SEDSolid Earth DiscussionsSEDSolid Earth Discuss.1869-9537Copernicus GmbHGöttingen, Germany10.5194/sed-7-909-2015Polyphase evolution of a crustal-scale shear zone during
progressive exhumation from ductile to brittle behaviour: a case
study from Calabria, ItalyFazioE.efazio@unict.ithttps://orcid.org/0000-0003-1802-8207OrtolanoG.https://orcid.org/0000-0001-5802-6922CirrincioneR.PezzinoA.VisalliR.Università degli Studi di Catania, Dipartimento di Scienze
Biologiche, Geologiche e Ambientali, Corso Italia 57, 95129 Catania,
ItalyE. Fazio (efazio@unict.it)6March20157190995530December20146February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/preprints/7/909/2015/sed-7-909-2015.htmlThe full text article is available as a PDF file from https://se.copernicus.org/preprints/7/909/2015/sed-7-909-2015.pdf
Mylonitic rocks involved within a polyphase crustal-scale shear
zone, cropping out in the Aspromonte Massif (Calabria, Italy), has
been investigated to reveal the meso- and micro-structural evolution
(from ductile- to brittle-type deformation) occurred during
exhumation trajectory. A relatively small area (about
4 km2) has been selected in the central-eastern part of
the massif to constrain the sequence of the structural features from
the earliest ones (Hercynian in age), almost totally obliterated by
a pervasive mylonitic foliation (plastic regime), up to recent ones,
consisting of various sets of veins typical of semibrittle to
brittle regime. The former ductile evolution was followed by
a compressive thin-skinned thrusting stage developed during the
Apennine phase of the Alpine Orogeny, interested by a second brittle
stage, consistent with the switching from compressive to extensional
tectonics. This last stage accompanied the final exhumation process
causing the activation of regional scale normal faults, which partly
disarticulated previous mylonitic microstructures. A suite of
oriented specimens were collected and analyzed to complete the
deformational history already recognized in the field. Quartz c axis
orientation patterns confirm the greenschist facies conditions of
the former ductile exhumation stage with a dominant top-to-NE sense
of shear. Microstructural investigations highlighted the progressive
development from plastic- to brittle-type structures, allowing to
constrain each step of the multistage exhumation history, and to
establish the relative timing of the stress field variation causing
thrusting and subsequent normal faulting. Obtained results support
a continue compressional exhumation of this sector since the opening
of Tyrrhenian basin (10 Ma).
Introduction
Crustal-scale shear zones are commonly rooted in the middle to lower
crust. They evolve over a considerable period of time exhibiting
a complex variety of fault-rocks microstructures, thus potentially
becoming a good tool to unraveling the changes in the deformation
mechanisms during progressive deformation often associated to
exhumation (Ramsay, 1980). For this reason, it is common to observe
a sequential evolution of the deformation passing from crystal-plastic
microstructures to brittle fracturing (Simpson, 1986). The
reconstruction of this sequence can allow to unravel the exhumation
history of originally deep seated crystalline basement rocks
constraining rheological behavior, variation in the stress-field
orientation as well as PT conditions of shearing activity.
In this view is inserted the present work that would to be analyze the
rheology, the kinematics and the PT conditions of the different
evolutionary steps of the Montalto Shear Zone (MSZ) (Cirrincione
et al., 2009). MSZ is indeed a beautiful example of a continuum of
deformation from ductile- to brittle-type with a lot of transitional
stages located in the central-eastern part of the Aspromonte
Massif. This massif (Cirrincione et a., 2013), together with the
Peloritani Mountains Belt (Cirrincione et al., 2012) represent the
most shortened southernmost segment of the Calabrian Peloritani Orogen
(CPO) (Pezzino et al., 2008). The evolution of this orogenic segment
occupies a key role in the kinematics of the entire orogen, unraveling
the Oligocene-Miocene opening of Mediterranean back-arc basins, which
developed within an overall collisional tectonic setting. In this
scenario several tectonic models were proposed (e.g. Rosenbaum et al.,
2002 and reference therein), which are mostly associated to the
syn-orogenic retreat of the subducting plate, justifying the observed
drifting phenomena of the present-day peri-Mediterranean thrust
belts. These models attempt to explain the exhumation mechanisms, such
as syn-convergent extension (Heymes et al., 2010) or extrusion by flow
channeling (Pezzino et al., 2008; Cirrincione et al., 2008a), which
operated during the uplifting process, involving the southern CPO
crystalline basement rocks.
Within these contrasting views our work aims to be deep the evolution
of the structural features from ductile- to brittle-types behaviors,
analyzing in detail the relationships between meso- and
micro-structural features of the MSZ. The study of the sequential
evolution of the MSZ can contribute indeed to constrain the kinematics
and the stress-field orientation variation, occurring during the
different stages of the exhumation trajectory of the mylonitic rocks
involved in the shear zone, eventually verifying the stepwise changing
of the regime from compressional to extensional one (e.g. normal
faults).
To this aim, recent detailed field investigations (Fazio et al., 2015;
Ortolano et. al., 2015), allowing to realize two 1:20 000 scale
geological maps of the Aspromonte Massif nappes-like edifice,
permitted to select a relatively small area of about 4 km2,
where a continuum passage from the early deep-seated metamorphic
evolution, up to the sequential transitional and final brittle
evolution of the MSZ has to be pointed out.
A complete sequence of structural features recognized during
a detailed structural field analysis together with a microtectonic
study of mylonitic samples collected from the same outcrop permitted
the tectonic history of the area to be reconstructed.
Quartz c axis pattern have also been investigated in order to infer
the sense of shear as well as thermal conditions of the deep seated
shearing zone activity, controlled by dominant deformation mechanisms
causing the progressive crystal lattice strain by the activation of
multiple slip-systems (e.g. Stipp et al., 2002; Heilbronner and
Tullis, 2006; Menegon et al., 2008). Moreover paleopiezometry based on
quartz grainsize allowed us to infer differential stress as well as to
estimate possible strain rate attained for the studied shear zone. The
study of the ductile-related microstructure, has been completed by the
detailed analysis of the semi-brittle and brittle deformation, here
consisting of a series of fracturing events, producing different sets
of veins, that contribute to reveal the complex tectonic history
(e.g. Woodcock et al., 2006) as in the case here reported.
Geological setting
Study area is inserted within the southern sector of the Calabrian
Peloritani Orogen (CPO, Pezzino et al., 2008). This is a ribbon-like
continental microterrane located in the central Mediterranean realm as
the result of the Alpine-Apennine amalgamation of different
microplates disarticulated from the original southern European
Hercynian chain (Stampfli and Borel, 2002; von Raumer, 2002,
Fig. 1a). According to Angì et al. (2010) and Ortolano
et al. (2013), this evolution has brought indeed to the present-day
lateral juxtaposition of different Hercynian microplate remnants,
characterized by a different tectono-metamorphic evolution, locally
involved by a strongly to weakly Alpine metamorphic overprint. The
whole orogen can be subdivided into three main sectors, from north to
the south: the Sila Massif; the Serre Massif; the Aspromonte Massif
and Peloritani Mountains (Fig. 1b). The study area is located within
the Aspromonte Massif, a nappes-like structure composed of three
crystalline basement units cropped out in the southern sector of the
CPO (Fig. 1c). These units, which have experienced a poly-orogenic
multistage history extended from Hercynian Orogeny up to the Apennine
stage of Alpine one (Ortolano et al., 2015), are distinguished from
top to the bottom in the Stilo (SU), Aspromonte-Peloritani (APU) and
Madonna di Polsi (MPU) Units (Cirrincione et al., 2008b; Fazio et al.,
2008; Pezzino et al., 2008). The uppermost unit consists of
low-greenschist to low-amphibolite facies Paleozoic metapelites
unaffected by Alpine metamorphic re-equilibration and formed during
Hercynian metamorphic cycle (Crisci et al., 1982; Bonardi et al.,
1984; Fazio et al., 2012, 2015). It has been superimposed on the
underlying APU through a brittle tectonic contact developed during the
last stages of Alpine orogenic cycle responsible for the stacking of
originally separated crystalline basement domains. APU is made up of
amphibolite facies Paleozoic rocks intruded by late- to post-Hercynian
granitoid bodies (Fiannacca et al., 2008) locally reworked by
Alpine-type metamorphism developed at about 25–30 Ma (Bonardi
et al., 1987). At the same time, a thick mylonitic horizon, marking
the contact between APU and the lowermost MPU and the beginning of
their shared tectono-metamorphic history, has developed (Ortolano
et al., 2005; Cirrincione et al., 2009). MPU crops out in three main
tectonic windows, namely the Cardeto window (Fazio et al., 2008, 2009,
2010), the Madonna di Polsi window (Pezzino et al., 1990) and the
Samo-Africo window (Ortolano et al., 2005). It is constituted by
lower-greenschist to lower-amphibolite facies
metapelitic-metapsammitic sequences characterized by a polyphase
Alpine-type metamorphism which has not been detected into the
overlying APU. According to Cirrincione et al. (2008b), this is
supported by pre-mylonitic relics found both in the APU and MPU
suggesting distinct tectono-metamorphic evolutions. The first ones
indicate a clockwise polyphase evolution at relatively HT/LP regime,
ascribable to the entire Hercynian metamorphic cycle, whereas the
second ones, seem to be affected by an early Alpine metamorphism at
HP/LT peak conditions. The evolution of the overlapping tectonics
along a regional detachment shear-zone, replaced the original
mylonitic fabrics, causing the shortening of the chain and the folding
of the original main shear-plane. This last structural evolution
further evolved in the more superficial conditions with the activation
of thrusting structures, fractally distributed at different scales
with formation of conjugate low-angle fractures (Ortolano et al.,
2015). The more recent evolution of CPO is characterized by the
activation of NE–SW brittle extensional fault system in response to
a change from the collisional processes to an extensional tectonics,
which took place in the upper Miocene (Monaco et al., 1996). In this
time occurs the Tyrrhenian basin opening linked with the coexistence
of extensional and compressional phenomena responsible for the
structuring of the whole Apennine-Maghrebian Orogen (Finetti and Del
Ben, 1986). Simultaneously, the NW subduction of the Ionian oceanic
crust through a slab sinking mechanism took place (Malinverno and
Ryan, 1986) accompanied by the formation of a regional strike-slip
tectonics averagely oriented NW–SE, known as South Tyrrhenian System
(Finetti et al., 1996; Lentini et al., 2002; Guarnieri et al., 2002),
locally still active in the central and eastern part of the
CPO. Finally, the last stages of the CPO structural evolution are
marked by a currently active extensional Quaternary tectonics which
resulted in an extensive regional uplift and in the sequential
activation of seismogenic normal fault systems. The latter extend from
the current Calabrian Tyrrhenian edge up to the Hyblean Plateau
crossing the Strait of Messina and forming the so-called
Siculo-Calabrian Rift Zone (SCRZ) (Monaco and Tortorici, 1995, 2000;
Tortorici et al., 1995). In the study
area, the different fault segments of the SCRZ system extend up to
several hundred of kilometers and are characterized by a prevalent
NNE–SSW trend typically observed also in the north-western Sicily and
in the Hyblean Plateau with the development of evident triangular and
trapezoidal facets (Ortolano et al., 2015).
Meso-structural analysis
A detailed field investigation from a selected area of about
4 km2 have been carried out in order to constrain the
temporal sequence and styles of the meso-structural features. During
this work, a total of 1696 structural measurements from 48 stations
(Table 1) were collected permitting to realize a detailed geological
map (Fig. 2). Successively, these data were processed, producing
stereoplots representative for the sequence of the recognized
meso-structural features subdivided per unit by means of OpenStereo
software (Grohmann and Campanha, 2010). To this aim, contouring was
used to plot main foliation, fold axis and lineation data set, whereas
rose diagrams were preferred to plot fractures. Abbreviations used in
the text are: Dn for the deformation phases, Mn for the
metamorphic episodes; Sn for schistosities, Bn for fold
axis, Ln for lineations, where the n index indicates the
relative chronology of the event. H and A subscripts are related to
Hercynian and Alpine orogenic cycle respectively,
Stilo unit (SU)
The basement rocks of SU cropping out in the study area are prevalently
fine-grained phyllites often showing quartz lenses and rods structure
elongated parallel to the main foliation (Fig. 3a), connected to an isoclinal
folding event and interrupted by low angle fracture systems. Locally,
a subsequently crenulation cleavage, highlighting by the presence of
micro-fold axial culmination lineation, was observed. Structural
investigations from 4 stations located in the south-western part of the
geological map (Fig. 2) suggest an exclusively syn-metamorphic deformation,
ascribable to the early Hercynian metamorphic evolutionary stages. These last
can be subdivided in two stages: (i) a first isoclinal folding event
(D1E – Fig. 4a) producing an ENE–WSW axial plane schistosity
(S1E – Fig. 4), which constitutes the main foliation; and (ii)
a second deformational stage (D2E – Fig. 4a) responsible for
a sub-mm-scale S1E crenulation producing the formation of a new
schistosity (S2E – Fig. 4a). The latter, is accompanied by the
development of axial microfolds culmination approximately oriented E–W
(B2E – Fig. 4b). Moreover, during the early Apennine evolution
(D5A – Fig. 4a) it was involved in cataclastic shear zone
processes linked to the overthrusting at upper crustal levels of the entire
SU on the underlying APU, leading to the formation of NE–SW compressive low
angle brittle joints (LAF – Fig. 4). Finally, dm- to m-scale high angle
brittle joints (HAF – Fig. 4), commonly NE–SW oriented, probably linked to
the South Tyrrhenian System (Lentini et al., 2002; Guarnieri et al., 2002)
(D6A – Fig. 4a) occur.
Other two normal fault families oriented N–S and E–W, respectively have
been further recognized, here interpreted as related to the activation of the
strike-slip tectonics which accompanied the drift of the Aspromonte Massif
during the opening of the Tyrrhenian basin, causing the present-day
lateral-juxtaposition of the CPO composite framework.
Aspromonte-Peloritani unit (APU)
Rock-types belonging to this unit constitutes the most areal outcrop
extension. Mylonitic para- and leuco-ortho-gneiss represent the main
outcropping litho-types. Weakly to strongly developed pervasive mylonitic
surfaces, constitutes the dominant foliation but relics of an early isoclinal
folding aligned to the latter one are locally preserved. In most of the
outcrops it is possible to see a constant alternation from proto- to
ultra-mylonite, (Fig. 3b) testifying a continuous repetition of the mylonitic
horizon. This is due to the bending phases occurs during the exhumation of
the chain inducing a thickening of the original mylonitic horizon actually
reaching up to 800 m. The results of field investigations were
derived from 29 stations clustered in the central part of the map (Fig. 2).
These outline a poly-orogenic structural evolution articulated in a first
event of the Hercynian cycle (D1E – Fig. 5a) during which the
oldest metamorphic surface develops (S1E – Fig. 5a),
subsequently microfolded during the second event (D2E – Fig. 5a)
to form a S2E schistosity (Fig. 5a), locally observed at outcrop
scale. Such structures are nearly totally obliterated by an Oligocene-Miocene
deformational event occurred during the early Alpine cycle (D3A
– Fig. 5a), in which the APU overthrust on the MPU along a deep-seated
compressional shear zone. At this time, a pronounced mylonitic schistosity
(S3A – Fig. 5) constituting the main foliation recognizable in
the outcrops, with a main shear planes oriented 260∘/38∘ and
85∘/32∘ (right hand rule notation) is produced (Fig. 5b).
Such surface is averagely oriented ENE–WSW with dip directions spanning from
15 to 85 ∘. Simultaneously, a pervasive stretching lineation
(L1A – Fig. 5) predominantly oriented SW–NE is formed, whose
kinematic indicators provide a top-to-NE sense of shear in the current
geographic coordinates. Post-mylonitic stage involves APU rocks in
a subsequent isoclinal folding phase (D4A – Fig. 5a) with the
development of a new axial plane foliation (S4A, B3A
– Figs. 3d and 5a) accompanied by the generation of an intersection
lineation (L2A – Fig. 5) almost perpendicularly oriented to the
L1A. During exhumation (D5A – Fig. 5a) a switching
from a ductile behavior, characterized by meter-scale SSE–NNW verging
asymmetric folds (B4A – Figs. 3e and 5), to a brittle one
consisting of NE–SW low angle brittle joints took place (LAF – Fig. 5). For
this reason, in different places of the map (Fig. 2) it is possible to
observe a repetition of the original contact between APU and MPU, leading to
an increased thickening of the whole mylonitic zone. The final record of the
tectonic history is the activation of normal faulting systems (HAF –
Figs. 3f and 5) indicating the beginning of an extensional regime, similarly
to that shown by SU (Fig. 4b).
Madonna di Polsi unit (MPU)
The lower-greenschist to lower-amphibolite facies metapelitic-metapsammitic
sequences of MPU (Pezzino et al., 1990, 2008) is characterized by a polyphase
Alpine metamorphism (Pezzino et al., 2008; Ortolano et al., 2005; Cirrincione
et al., 2008b, 2010a; Fazio et al., 2008). Predominant lithotypes defining
the basement rocks of MPU are phyllites and garnet-micaschists (Fig. 3c).
Structural investigations were carried out on 15 stations scattered across
the map (Fig. 2). Results suggest that these rocks have experienced several
deformational episodes characterized by a first isoclinal folding
(D1A – Fig. 6a) indicated by hinge relics of deformed quartz
producing an axial plane schistosity (S1A – Fig. 6a), which,
according to Puglisi and Pezzino (1994) and Cirrincione et al. (2008b) is
attributable to the early Alpine deformation. A second event (D2A
– Fig. 6a) determining a new surface (S2A – Fig. 6a) derived by
the crenulation of the first foliation is only locally observed at outcrop
scale. Rather, the most extensive deformational stage is related to the shear
event (D3A – Fig. 6a) responsible for the formation of the
pervasive mylonitic foliation (S3A) mostly oriented ENE–WSW, the
stretching lineation (L1A) having an average plunge of
210∘/25∘ (azimuth/dip direction), intrafolial folds and S-C
fabrics. These structural features together with the later isoclinal folding
deformation of the mylonitic foliation suggest that MPU rocks have shared,
since this age, the same tectono-metamorphic history with the overlying APU.
Such an evolution is therefore characterized by the sequential formation of
asymmetric folds (B4A) characterized by the development of
a ENE–WSW conjugate low angle joint system, linked with the already observed
brittle thrusting (LAF) (Figs. 3e and 6), coeval with the final stages of the
progressive exhumation. As in the other units, these structure are repeatedly
cut by high angle fracture cleavages (HAF – Fig. 6) developed during the
last step of the Alpine evolution (D6A – Fig. 6a), but in this
case it was possible to distinguish just two of the three families previously
highlighted, with E–W and NE–SW trends.
Microstructural analysisPetrography and mineral assemblages
In this chapter we focused our attention on metamorphic episodes developed
during the shear deformation (i.e. from D3A to D6A,
see Table 2), thus the early deformational and blastesis events (from
D1H to D2A and from M1H to
M2A) will be not furthermore discussed. In this context a suite
of oriented specimens were collected from the same outcrop (see Fig. 2 for
location and Fig. 7 for petrographic features). Usually two orthogonal thin
sections were prepared and analyzed for each sample: one was cut parallel to
the stretching lineation (X direction, L1A), the other was cut
orthogonal with respect the lineation, both of them are perpendicular to the
mylonitic foliation (XY plane, S3A).
The uppermost tectono-metamorphic unit outcropping in the study area, the
Silo Unit (SU), is essentially made by Variscan low-grade shales and
phyllites (Crisci et al., 1982; Bonardi et al., 1984; Fazio et al., 2015).
The typical assemblage is given by
Qtz + Wm + Pl + Chl + Bt + Ilm (Fazio et al., 2012,
2015). Since this unit was not affected by the shear zone its microstructural
features will be not further treated in the following.
The intermediate Aspromonte-Peloritani unit (APU) is composed essentially by
middle crustal biotite paragneisses and leucocratic augen gneisses with minor
amphibolites, mica schists and marbles. Rocks belonging to the APU collected
for this study are highly foliated ultramylonites, mylonites, and
cataclasites (Figs. 7 and 8) showing good examples of brittle deformation
overprinting an earlier ductile fabric. The main assemblage is given by
Qtz + Pl + Kfs + Bt + Wm + Chl + Ep + Grt +Tur
developed at about 540 ∘C and 0.45 GPa during the syn-shear
metamorphic episode followed by a retrograde metamorphism characterised by
thermobaric condition ranging from 500 ∘C–0.5 GPa to
450 ∘C–0.25 GPa (PT estimates after Cirrincione et al., 2008b).
Thin sections cut parallel to NE–SW show a top to the NE sense of shear
(Fig. 8). At the sub-centimetric scale it is possible to observe
ultramylonite levels interbedded with mylonite and cataclasite portions of
the same rock depicting at places a cataclastic flow. Spectacular intrafolial
oblique folds with disharmonic appearance, sometimes resembling micro-sheath
folds, have also been recognized (Fig. 9, CD thin section of SDA3). Mica fish
are widespread. Quartz occurs as ribbons, sometimes showing micro-folds with
axial planes sub-parallel to the mylonitic foliation (S3A).
Quartz domains are also characterized by smaller recrystallized
grains forming an oblique foliation (Figs. 7a, 8d, 9d) suggesting a dominant
subgrain rotation recrystallization (SGR) and a locally grain boundary
migration recrystallization (GBM). Smaller quartz grains recrystallizing at
the edges of larger sigmoid shaped quartz grain suggest also the activation
of bulging (BLG). Quartz-rich domain (e.g. ribbon-like layers) sometimes are
widely truncated by a pervasive network of veins suggesting hydraulic
fracturing (Fig. 7e). Sometimes larger grains of quartz show
intra-crystalline plasticity with classical chessboard pattern and
sporadically deformation lamellae. Feldspars, usually forms porphyroclasts,
often very elliptical, elongated and rounded testifying an intense reworking
during shearing activity (Fig. 9e). Domino-type micro-textures are widespread
in microcline showing deformation twins, forming fragmented augens with
micro-cracks filled by chlorite and sericite. K-feldspar (adularia) also
crystallizes as rhombic shaped blocky grains (long axes about 200–300
microns) with ondulatory extinction into a set of paraconcordant lenticular
veins (Fig. 7d), arranged into layers sub-parallel to the mylonitic foliation
(S3A). These crystals (i.e. Ad) are characterized by
a mosaic-like texture. Kink bands are shown both in biotite flakes, sometimes
retrogressed into chlorite, and in plagioclase highlighted by folded albite
twins. Plagioclase and K-feldspar within leucocratic mylonitic gneiss show
sometimes an extreme lattice bending (SDA11, Fig. 7c). In the same rocks
layers of fragmented garnet enveloping pre-kinematic kinked feldspars also
occur.
Discordant veins with large crystals of quartz and feldspar crosscut at high
angle both the mylonitic foliation and the pseudotachylyte (Pst) portions
which are sub-parallel up to slightly discordant with respect to the
S3A (sample SDA14). Late-fractures dislocating the mylonitic
foliation and plagioclase porphyroclasts showing microns-sized dip-slip at
thin section scale are widespread (Fig. 7f).
The typical assemblage of the MPU metapelites is given by
Qtz + Wm + Pl + Grt + Kfs + Chl + Amph + Tur.
Layering (S3A) is due to the alternance of micaceous- and
quartz-rich levels isoclinally folded (D4A) (leading to the
transposition of an early foliation, S1A). Often a cataclastic
flow overprints the mylonitic fabric (S3A). When the cataclastic
process is very pervasive, coupled with a smaller pre-mylonitic grainsize
with respect to the APU rocks, the earlier ductile features are fully
obliterated. White mica fish are diffuse. Garnet grains are very rounded
suggesting erosion during transitional ductile-brittle shear regime;
ribbon-like quartz (SGR + GBM, ondulatory extinction) showing internal
oblique foliation are diffusely occurring; pre-kinematic Pl with core-rim
(dusty-limpid).
Quartz LPO
On a suite of collected samples the lattice preferred orientation (LPO) of
recrystallized quartz grains was inferred by the CIP method (computer
integrated polarization microscopy; Heilbronner, 2000), widely adopted in
microstructural analysis of mylonitic rocks (e.g. Heilbronner and Tullis,
2006; Menegon et al., 2008; Fazio et al., 2010; Pennacchioni et al., 2010).
In particular, eight domains of oriented specimens have been investigated in
order to retrieve information about the orientation of the optical c axes
of recrystallized quartz grains. Quartz c axis patterns are shown on
Fig. 10.
The dominant crystallographic distribution of quartz c axis is
represented by a diffuse basal 〈a〉 slip system
(Fig. 10a–h) suggesting a main recrystallization episode happened
under shear strain at greenschist facies condition (Schmid and Casey,
1986; Stipp et al., 2002). At a lesser extent the rhomb 〈a〉 and prism 〈a〉 slip systems are also developed
(Fig. 10a, e and g). Minor prism 〈c〉 slip system has
been also registered (samples SDA1, SDA11; Fig. 10a and e).
In one case a late discordant vein (Fig. 10h′′)
crosscutting the earlier mylonitic foliation displacing ribbon-like
quartz levels (Fig. 10h–h′) has also been investigated.
Results, thus not correlated with the shear zone activity but
undoubtedly to a subsequent episode, show a peculiar pattern with
a maxima between the X and Z directions and a general rhombic
symmetry linked to the specific orientation of maximum stress during
crystals growth into the opening vein (Fig. 10h′′).
Qtz deformation mechanisms
The dynamic recrystallization of quartz domains testifies the
activation and protracting of deformation by means of dislocation
creep mechanisms which produced characteristic fabrics in the Montalto
sheared rocks. In particular, prevalent subgrain rotation
recrystallization (SGR), with minor bulging (BLG) and sporadic grain
boundary migration (GBM), has been recognized as the main
recrystallization process.
Recrystallization microstructures typically developed in crustal scale
shear zone affecting quartz-feldspar-mica bearing rocks denoting
quartz intracrystalline plasticity were observed. Oblique foliation
developed at about 15∘ with respect the shear zone boundaries
(c surfaces) is delineated by new recrystallized smaller grains
(average diameter ca. 50 microns) suggesting a dominant SGR
recrystallization process (Fig. 11a). This fabric has been observed
thorough almost all samples collected from the Montalto shear zone
(Fig. 10b–g). Larger grains forming ribbons are elongated with major
axis parallel to the main foliation (long axis about 500 microns,
Figs. 10b, d, e, and 11b). They exhibit optically visible subgrains
and show at their borders tiny recrystallized grains typical of SGR
(core and mantle texture, Fig. 11c).
Syntectonic recrystallization of quartz probably has proceeded towards
the last exhumation phases of the shear zone, during progressive
cooling, causing the overprint of lower temperature fabric over higher
temperature ones. This causes that mylonites pass through the BLG–SGR
transition, which has been estimated to occur approximately at
280 ∘C (Dunlap et al., 1997), 400 ∘C (Stipp et al.,
2002). Moreover fracturing also occurs (Fig. 7e) in alternation to
dynamic recrystallization as a response to the rising of local
differential stress up to the magnitude of confining pressure (Kuster
and Stockert, 1999). Interestingly mica flakes occurring at the
boundary of these fragmented quartz domains, at the end of opening
veins show local kinking (Fig. 11d) suggesting that deformation
(intracrystalline plasticity) has continued after the fracture
propagation.
Qtz paleopiezometry
Paleopiezometry calculation based on the mean grainsize of
a recrystallized quartz-rich domain from an highly deformed mylonite
was also performed (SDA3 sample, Fig. 12). This was useful to infer
the differential stress (Δσ) active during the shearing
deformational phase (Poirier, 1985). To this aim a grain boundary map
was obtained from a stack of three CIP misorientation images adopting
the Lazy grain boundary macro (Heilbronner, 2000) with the Image SXM
software (Barrett, 2010). The selected domain containing 3542
particles can be considered statistically valid to infer differential
stress (Herwegh et al., 2011). Applying an empirical equation (Stipp
and Tullis, 2003) corrected by Holyoke et al. (2010), considering the
mean grain size of 42 microns, a differential stress (Δσ)
of 0.25ß,GPa was inferred.
Since the value of differential stress is known and the deformation
mechanisms correlated with dislocation creep regime which produced
characteristic related microstructures (BLG, SGR) were recognized, the
strain rate (ε˙) can be retrieved by adopting a flow
law equation (Bürgmann and Dresen, 2008) in the following form:
ε˙=Aσnd-mfH2Oe-Q+pVRT
where A is a material constant, σ is the applied stress, n
is the (power-law) stress exponent, Q is the activation energy, p
is the pressure, V is the activation volume, T is the absolute
temperature, R is the molar gas constant, d is the grain size, m
is the grain-size exponent, fH2O is the water fugacity,
and r is the fugacity exponent.
Our estimates are calibrated on the experimental data obtained for
quartzite under wet conditions (Hirth et al., 2001), realistically
approximating conditions of natural shear zones with associated
mylonites (Stipp et al., 2002), by adopting values of A, Q, and
n as follows: 6.30957×10-12(MPa-ns-1),
135 000 (Jmol-1), and 4 respectively. Indeed,
petrographical evidences confirm that hydrous minerals
(e.g. muscovite) are constituents of syn-shearing assemblage.
In order to obtain reliable estimates for water saturated conditions,
appropriate water fugacity values have been selected (Tödheide,
1972) by taking into account the thermal range deduced from quartz
c axis patterns, which is comprised between 350 and 550 ∘C,
being basal 〈a〉 the dominant active slip system. These
thermal estimates are also consistent with those proposed by previous
studies (Cirrincione et al., 2008b) who also determined pressure
conditions for the Montalto shear zone in the range 0.3–0.45 GPa.
We then calculated the strain rate values at the following PT shearing
conditions: P (0.45 GPa); T (550 ∘C), for which a value
of fH2O=4.82 GPa has been selected (Tödheide,
1972). The strain rate (ε˙) found for the Montalto
shear zone (SDA3 sample) is 3.21×10-12(s-1),
which is in agreement with typical values of actively deformed regions
(10-12 to 10-15s-1; Behr and Platt, 2011) and
natural shear zone (10-9 to 10-13s-1;
Bürgmann and Dresen, 2008). Nevertheless the above mentioned PT
conditions were attained during the initial phase of shearing but
deformation has continued up to shallower crustal conditions. For this
reason we also made a further paleopiezometry calculation attempting
to estimate the last phase of the same shearing event at lower PT
conditions (0.30 GPa and 350 ∘C). Such calculation,
considering a value of fH2O=1.35 GPa (Tödheide,
1972), gives a strain rate value rate (ε˙) of 1.62×10-15(s-1).
Brittle microstructures
Besides cataclastic flow, which has been recognized in a couple of
samples (SDA14, SDA10), three sets of joints/veins have been
recognized (sample SDA1, Figs. 11–13):
Millimeter-thick lentoid shaped epithermal veins (Fig. 11),
usually paraconcordant with respect the mylonitic foliation,
consisting of polygonal aggregates of K-feldspar (adularia, Ad), at
places truncating both micaceous and quartz rich domains delineating
the mylonitic foliation (S3A).
Thinner massive veins (Fig. 11a–d) characterized by walls with
a high concentration of opaque minerals (Fe-oxides) at high angle
with respect the mylonitic foliation (S3A) filled by
feldspar, chlorite, and quartz. Sealing quartz is sometimes
characterized by ondulatory extinction, testifying that deformation
was still active at subsolidus conditions. This second set of tiny
veins (Fig. 11d), truncating both the early mylonitic microfabric
(S3A) and the first set of veins, sometimes appears as
suture joint (teeth-saw profile) resembling stylolites, coherent
with the same sigma1 orientation active during the first vein
opening, oriented parallel to the main foliation. Possibly this set
of joint is also related to the final stage of the D5A
brittle episode.
The last microstructures related to the fragile regime
(Figs. 11–14) are breccias, pseudotachylytes and close
fractures. The latter crosscut at an average angle of 60∘
the mylonitic foliation causing its displacement
(Fig. 7f). Sometimes such microcracks produce fibrous opening veins
filled by biotite and sericite (Fig. 12) or sporadically also quartz
veins (Fig. 13) suggesting that both of them are related to an
extensional tectonic event (D6A) that became active at
relatively deep seated conditions (T>250∘C; P>0.3 GPa) at least during its initial phase.
Discussion and timing of deformational phases
The relationships between ductile and fragile structures are here
described to highlight the possible evolution of the shearing MSZ
activity trough the reconstruction of the overprinting and dispersion
relations among the different evolutionary stages, controlled
sequentially the exhumation trajectory of the Aspromonte Massif
crystalline basement units (Fig. 15, Table 2). From the integrated
analysis at the field- and thin section-scale of the two deep seated
outcropping units (i.e. APU and MPU) the following possible tectonic
evolution can be delineated.
The most developed deformational phase (D3A), here
obliterating most of the previous structures, is typical for the
development of structures during the evolution of a non-coaxial
plastic-type deformation with formation of a weakly to strongly
mylonitic foliation (S3A) and a stretching lineation
(L1A, parallel to the tectonic transport direction, see
SDA3 thin section, Figs. 8 and 9) and a layering given by alternating
micaceous and quartz-feldspar rich domains. Syn- to post-mylonitic
asymmetric microfolds of quartz layers are widespread. Mica fish,
S-C-C′ textures, ribbon like quartz, book-shelf feldspar are
characteristic microstructures of this stage. Ultramylonites develop
during this tectonic phase.
The second phase is related to the ductile-brittle transition coeval
with the shear zone uplifting (a possible change and/or decrease of
the main shear plane inclination occurred at this time). The tectonic
transport (D4A) causes overpressure in the pile nappes
edifice with development of a set of veins (joints) overprinting
mylonites (Fig. 15, see SDA1 thin section). Two sets of veins are
characteristic of this phase: syntaxial veins (V1A –
sigma1 oriented parallel w/r to S3A)
of blocky feldspar and quartz (similar to the Ad + Ab + Qtz
“Alpine veins” of Rossi et al., 2005, possibly related to high K-Na
fluid circulation) usually para-concordant with respect to the
mylonitic foliation. Large grainsize of such sealing crystals are due
to a rising temperature effect associated with a considerable
enrichment of fluids pressure. These veins (V1A veining
episode, Table 2) sometimes are not fractured by subsequent veins
(V2A stylolite-like veins) probably for a mineral-rheology
related contrast of competence between surrounding micaceous matrix
constituting the host rock and the vein itself. Adularia
crystallization (Rossi et al., 2005) into paraconcordant veins is
possibly linked to high-K-Al-Si fluid circulation (upflow zones –
most likely Ad takes place under dynamic conditions that are
characteristic of an open system such as fault fissures carrying
geothermal fluid – i.e. a hydrothermal K-feldspar precipitated by the
geothermal fluid, and forming incrustations on fissured wall
rocks). Probably such epithermal veins (V1A) opened at
ca. 300 ∘C, after Steiner, 1970). Sigmoidal recrystallized
quartz observed in one sample (SDA1) into a quartz-rich domain is
considered coeval with this veining stage (Fig. 10e). Indeed the
surrounding quartz domain is characterized by a classical low
temperature quartz c axis pattern (Fig. 10), whereas the new quartz
grains, with a different crystallographic orientation, have tails
sub-parallel to the main foliation which should be coincident with the
sigma 1 direction at the time of recrystallization coevally with
respect the Apennine thrusting phase (D5A). The
V1A set of veins has been interpreted as syn-tectonic
hydraulic fractures parallel to the sigma1 direction probably coeval
with the D4A tectonic phase responsible for the verging to
isoclinal folding of S3A evolving to the thrusting phase
(D5A) responsible for the low angle faults (LAF) visible
at the outcrop scale. Indeed, fluid pressure may increase during
thrust sheet emplacement. Then a network of stylolite-like massive
veins (V2A – sigma1 oriented parallel
w/r to S3A) sub-coeval with the
previous ones develops. They often appear as suture joints at high
angle with respect S3A. Elevated fluid pressure, which led
to fracturing, dilation, and fault initiation, coupled with
progressive decreasing temperature, resulted in the transition from
ductile folding to brittle thrusting (e.g. Gibson, 1985). Changing
physical conditions probably reflect erosional unroofing during uplift
and late Apennine thrust sheet emplacement.
The subsequent tectonic collapse of the chain (D6A)
favored the variation of the sigma 1 orientation, changing from
sub-horizontal to sub-vertical (switching of sigma1 with sigma3), and
the development of both V3A veins at microscale and of
normal faults connected to the last brittle episode (D6A)
at the regional scale, causing the recent horst and graben structure
of the entire Aspromonte Massif crystalline edifice (Fig. 3g). At the
onset of the maximum stress orientation switching, characterized by an
increasing vertical loading, a set of veins marked this stage
(V3A – sigma1 oriented perpendicular
w/r to S3A): tiny fuzzy antitaxial
fibrous veins (Chl, Wm, Bt) with Fe-oxides at walls, and massive veins
filled by large quartz crystals crosscutting at high angle ribbon like
quartz parallel to the mylonitic foliation. These microstructures are
related to the syn- to post-exhumation vertical loading of the entire
basement edifice marking also a possible connection with the
recognized meso-structural scale high angle faults (HAF) (Fig. 3).
The third phase (D6A) has developed under the classic
brittle regime s.s. Two brittle sub-stages can be distinguished:
(first stage) cataclastic flow and breccias overprinting mylonites and
development of pseudotachylyte (Pst) crosscutting mylonitic layers
(parallel to the mylonitic foliation), connecting the V3A
set of veins with large quartz grains. Sometimes injectites have been
recognized; (second stage – sigma1 oriented perpendicular
w/r to S3A): high angle micro faults
(jogs, dilation veins with microscale pull apart basin) with dip-slip
or displacement visible at thin section scale suggesting a connection
with mesostructural high angle faults (HAF).
Conclusions
The structural analysis conducted both at the meso- and micro-scale
allowed us to detect a complex tectono-metamorphic evolution of the
Montalto shear zone, evolving from ductile to brittle regime, passing
through the ductile-brittle transition. Several evidences, like
microstructures, mineral rheology, quartz LPO pattern, suggest for
this crustal scale shear zone a mylonitic stage at greenschist facies
condition (up to lower amphibolite facies, Cirrincione et al., 2008b)
with associated typical ductile strain features like quartz ribbons,
mica fish, rounded porphyroclasts, oblique foliation, isoclinal folds
with recrystallized grains of quartz with their longest axis sub
parallel to the fold axial surface. Moreover, the study of the
crystallographic orientation of recrystallized quartz grains of
selected domains by the CIP method permitted us to confirm greenschist
facies thermal range of the shear zone activity as well as to gain
information about the sense of shear in the nowadays geographic
coordinates, giving an average tectonic transport vergency towards NE.
Concerning the quartz LPO, the dynamically recrystallized grains
develop a broad maximum near the Z direction compatible with
a dominant basal 〈a〉 slip occurred at about
300–350 ∘C (greenschist facies conditions) with subordinate
rhomb 〈a〉 and prism 〈a〉 slip systems
developed under 450–550 ∘C (amphibolite facies
conditions). The geologically constrained dislocation creep flow law
of Hirth et al. (2001) for wet quartz has given realistic results in
the calculation of strain rate in the Montalto sheared rocks. Indeed
when considering the shearing conditions (temperatures and stresses)
of the studied area, the mylonites should have deformed at strain
rates (ε˙) of 3.21×10-12 to 1.62×10-15 (s-1) for differential stress (Δσ)
of 0.25 GPa estimated by paleopiezometry (Stipp and Tullis,
2003). These strain rate values inferred from quartz paleopiezometry
can be considered to be realistic if compared to the geological
literature data for crustal-scale shear zones.
At the transition between a typical ductile rock behavior and an
initial fragile regime, a mixture of features belonging to the
opposite finite strain products is observed. In particular the careful
crosscutting relationships between several sets of veins at the
microscale allowed us the whole tectonic history to be better
constrained. Massive veins, related to fragile rock behavior, sealed
with adularia, quartz (syntaxial vein) and biotite (antitaxial vein)
suggesting thermal condition above 250 ∘C, referable to the
ductile regime up to the ductile-brittle transition. The peculiar
orientation of such veins, sub-parallel to the main mylonitic
foliation, suggests that exhumation has developed fundamentally in
a compressional stress field rather than in an extensional one. Up to
this stage both meso- and micro-scale structures related to ductile
regime as well as to transitional and semibrittle ones furnish
evidences of a continue deformation developed in a compressional
settings. Then characteristic brittle structures such as cracks,
micro-faults, dilation veins and joints are superimposed on the
clearly plastic deformational features acquired during early stages of
the tectonic history. Such microstructural observations have their
correlative features both at the meso- and mega-scale, especially
regarding the low-angle faults (LAF) and high-angle faults (HAF)
linked to the thrusting phase and the subsequent normal faulting of
the entire Aspromonte Massif edifice, respectively.
We can conclude that the whole structural dataset provide evidences
for an articulated, but relatively continued polyphase evolution of
the investigated fault-rocks involved within the MSZ evolution, which
has been delineated thanks to a careful geological and structural
analysis supported by microscopic observations. Results are concordant
with an exhumation activity substantially developed in a compressional
regime operating from the Oligocene-Miocene deepest mylonitic shearing
activity (Alpine phase, D3A–D4A) and
continued up the former stages of the thin skinned Apennine
evolutionary stages (Apennine phase, D5A). Our
observations suggest then that the switching from compressional to
extensional regime, can be ascribed only to the still active final
evolutionary stage (i.e. Apennine phase, D6A) causing the
activation of a dip-slip tectonics which brought to the formation of
the present-day horst and graben structure of the Aspromonte Massif.
The Supplement related to this article is available online at doi:10.5194/sed-7-909-2015-supplement.
Acknowledgements
We thank Renèe Heilbronner for making available the CIP
facilities at the University of Basel. Daniele Castelli is greatly
acknowledged for making available devices to take microphotographs
at the Earth Science Department of Turin University.
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Total stations and structural data reported in the geological map of
Fig. 2.
Synthesis of geological setting: (a) areal
distribution of Alpine belt in the central Mediterranean realm;
(b) geological sketch map of Calabrian Peloritani Orogen
(CPO) (modified after Angì et al., 2010; Cirrincione et al.,
2011); (c) geological sketch map of the Aspromonte Massif
(after Pezzino et al., 1990, 2008; Ortolano et al., 2005; Fazio
et al., 2008).
Detailed geo-structural map of the study area with outcrop
location (scale 1:10 000).
Outcrop features: (a) Stilo Unit phyllites;
(b) alternance of mylonitic ortho- and para-gneiss of the
Aspromonte Peloritani Unit (inset showing detail of the stretching
lineation); (c) Madonna di Polsi Unit mylonitic garnet
bearing micaschist; (d) isoclinal post-mylonitic folding
(mylonitic paragneiss of the APU); (e) asymmetric
syn-D5A fold, evolving to brittle low angle fracture
(LAF) both in the APU (to the left) and in the MPU rock-types (to
the right); (f) conjugate high angle joint system in the
mylonitic leucogneiss of the APU; (g) panoramic view of the
early mylonitic contact between APU and MPU surfacing along the
Zillastro landslide affected by the later brittle tectonics (see
Fig. 2 for location).
Schematic representation of Stilo unit (SU) structural
evolution: (a) sequential blasto-deformational evolution
with associated the orogenic stages from ductile to brittle
sequence; (b) stereoplots of the main structural features
observed in outcrops. N= number of measurements, 1st, 2nd, 3rd = high angle families hierarchy – longer segments corresponds to
a greater number of data.
Schematic representation of Aspromonte-Peloritani unit (APU)
structural evolution: (a) sequential blasto-deformational
evolution with associated the orogenic stages from ductile to
brittle sequence; (b) stereoplots of the main structural
features observed in outcrops. N= number of measurements, 1st,
2nd, 3rd = high angle families hierarchy – longer segments
corresponds to a greater number of data.
Schematic representation of Madonna di Polsi unit (MPU)
structural evolution: (a) sequential blasto-deformational
evolution with associated the orogenic stages from ductile to
brittle sequence; (b) stereoplots of the main structural
features observed in outcrops. N= number of measurements, 1st,
2nd, 3rd = high angle families hierarchy – longer segments
corresponds to a greater number of data.
Microphotos: (a) classical view of an APU
mylonite. The main foliation is characterized by alternating
mica-rich (Bt “fish” shaped) and quartz-rich domains (ribbon-like)
characterised by ondulouse extinction, very fine grainsize (SGR-GBM)
and a marked oblique foliation; (b) K-feldspar
porphyroclasts showing domino-type structures (both synthetic and
anthitetic fractures are developed) floating into a very fine
ultramylonite matrix (sample SDA13). Clasts have usually a flattened
aspect, the elliptical shape is due to erosion of external edges
during shear deformation. (c) Severely deformed
pre-mylonitic K-feldspar showing intracrystalline plasticity
associated with frictional sliding; (d) blocky adularia
veins subparallel to the mylonitic foliation; (e) hydraulic
fracturing of a quartz microlithon crosscut by a network of veins
(filled by chlorite and sericite) which usually are not propagating
to the surrounding micaceous layer; (f) mylonitic fabric
overprinted by a subvertical conjugate set of fractures
(micro-graben like structure).
Mylonite of APU and associated microstructures (SDA3 sample
– AB section): (a) sketching draw of oriented specimen
with the trace of cut (dotted line) parallel to the stretching
lineation (L1A) and relative stereoplot with the
mylonitic foliation (S3A, red plane); (b) feldspar clast
floating into a fine micaceous matrix (crossed
polarizers); (c) same of (b) ultramylonite layer
is visible in the upper part of the picture (parallel polarizers);
(d) quartz-rich and micaceous-rich domains enveloping
feldspar augens (crossed polarizers). Note the oblique foliation in
the upper quartz ribbon coherent with a sinixtral sense of shear
(top-to-NE); (e) same as (e), note the elongated
and rounded shape of the large feldspar at the bottom-right corner
(parallel polarizers).
Mylonite of APU (sample SDA3 – CD thin section orthogonally
cut with respect the Fig. 8): (a) block diagram showing
thin section cut direction together with stereoplot; (b)
ultramylonite (half bottom part) interbedded with mylonite (half
upper part), a set of high angle veins disrupt the horizontal
foliation continuity; (c) detail of a (squared area)
showing an intrafolial fold (crossed polarizers); (d)
oblique foliation in quartz domain; (e) extremely rounded
porphyroclast; (f) complex refolding microfabric (parallel
polarizers); (g) complex geometry of folds affecting quartz
layers (crossed polarizers).
Quartz c axis patterns inferred by the CIP method on
selected quartz-rich domains. In the crystallographic orientation
images (coi) the azimuth and dip of each pixel is color coded based
on the shown look-up-table: (a) sample SDA11; (b)
sample SDA10 (section AB); (c) section SDA10 (section CD);
(d) SDA8 sample (MPU); (e) SDA1 sample;
(f) SDA3 sample (section AB); (g) sample SDA7;
(h) sample SDA14: coi of a complex domain consisting of
sub-horizontal syn-mylonitic quartz and post-mylonitic crosscutting
subvertical quartz vein; (h′) restricted
subdomain of (h) focusing on the syn-shear
ribbon-like quartz; (h′′)
subdomain of (h) showing the quartz recrystallized into the
discordant vein. Legend: Pg: paragneiss; Lg: leucocratic gneiss;
Um: ultramylonite; My: Mylonite; Pst: pseudotachylite; MPU: Madonna
di Polsi Unit.
(a) Grain boundary map of the quartz domain selected for
paleopiezometry (sample SDA3). The colour lookup table is the same as
Fig. 10; (b) quantitative microstructural analysis of the domain
depicted in (a) considering a total of 3542 grains: the plot (aspect
ratio vs. number of grains) shows an unimodal distribution of grains
clustered around the 1.5 value.
(a) Typical SGR recrystallization showing lobate grain
boundaries and recrystallized grains which are slightly larger than sub
grains (visible at lower left corner of the picture); (b) BLG
recrystallization process showing undulose extinction of elongate grains and
grain boundary bulges concentrated at the sutured edges of ribbon-like
quartz; (c) undulose extinction visible within a sub-horizontal
ribbon grains with associated deformation lamellae between a quartz-rich
domain (lower part) and a micaceous domain (upper part) showing a mortar
texture characterized by recrystallized smaller quartz grains at its margins
(white arrows); (d) large quartz microlithon showing pervasive
fracturing. Note here as kinking of biotite flakes are concentrated at the
end of veins crosscutting the quartz layer.
(a) 1st and 2nd set of veins (V1A and
V2A); (b) detail of a (parallel polarizers);
(c) the same domain depicted in (b) (crossed polarizers);
(d) sketch draw of a showing horizontal mylonitic foliation
(S3A), subparallel 1st set of adularia veins (black
region), and 2nd set of stylolite-like veins (spotted line) produced
with principal stress orientation as depicted in the central inset;
(e) sigmoidal 1st set of veins truncating mylonitic fabric
(parallel polarizers); (f) detail of (e) –
squared region – showing blocky grains of adularia (crossed
polarizers); (g) layering given by ribbon-like quartz,
micaceous-rich layers and adularia paraconcordant veins. Note how
a subvertical secondary vein is deflected around the early formed
adularia vein, whereas it is passing through quartz domains (upper
right part of the picture, crossed polarizers).
3rd set of veins (V3A): (a) fibrous
veins; (b) detail of (a, c, d)
sketch draw of (a, b) respectively, suggesting a sinixtral
shear sense around the fracture; (e) other example of
biotite fibrous vein; (f) high angle fibrous vein with
respect mylonitic foliation.
(a) Rough grain boundary map of the same domain of Fig. 9h
(sample SDA14) investigated by means of the CIP method; (b)
quartz c axis pattern and inferred sigma1 orientation of the coarser quartz
grains composing the vein on the right cutting at an angle of about 60 the
early ribbon quartz shown on the left.
(a) Ultramylonite (sample SDA3) characterised by a complex
geometry of quartz layers; (b) detail of (a) showing
a complex fold (red square region near the center of the picture, crossed
polarizers); (c) like (b) (parallel polarizers);
(d) sketch draw with interpretation of microstructures in
(a); (e) interpretative schematic draw of (d)
showing superposition of ductile-regime related shear zone (black-colored
sigma1) and brittle-regime related veins (red-colored sigma1).
Synoptic scheme illustrating the tectonic evolution of the
Montalto Shear Zone also showing the linkage between mesostructural
features (outcrop scale) and microscopic structures (thin section
scale) during progressive deformational phases (Table 2).