The Vinschgau Shear Zone (VSZ) is one of the largest and most significant
shear zones developed under plastic conditions within the Austroalpine domain,
juxtaposing the Ötztal and the Texel units to the Campo, Scharl and
Sesvenna units during the building of the Eo-Alpine Orogen. The VSZ
dominates the structural setting of a large portion of the central
Austroalpine Late Cretaceous thrust stack. In order to fully assess the
evolution of the VSZ, a multi-faceted approach based on detailed multiscale
structural and petrochronological analyses has been carried out across
representative transects of the shear zone in the Vinschgau Valley. The
research has been performed with a view to characterizing kinematics,
Our fieldwork-based analyses suggest that the dip angle of mylonitic
foliation increases from west to east with an E–W-trending stretching lineation
which dips alternatively to the west and to the east, due to later folding related
to the Cenozoic crustal shortening. The dominant top-to-W shear sense of
the mylonites recognized in the field and confirmed by microstructural
analyses led to exhumation of the upper Austroalpine nappes in the hanging
wall of the shear zone; the Texel unit with Late Cretaceous eclogites and the
Schneeberg and Ötztal units were all affected by Eo-Alpine
amphibolite-facies metamorphism. Chemical and microstructural analyses
suggest deformation temperatures of ca. 350–400
The Vinschgau Shear Zone (VSZ), extending along the homonymous valley (NE Italy; Fig. 1a–b), is one of the most important tectonic structures developed within the Austroalpine domain of the Alps (Figs. 1 and 2). Starting from the first systematic studies of the Alpine belt, this large shear zone, firstly defined as the “Schlinig Thrust”, was interpreted as a top-to-W thrust plane (Spitz and Dyhrenfurth, 1914). Although several authors later rejected this interpretation (Heim, 1922; Staub, 1937), modern studies (Schmid and Haas, 1989; Froitzheim et al., 1994, 1997) carried out since the end of the last century demonstrated the validity of the first assumptions. Recent structural analyses (Brunel, 1980; Ratschbacher, 1986; Ratschbacher et al., 1989; Schmid and Haas, 1989; Pomella et al., 2016) recognized indeed that the entire central Austroalpine nappe stack was affected by W-directed tectonic transport during the first stages of the Late Cretaceous Alpine deformations. Along the VSZ, the Austroalpine tectonometamorphic units with a dominant metamorphism of Alpine age overthrust Austroalpine units (Sesvenna and Campo–Ortler) that were only slightly affected by Alpine metamorphism (up to greenschist facies) and deformation during the Eo-Alpine stage (Thöni, 1981), still largely preserving features acquired during the Variscan orogeny.
Geological setting of the eastern–central Alps.
The VSZ is almost continuously exposed for more than 50 km, mainly on the northern flank of the Vinschgau Valley (Fig. 1), reaching a maximum thickness of about 550 m close to Eyrs (Figs. 1 and 2). These features make the VSZ one of the largest ductile thrust-sense shear zones now exposed in the Alps, together with the Periadriatic Fault (Schmid et al., 1987; 1989) and the Orobic Thrust in the southern Alps (Zanchetta et al., 2011; D'Adda and Zanchetta, 2015). The Noric Thrust in the eastern Alps (Ratschbacher, 1986) is another important ductile thrust, the first one studied by modern kinematic and structural methods. Due to its complete exposure and accessibility, the prominent VSZ represents an ideal natural laboratory for the study of cumulative shear strain distribution during the development of a large mature intra-basement shear zone and to evaluate the evolution in terms of shear strain localization, coaxiality, kinematic and lifetime of activity (e.g., Xypolias, 2010; Xypolias and Koukouvelas, 2001; Law et al., 2013; Fossen and Cavalcante, 2017; Oriolo et al., 2016, 2018).
Large-scale thrust- or normal-sense shear zones developed within collisional settings display huge along-strike exposures such as the ca. 2500 km of the Main Central Thrust and South Tibetan Detachment in the Himalayan orogen (e.g., Caby et al., 1983; Searle et al., 2008), the ca. 30 km of the Simplon Shear Zone (Mancktelow, 1985) and Brenner Fault (Rosenberg et al., 2018) in the Alps, or the Great Slave Lake shear zone (Hanmer et al., 1992) in northern Canada. Due to its peculiar exposure along the strike, the VSZ shows different features from shallow depth conditions in its western portion and deeper conditions at the eastern end (Schmid and Haas, 1989), providing a complete crustal section of a large-scale shear zone. This kind of exposure provides insights not only on the different deformation mechanisms and behaviors of the shear zone at different crustal levels, but also on its evolution through time.
In this work, we applied a quantitative approach to reconstruct the
evolution through space (depth) and time of the VSZ.
The study area (Figs. 1 and 2) is located along the Vinschgau Valley (NE Italy), entirely extending within the central Austroalpine domain, between the northern Calcareous Alps to the north and the Periadriatic Fault to the south (Fig. 1). Here, the Austroalpine domain consists of tectonometamorphic units that have been identified based on paragenesis, deformation history, metamorphism and relative ages. These units are the Pejo and Laas units (belonging to the Campo–Ortler nappe system) and the Ötztal–Stubai complex, the Matsch unit, and finally the Texel and the Schneeberg units (belonging to the Koralpe–Wölz high-pressure nappe system; Schmid et al., 2004; Handy et al., 2010; Pomella et al., 2016; Klug and Froitzheim, 2022). The E–W-striking VSZ separates the Pejo and Laas units to the south, the footwall of the VSZ characterized by greenschist-facies Alpine metamorphism, from the Ötztal, Matsch, Texel and Schneeberg units forming the hanging wall, characterized by amphibolite- to eclogite-facies Alpine metamorphism (Fig. 2). Therefore, the VSZ together with the Passeier Fault, the Jaufen Fault and the Defereggen–Antholz–Vals Fault (PSF, JF and DAV in Fig. 1) has been considered to form the southern limit of Alpine metamorphism (SAM; Hoinkes et al., 1999), a large fault system defining the southern border of the high-grade Alpine metamorphism in the Austroalpine domain of the eastern Alps.
The VSZ has been described as a ductile-to-brittle fault formed by a thick
zone of mylonites and phyllonites exposed mainly along the left hydrographic
side of the Vinschgau Valley (Fig. 2; Schmid and Haas, 1989; Conti, 1997;
Froitzheim et al., 1997; Thöni, 1999; Sölva et al., 2005; Pomella et
al., 2016; Koltai et al., 2018; Klug and Froitzheim, 2022). Schmid and Haas (1989) defined the main structure as a thick intra-basement shear zone
dominated by intracrystalline plastic processes, showing different thermal
conditions ranging from 300
Four tectonic units mainly consisting of polyphase metamorphic crystalline basement rocks, which were deeply involved in the Alpine deformation and metamorphism, form the hanging wall of the shear zone. They consist of the western termination of the Texel and Schneeberg units and of the Ötztal unit, one of the largest nappes of the Late Cretaceous Austroalpine thrust stack, which is overthrust by the Matsch unit, forming a folded klippe atop the VSZ mylonites.
The age pattern of the Alpine metamorphic peak of the tectonometamorphic
units in the hanging wall of the VSZ displays almost coeval ages in the
Texel and the Schneeberg units. Partially amphibolitized eclogite boudins,
preserved within the mica schists and paragneisses of the Texel unit, point
to metamorphic peak conditions during the Alpine orogenesis of 540–630
The age and the peak metamorphic conditions experienced by the Ötztal
unit during the Alpine metamorphism are far less constrained. Mid-Cretaceous to Late
Cretaceous K–Ar mica ages (100–110 Ma) have been obtained by Thöni (1980) from the basement in the hanging wall of the VSZ, and a whole-rock
Rb–Sr age of 83
The westernmost portion of the VSZ is crosscut by the Glurns Fault (GF in Fig. 2), along which the Ötztal basement is in contact with the Sesvenna unit, mainly consisting of orthogneiss with a Variscan medium-grade metamorphic imprint and its Permian–Mesozoic sedimentary cover (Froitzheim et al., 1994, 1997).
The entire VSZ has been individuated and followed in the field from Naturns (east) to Glurns (west) (Fig. 2). The maximum thickness is reached close to its western ends, at Eyrs, where it is estimated to be of about 600 m. To the east of Naturns (Fig. 2) the VSZ widens and branches out in several shear zones that wrap around the rigid body of the Partschinser orthogneiss of the Texel unit, as already noticed by Schmid and Haas (1989), which has given a Rb–Sr radiometric age of about 450 Ma for Partschinser orthogneiss (Zantedeschi, 1991). Here, in the Meran area, the VSZ is crosscut by most recent shear zones and faults (Bargossi et al., 2010).
Detailed field structural analyses and sampling of the VSZ were performed
along three selected transects (Fig. 2) in the localities, from east to west, Juval, Schlanders and Eyrs. The three studied geological sections were
chosen through field surveys and structural analyses. They are considered to
be representative of the entire VSZ at different depths of exposure
(shallowest conditions at Eyrs and deepest at Juval) and also offer the
possibility of studying and sampling the shear zone in continuity, due to the
good bedrock exposure. Each transect (Fig. 2) has been mapped at a
The easternmost transect is completely within the Texel unit. The cross-section extends southeast–northwest, from the bottom of the Vinschgau Valley to the Juval
Castle. Here the road is entirely excavated in the bedrock, offering a
continuous exposure of the entire shear zone. The bedrock mainly consists of
granitoid orthogneiss (Partschinser orthogneiss) showing different
mylonitization degrees. This transect almost corresponds to the westernmost
termination of the Texel unit (Fig. 2). Besides the Partschinser
orthogneiss, the Texel unit here consists of garnet-, staurolite- and kyanite-bearing paragneisses that are also affected by mylonitization. They chiefly
occur in the upper and central part of the transect, alternating with the
orthogneiss. Some amphibolite boudins (Fig. 3d) are also exposed within the
paragneiss along the road that leads to the Juval Castle. Paragneisses
display a decrease in grain size with respect to the ones outside the VSZ,
especially in the central part of the transect, where they can be classified
as protomylonites. Analysis of the shear strain distribution highlights a
symmetric increase, from rims to core, with ultramylonites and mylonites
(Fig. 3) concentrated in the central part of the shear zone, whereas
protomylonitic textures mainly occur close to both the structurally higher
and lower margins. The mylonitic foliation is defined by the SPO (shape-preferred orientation; Passchier and Trouw, 2005) of biotite. The mylonitic
lineation visible in outcrops mainly consists of elongated quartz aggregates
and aligned biotite crystals (see Sect. 3.2 for details). Foliation dips
towards the north-northwest, with mylonitic lineations that are nearly horizontal trending
east-northeast–west-southwest (Fig. 2b). Moving toward the core of the shear zone, the
clast–matrix ratio decreases, as does the size of K-feldspar porphyroclasts
(Fig. 3). Ultramylonites appear as dark-gray bands within mylonites, a few
centimeters up to 3–4 m in thickness (Fig. 3c). Quartz ribbons, a few
millimeters in thickness and up to several decimeters in extension,
frequently occur (Fig. 3e). Porphyroclasts are scarce within the
ultramylonites, with a mean size not exceeding a few millimeters, whereas
they commonly display a mean size of 20–30 mm outside the shear zone.
Kinematic indicators represented by
The Schlanders transect is located about 20 km to the west of the Juval section (Fig. 2). This section of the VSZ is entirely within the Ötztal polymetamorphic basement, here chiefly consisting of granitoid orthogneiss and minor two-mica paragneiss.
The upper portion of the structural transect extends outside the VSZ, where orthogneisses still preserve their metamorphic regional foliation of Variscan age (Schmid and Haas, 1989; Hoinkes et al., 1999; Thöni, 1999) gently dipping to the east. The cumulative shear strain distribution visible along this transect is symmetric, as was the case in the Juval section. Shear strain increases from the margins toward the core of the shear zone, with a decreasing grain size of both matrix and K-feldspar porphyroclasts. On the basis of the matrix–clast ratio (Simpson and De Paor, 1993), the orthogneiss is protomylonitic. Only towards the core of the shear zone do mylonitic bands occur, ranging in thickness from 10 to 50 cm. Ultramylonites, which are frequent along the Juval transect, occur here only as 2–10 cm thick bands, with a dark-gray color and an intense grain-size reduction (Fig. 3f). The mylonitic foliation dips to the north-northwest with a variable dip angle (Fig. 2b). Dip variations result from the occurrence of late-stage S-facing folds with E–W-trending fold axes that refold the mylonitic foliation. The lineation associated with top-to-W shearing is nearly sub-horizontal or gently dipping to the west (Fig. 2b), identified in the field by elongate quartz aggregates and white micas.
Field photographs of Juval, Schlanders and Eyrs outcrops.
The Eyrs transect covers the western part of the exposed VSZ, close to the Schlinig Normal Fault and Glurns Fault (Figs. 1 and 2) that crosscut the shear zone. The exposed part of the VSZ is entirely developed again here within the Ötztal polymetamorphic basement, chiefly made of granitoid orthogneiss. Protomylonites and mylonites are preserved only along the upper margin of the shear zone, whereas the remaining part, from 1300 down to 900 m a.s.l. (meters above sea level), consists almost entirely of light-gray to whitish phyllonites (Fig. 3g). The phyllonites' protolith is hardly identifiable in the field, but the widespread occurrence (see Sect. 3.2) of K-feldspar suggests that mylonites and ultramylonites developed on pre-existing granitoid orthogneisses. Phyllonites are also exposed on the right side of the Vinschgau Valley (Fig. 2), northwest of Prad am Stilfserjoch. This part of the VSZ was probably the best known to past authors, and its fault rocks were previously known as the “Eyrs phyllites”. The occurrence of dispersed carbonates (Fig. 3h) within the phyllonites led some authors to suppose the occurrence of Permian–Triassic carbonate sediments entrapped within the VSZ (Schmid and Haas, 1989).
Mylonitic foliation dips north-northwest with a variable dip angle due to the occurrence of S-facing folds, as described in the Schlanders transect (Fig. 2b). The mylonitic lineation, here mainly identified by iso-oriented sericite crystal on the foliation planes, is far less evident than in other sectors of the VSZ, but a WNW–ESE trend (Fig. 2b) is generally recognizable. The SPO of chlorite, white mica and quartz defines the mylonitic foliation. Extremely fine-grained quartz bands, a few millimeters thick, commonly occur (Fig. 3h). The light color of the phyllonites frequently turns into a brownish aspect (Fig. 3h). This is due to the occurrence of dispersed fine-grained carbonates (mainly calcite and Fe-dolomite; Fig. 3h) that likely originated from secondary fluids circulating along the shear zone. Secondary carbonates have also been found within orthogneiss-derived mylonites of the Juval transect.
Field structural analyses served as a basis for sampling of the different structural facies recognized along the studied VSZ transects. Samples were collected (Supplement Table S1) all along the three transects at regular distances in order to obtain a complete representation of the whole exposed shear zone.
Granitoid orthogneiss, the main protolith of the mylonites of the VSZ, does
not represent the best opportunity for
In the Juval transect poorly deformed orthogneiss and proto- and mylonitic
orthogneiss consist of plagioclase, quartz, K-feldspar, white mica, biotite
and chlorite. Rutile, apatite, titanite and zircon occur as accessory
phases. The regional foliation is made by the SPO of Bt
In the poorly deformed orthogneiss, quartz is recrystallized mainly via the bulging (BLG; Passchier and Trouw, 2005) recrystallization mechanism (Fig. 4a). Within proto- and mylonitic orthogneiss the dominant mechanism is subgrain rotation (SGR; Passchier and Trouw, 2005; Fig. 4b) recrystallization. A well-developed SCC' fabric (Fig. 5), together with various groups of mica fish (group 1 to 5 of Passchier and Trouw, 2005; group 5 in Fig. 4c) and asymmetric K-feldspar porphyroclasts (Fig. 4d), points to a top-to-W shear sense, as already observed in the field.
Representative photomicrographs from the VSZ, crossed polars.
In the Schlanders transect, chlorite substitutes biotite along the mylonitic
foliation, pointing to lower temperatures during shearing than in the Juval
section. The syn-mylonitic foliation is defined here by the SPO of
Ms
In the Eyrs transect, the westernmost and, following Schmid and Haas (1989), the shallowest transect of the VSZ, phyllonites completely substitute mylonites. The whole of the shear zone consists of extremely fine-grained light-gray to whitish phyllonites. Phyllosilicate domains reach up to 60 % of the rock volume, with white mica as the major mineral phase. The abundance of calcite increases, reaching up to 10 % in several outcrops. Calcite and Fe-calcite crystal show type 1 and type 2 deformation twinning (Ferrill et al., 2004; Fig. 4f). Quartz recrystallizes via BLG; no relicts of SGR and GBM (grain boundary migration; Passchier and Trouw, 2005). P textures have been observed in samples from the Eyrs transect. SCC' fabric, foliation fish, mica fish (group 3 and 4) and K-feldspar porphyroclasts occur as kinematic indicators, pointing to a top-to-W sense of shear.
Vorticity estimates through SC
To define the type of flow within the VSZ, kinematic vorticity analyses were performed on five samples (JVL-16, JVL-12, JVL-11, JVL-6, JVL-14) collected along the Juval transect oriented perpendicular to the shear zone boundaries. The vorticity analysis was carried out on Juval samples, as they are suitable for vorticity estimates. Unfortunately, Schlanders and Eyrs mylonites are unsuitable for the application of any of the methods of vorticity estimates due to the lack of fabrics which allow application of vorticity estimate methods (e.g., shear bands fabric, porphyroclasts or oblique foliation in quartz).
The analyses were performed on thin sections cut perpendicular to foliation
and parallel to lineation (i.e., the
The C
Electron microprobe analyses (EMPAs) were carried out using a JEOL 8200 Super
Probe EMP at the Dipartimento di Scienze della Terra “A. Desio”,
Università degli Studi di Milano. Quantitative chemical analyses were
performed on carbon-coated petrographic thin sections. Data acquisition was
performed using an accelerating voltage of 15 kV, a beam current of 5 nA
and a spot size of 1
Quantitative chemical analyses were performed on a non-mylonitic orthogneiss
sample (JVL-15), on mylonitic orthogneiss samples (JVL-1, JVL-7, JVL-13,
SCH-4) and on a phyllonite sample (ERY-11), with a total of about 100
points. Chemical analyses are reported in Fig. 6 and Supplement Table S2. Both the first and second generations of micas were analyzed; the
microstructurally older white mica and biotite generation forming the main
mylonitic foliation (hereafter Wm
Considering all six samples, some compositional variations around the
muscovite–celadonite join can be observed, with Si ranging between 3.1 and
3.4 apfu and Al ranging between 2.17 and 2.71 apfu (Fig. 6a). White mica in
ERY-11 is characterized by the highest
The Na
Analyses were performed on biotite from five samples since it was absent
from sample SCH-4. In the sample ERY-11 the
The Si content of biotite in sample ERY-11 is homogenous, while in samples JVL-1, JVL-7, JVL-13 and JVL-15 it shows a cluster characterized by Si content between 2.74 and 2.83 apfu (Fig. 6c). The Ti content in biotite from samples JVL-1, JVL-7, JVL-13 and JVL-15 is clustered at values between 0.08 and 0.17 apfu (Fig. 6d), apart from a biotite-2 from sample JVL-7, which shows a remarkably lower value (0.01 apfu), like sample ERY-11 (0.001–0.006 apfu).
The K content (Supplement Table S2) reveals that in sample ERY-11, all the spot
analyses yielded low, sub-stoichiometric K in Bt
EPMA (electron microprobe analysis) results showing compositional variation in white mica
White mica and biotite used for
Mica samples were irradiated in the McMaster University Research Reactor
(Hamilton, CA), carefully avoiding Cd shielding.
The age spectrum, the correlation diagram (
The
The phyllonites (ERY-3 and ERY-8) contain dispersed carbonates, evidence of
massive fluid circulation (Fig. 3h). Moreover, due to the lower strain rate
and/or lower temperature, the micas did not fully recrystallize during the
Cretaceous faulting and give meaningless mixed ages with substantial Ar
inheritance, and white mica ages are geologically meaningless (Fig. 9a–d).
The age of mylonitic foliation has been constrained to be 92.58
Although the Vinschgau Shear Zone (VSZ) is one of the largest thrust-sense
shear zones exposed in the Alps, no age constraints existed on the shearing
activity of this huge intra-Austroalpine thrust, with its Late Cretaceous
age only inferred on the basis of indirect evidence (Schmid and Haas, 1989)
and a single Rb–Sr whole-rock age of a deformed pegmatite (Thöni, 1986).
Our
The evolution of shear zones has been deeply investigated in terms of
spatial variation, especially concerning their length and thickness (Hull,
1988; Mitra, 1992; Means, 1995; Vitale and Mazzoli, 2008, 2010; Fossen,
2016; Fossen and Cavalcante, 2017). This effort was aimed at defining the
parameters that may influence the evolution of one type of shear zone with
respect to another one. If the growth in length of a shear zone is
essentially due to linkage of different branches forming a composite system
of shear zones (Fossen and Cavalcante, 2017), the growth in thickness may be
influenced by different mechanisms. Four ideal models of shear zone
evolution have been proposed and discriminated based on shear strain
gradient, kinematic vorticity, and plane or triaxial strain (Vitale and
Mazzoli, 2008; Fossen and Cavalcante, 2017). Processes of strain hardening
or strain softening promote the thickening and the thinning of the shear
zone, respectively type 1 or type 2 models. In the type 1 model, the deformation concentrates
in the margins of the shear zone, leaving the inner portion inactive. In contrast, in the type 2 model the deformation shifts and concentrates in the inner
portion of the shear zone as strain accumulates, leaving the margins
inactive. In addition, the type 3 model is related to a strain-weakening process, even
if its active thickness remains constant with time. The type 4 model expands in
thickness, but, unlike type 1, all the thickness remains active through time.
According to the several models proposed for shear zone evolution (Fossen
and Cavalcante, 2017, with references), the VSZ followed a type 2 evolutionary
model, with increasing cumulative shear strain from margins to the core of
the shear zone (Fig. 10). This pattern of shear strain distribution is
demonstrated for the VSZ by the occurrence of ultramylonites at the core of
the Juval transect (Fig. 10), whereas protomylonites derived from the
Partschinser orthogneiss are preserved only at the margins. The kinematic
vorticity of flow follows the same symmetric distribution, with
Schematic cross-section of Juval transect showing the occurrence
of protomylonites, mylonites and ultramylonites developed along the strain
gradient from the rim to the core of the shear zone, with
The VSZ deepens from west to east, as suggested by previous workers (Schmid and
Haas, 1989) and confirmed by the present data, with white mica–chlorite
phyllonites overprinting white mica–biotite mylonites in the western part of
the shear zone. Broadly, the
Beyond the deformational path of the VSZ, which clearly follows a strain-softening type 2 shear-zone-evolution model, the novelty of our work is the integration of microstructural and kinematic analyses with age profiling of the shear zone along its transport direction, along its depth and across its strike (Fig. 11). The obtained age pattern allows the reconstruction of a time-resolved evolution of the shear zone during its progressive activity and exhumation.
The
The evolution of the Eo-Alpine orogenic wedge of the eastern Alps is generally related to the closure of the Meliata Ocean, located in an intra-Austroalpine position (e.g., Schmid et al., 2004) or separating the former Austroalpine and Southalpine domain (e.g., Neubauer et al., 2000). Other interpretations consider instead the possibility that the entire Austroalpine orogenic wedge formed in the Late Cretaceous in a pre-collisional setting (Zanchetta et al., 2012, 2015).
Irrespective of the geodynamic scenario of the Eo-Alpine orogen in the eastern Alps, the VSZ acted as a crustal-scale shear zone promoting nappe stacking and exhumation within an orogenic wedge chiefly made of continent-derived tectonic units. The ages of shearing along the VSZ indicate that the shear zone was already active at 97 Ma (Figs. 7, 8 and 11), at least 7–8 Myr before the pressure peak recorded by the Texel eclogites and the amphibolitic peak in the Schneeberg unit. The ages of the VSZ in the Schlanders transects overlap with published/available ages related to the peak of Alpine metamorphism in the Ötztal basement, suggesting that thrusting within the Austroalpine domain started where units in the hanging wall of the VSZ had already reached (Ötztal) or were close to the metamorphic peak (Texel and Schneeberg units). This age overlap between shearing and metamorphic peak is explained by a rapid exhumation of these HP rocks via thrusting in the Eo-Alpine orogenic wedge. Handy et al. (2010) argued that the Eo-Alpine orogen, in the time span 118–84 Ma, was subjected to intracontinental subduction and basement nappe stacking, whose evidence is the E–W-trending belt of Late Cretaceous, HP rocks of the Koralpe–Wölz nappe complex (Schmid et al., 2004; Thöni et al., 2008), to which the Texel and Schneeberg units belong. The exhumation of these HP rocks was both westward and northward (Handy et al., 2010).
Evolution of the Vinschgau Shear Zone in the orogenic Eo-Alpine
wedge during the Late Cretaceous. Activation of the VSZ
The WSW-directed Late Cretaceous thrusting (Eisbacher and Brandner, 1996; Ratschbacher, 1986; Schmid and Haas, 1989; Viola et al., 2003; Handy et al., 2010) and consequent nappe stacking under pressure-dominated, amphibolite- to greenschist-facies conditions are unusual if compared to major NNW-directed direction of transport for other sectors of the Alps. This scenario has been explained through a complex microplate configuration at 94 Ma, which necessarily invokes the faster eastward accommodation of the Iberia microplate with respect to the Adria microplate, also moving eastward with respect to Europe, but at lower rates (Handy et al., 2010).
The age of the pressure metamorphic peak within HP units has been reported to be between 95 and 89 Ma (Thöni, 2002; Habler et al., 2006; Miller et al., 2005; Thöni et al., 2008; Janák et al., 2009; Zanchetta et al., 2013), with partially overlapping ages related to their rapid exhumation (e.g., Fügenschuh et al., 1997; Sölva et al., 2005).
As the Texel and the Ötztal units are now in tectonic contact at the
eastern termination of the VSZ, at least 20 km of vertical exhumation of the
Texel unit should have been accommodated along the VSZ, considering the
difference in peak metamorphic pressure recorded by the two units (Fig. 11a): 1.2–1.4 GPa for the Texel unit (Habler et al., 2006) and 0.5–0.6 GPa
for the Ötztal basement (Purtscheller and Rammlmair, 1982). Considering
a lithostatic pressure gradient of 0.03 GPa km
The VSZ yields ages in the range of 80 Ma (the youngest age at the inner portion of the Juval transect) to 97 Ma (the oldest age in the Schlander area), almost coeval with the peak metamorphic age in the Ötztal and Texel units at ca. 85–90 Ma (Zanchetta et al., 2013; Fig. 11). Hence, we argue that the VSZ has been active at least for 17 Ma, promoting the rapid exhumation of the Texel eclogite in the Late Cretaceous. Syn-shearing exhumation of the Texel and Schneeberg units continued at least up to 76 Ma, as testified by the age of greenschist-facies mylonites along shear zones within these two units (Sölva et al., 2005). This scenario was also depicted by Handy et al. (2010), who suggested a rapid exhumation of the Koralpe–Wölz units during the Cenomanian–Santonian (ca. 94–84 Ma), when the Gosau Group (syn- to post-orogenic clastic sediments sometimes deposited in intra-orogenic extensional basins) sealed thrusts in the Austroalpine basement and in the northern Calcareous Alps.
Exhumation models for crystalline rocks have been extensively proposed; modeled; and tested for Himalayan orogen, where the exhumation of the metamorphic core of the belt was a matter of debate in the literature (e.g., Montomoli et al., 2015; Carosi et al., 2018; Montemagni, 2020). Regardless of the specific model, the scientific community agrees that exhumation in the Himalayas has been driven by two opposite shear zones bounding the crystalline core: a normal-sense at its top and a thrust-sense shear zone at its bottom, respectively (e.g., Godin et al., 2006; Montomoli et al., 2013, for reviews). Based on structural and geochronological data, a similar model was also proposed for the eclogite type locality in the Saualpe region (Wiesinger et al., 2006). A recent study (Schulz and Krause, 2021) documented the younger age in amphibolite-grade footwall units underlying the eclogite-bearing unit.
From this perspective, we argue that the VSZ played a key role in the
exhumation of HP rocks in the eastern Alps, and we can speculate that the
units containing HP rocks have been exhumed by a (not necessarily coeval)
shearing of a thrust-sense shear zone at the base and a normal-sense shear
zone at the top of the orogenic wedge, i.e., above the Ötztal–Stubai
complex. Considering the jump in the metamorphic grade and age of
metamorphism between the underlying Eo-Alpine Ötztal–Stubai complex
(garnet amphibolites facies) and the quartz-phyllites (ca. 350
Besides the geodynamic interpretation, the presented data indicate that the
VSZ had a long-lasting evolution of at least 17 Myr. The minimum
accommodated displacement of ca. 40 km implies a displacement rate of 2–2.5 mm yr
The VSZ is one of the prominent intra-basement thrust-sense shear zones developed in the Alps, promoting the exhumation of HP rocks within the Eo-Alpine orogenic wedge.
Our approach fully constrains its kinematic and temporal evolution:
Cumulative shear strain and kinematic vorticity values reveal an evolution
compatible with a type 2 thinning shear zone. The novelty of our work is the combination of microstructural and kinematic
analyses with age profiling of the shear zone both along its transport
direction and across its strike.
Our original data (EMPA analyses on micas and Ar–Ar data) are provided in the Supplement with public access (Tables S2 and S3).
The supplement related to this article is available online at:
SZ, AZ and CM designed the study, and CM and MR carried it out. The paper was prepared by CM and SZ and revised by AZ with the contribution of all co-authors. All authors participated in fieldwork and in the various scientific discussions.
The contact author has declared that none of the authors has any competing interests.
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We warmly thank the reviewers Franz Neubauer and Paolo Conti for their careful and significant comments that improved the paper and Yang Chu for the editorial handling of the manuscript. We are also grateful to Hannah Pomella for corrections of the German toponyms and discussion about the Vinschgau geology that helped us to clarify some aspects. We thank Andrea Risplendente (Università degli Studi di Milano) for his support during the electron microprobe analyses and Valentina Barberini (Università degli Studi di Milano – Bicocca) for the maintenance of the mass spectrometer.
This research has been supported by the Ministero dell'Università e della Ricerca (grant no. 2021-NAZ-0299, CUP: J33C22000170001) and the Provincia Autonoma di Bolzano – Alto Adige (grant no. 2022-NOECO-0131 PROGETTO MALLES).
This paper was edited by Yang Chu and reviewed by Paolo Conti and Franz Neubauer.