986) Earthquakes in the ductile
regime. Pure Appl Geophys 124(1–2):309–336
Hubbert MK, Ruby WW (1959) The role of fluid pressure in
mechanics of mechanics of overthrust faulting. Geol Soc Am
Bull 70:115–166
Jackson J (2002) Strength of the continental lithosphere: time to
abandon the jelly sandwich? GSA TODAY: 4–9
Kennedy GC, Wasserburg GJ, Heard HC, Newton RC (1962) The
upper three-phase region in the system SiO2-H2O. Am J Sci
260:501–521
Kim JW, Ree JH, Han R, Shimamoto T (2010) Experimental evidence
for the simultaneous formation of pseudotachylyte and mylonite
in the brittle regime. Geology 38:1143–1146
Kretz R (1983) Symbols for rock-forming minerals. Am Mineral
68:277–279
Kruhl JH, Huntemann T (1991) The structural state of the former
lower continental crust in Calabria (S.Italy). Geol Rdsch
80:289–302
Langone A, Gueguen E, Prosser G, Caggianelli A, Rottura A (2006)
The Curinga-Girifalco fault zone (northern Serre Calabria) and
its significance within the Alpine tectonic evolution of the
western Mediterranean. J Geodyn 42:140–158
Langone A, Godard G, Prosser G, Caggianelli A, Rottura A, Tiepolo
M (2010) P-T–t path of the Hercynian low-pressure rocks from
the Mandatoriccio complex (Sila Massif Calabria Italy): new
insights for crustal evolution. J Metam Geol 28:137–162
Lin A (2008a) Fossil earthquakes: formation and preservation of
pseudotachylytes. Springer, Berlin
Lin A (2008b) Seismic Slip in the Lower Crust inferred from
granulite-related pseudotachylyte in the Woodroffe Thrust
Central Australia. Pure Appl Geophys 165:215–233
Lin A, Sun Z, Yang Z (2003) Multiple generations of pseudotachylyte in
the brittle to ductile regimes Qinling-Dabie Shan ultrahigh-pressure
metamorphic complex central China. Island Arc 12:423–435
Lin A, Maruyama T, Stallard A, Michibayashi K, Camacho A, Kano
K (2005) Propagation of seismic slip from brittle to ductile
regimes: evidence from the pseudotachylyte of Woodroffe thrust
central Australia. Tectonophys 402:21–35
Longhi J, Fram MS, Vander Auwera J, Montieth JN (1993) Pressure
effects kinetics and rheology of anorthositic and related magmas.
Am Mineral 78:1016–1030
Longhi J, Vander Auwera J, Fram MS, Duchesne JC (1999) Some
phase equilibrium constraints on the origin of Proterozoic
(massif) anorthosites and related rocks. J Petrol 40:339–362
Lund MG, Austrheim H (2003) High-pressure metamorphism and
deep-crustal seismicity: evidence from contemporaneous forma-
tion of pseudotachylytes and eclogite facies coronas. Tectono-
phys 372:59–83
Magloughlin JF (2005) Immiscible sulfide droplets in pseudotachy-
lyte: evidence for high temperature ([1,200°C) melts. Tectono-
phys 402:81–91
Moecher DP, Steltenpohl MG (2009) Direct calculation of rupture
depth for an exhumed paleoseismogenic fault from mylonitic
pseudotachylyte. Geology 37:999–1002
Contrib Mineral Petrol
123

Moresi M, Paglionico A, Piccarreta G, Rottura A (1979) The deep
crust in Calabria (Polia Copanello Unit): a comparison with the
Ivrea-Verbano zone. Mem Soc Geol Padova 33:233–242
Paglionico A, Piccarreta G (1976) Le Unita
`
del Fiume Pomo e di
Castagna nelle Serre Settentrionali (Calabria). Bol Soc Geol Ital
95:27–37
Passchier CW (1982) Pseudotachylyte and the development of
ultramylonite bands in the Saint Barthe
´
lemy Massif French
Pyrenees. J Struct Geol 4:69–79
Passchier CW, Trouw RAJ (1996) Microtectonics. Springer, Berlin
290p
Pennacchioni G, Cesare B (1997) Ductile-brittle transition in pre-
Alpine amphibolite facies mylonites during evolution from
water-present to water-deficient conditions (Mont Mary Nappe
Italian Western Alps). J Metam Geol 15:777–791
Petzold A, Hinz W (1976) Silikatchemie. VEB Verlag, Leipzig 222p
Pittarello L, Pennacchioni G, Di Toro G (2010) Deep-seated
pseudotachylytes from the Ivrea Zone metagabbros (Southern
Alps Italy). Trab Geol 30:134–139
Pittarello L, Pennacchioni G, Di Toro G (2012) Amphibolite-facies
pseudotachy-lytes in Premosello metagabbros and felsic mylo-
nites (Ivrea Zone Italy). Tectonophys. doi:10.1016/j.tecto.
(2012)08.001
Rosenbaum G, Lister GS (2004) Neogene and Quaternary rollback
evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian
Maghrebides. Tectonics. doi:10.1029/2003TC001518
Rossetti F, Faccenna C, Goffe
´
B, Monie
´
P, Argentieri A, Funiciello R,
Mattei M (2001) Alpine structural and metamorphic signature
of the Sila Piccola Massif nappe stack (Calabria Italy): insights
for the tectonic evolution of the Calabrian Arc. Tectonics
20:112–133
Schenk V (1980) U-Pb and Radiometric dates and their correlation
with metamorphic events in the granulite-facies basement of the
Serre, southern Calabria (Italy). Contr Min Petrol 73:23–38
Schenk V (1984) Petrology of felsic Granulites metapelites metaba-
sics ultramafics and metacarbonates from Southern Calabria
(Italy): prograde metamorphism, uplift and cooling of a former
lower crust. J Petrol 25:255–296
Schenk V (1990) The exposed crustal cross section of southern
Calabria Italy: structure and evolution of a segment of Hercynian
crust. In: Salisbury MH, Fountain DM (eds) Exposed cross-
sections of the continental crust. Kluwer, Dordrecht, pp 21–42
Sibson RH (1980) Transient discontinuities in ductile shear zones.
J Struct Geol 2:165–171
Sibson RH, Toy VG (2006) The habitat of fault-generated pseudot-
achylyte: presence vs. absence of friction-melt. AGU Geophys
Mono 170:153–166
Spiegel C (2003) Mylonitization, dry shearing and ex-humation of the
lower crust: the Curinga Girifalco Line (Calabria South Italy).
In: Meschede M and Kuhlemann J (eds) Plate margin and
orogen: comparative studies. N Jb Geol Pala
¨
ont 250: 359–390
Spray JG (1992) A physical basis for the frictional melting of some
rock-forming minerals. Tectonophys 204:205–221
Stipp M, Tullis J (2003) The recrystallized grain size piezometer for
quartz. Geophys Res Lett. doi:10.1029/2003GL018444
Stipp M, Tullis J, Scherwath M, Behrmann JH (2010) A new
perspective on paleo-piezometry: dynamically recrystallized grain
size distributions indicate mechanism changes. Geol 38:759–762
Takagi H, Goto K, Shigematsu N (2000) Ultramylonite bands derived
from cataclasite and pseudotachylyte in granites northeast Japan.
J Struct Geol 22:1325–1339
Techmer KS, Ahrendt H, Weber K (1992) The development of
pseudotachylyte in the Ivrea-Verbano zone of the Italian Alps.
Tectonophys 204:307–322
Terzaghi K (1943) Theoretical soil mechanics. Wiley, New York
Thomson SN (1994) Fission track analysis of the crystalline basement
rocks of the Calabrian Arc southern Italy: evidence of Oligo-
Miocene late-orogenic extension and erosion. Tectonophys 238:
331–352
Thomson SN (1998) Assessing the nature of tectonic contacts using
fission-track thermochronology: an example from the Calabrian
Arc southern Italy. Terra Nova 10:32–36
Toyoshima T (1990) Pseudotachylite from the Main Zone of the
Hidaka metamorphic belt Hokkaido northern Japan. J Metam
Geol 8:507–523
Twiss RJ (1977) Theory and applicability of a recrystallized grain
size paleopiezometer. Pure Appl Geophys 115:227–244
White JC (1996) Transient discontinuities revisited: pseudotachylyte
plastic instability and the influence of low pore fluid pressure on
deformation processes in the mid-crust. J Struct Geol 18:
1471–1486
Xue S, Morse SA (1994) Chemical characteristics of plagioclase and
pyroxene megacrysts and their significance to the petrogenesis of
the Nain Anorthosites. Geochim Cosmochim Acta 58:4317–4331
Zechmeister MS, Ferree
´
EC, Cosca MA, Geissman JW (2007) Slow
and fast deformation in the Dora Maira Massif Italian Alps:
pseudotachylytes and inferences on exhumation history. J Struct
Geol 29:1114–1130
Contrib Mineral Petrol
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