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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, P–T conditions and timing of motion of the VSZ. 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 ∘C during shearing. Timing of deformation along the VSZ has been constrained for the first time through 40Ar/39Ar dating of syn-shearing micas, which reveal a Late Cretaceous age of the VSZ mylonites with ages ranging between 80 and 97 Ma. A systematic younging age of deformation occurs towards the central part of the shear zone in the studied transects. Vorticity analysis shows a clear decrease in the simple shear component correlated to the younging of mica ages towards the core of the shear zone. This evolution is consistent with the growth of a shear zone where shear strain localizes into its central part during deformation. The defined evolution of the VSZ sheds new light on how large-scale thrust-sense shear zones act and how much exhumation they can accommodate in the frame of an evolving orogenic wedge.

The WSW-ENE Meran-Mauls Fault (MMF) represents a break in the broadly E-W trending Periadriatic Fault (PAF), separating the Southern Alps from the N-verging wedge of the European Alps. Understanding the MMF evolution is mandatory to reconstruct the role of the entire PAF during the evolution of the belt since the late Cenozoic. Structural and microstructural analyses and paleostress calculations based on fault-slip data suggest the occurrence of four evolutionary stages for the MMF: 1) top-to-the-SE mylonites; 2) top-to-the-SE brittle faulting; 3) dextral faulting with re-activation of previous structures; 4) N–S compression associated with conjugate sets of mainly strike-slip fault systems. Paleostress reconstructions point to a σ2-σ3 permutation from stage 2 to stage 3, resulting in the switch from pure thrusting to strike-slip, followed by an anticlockwise rotation of the principal stress axes during stage 4, in a strike-slip regime related to N–S compression. Geochronological and thermochronological data point to 39-22 Ma age for stage 1, 22-17 Ma for stage 2, less than 17 Ma for stage 3. Our reconstruction strongly supports the interpretation of the MMF as a restraining bend of the PAF, which was weakly reactivated along its eastern portion as a dextral fault and later displaced by NNE-SSW left-lateral faults.

This contribution focuses on geochronological and kinematic data on the Main Central Thrust zone (MCTz) in the Garhwal Himalaya (NW India). A multidisciplinary approach combining detailed 2D and 3D microstructural observations with a new interpretation key of geochronological data on micas is here applied to reconstruct the polyphase evolution of a crustal-scale shear zone. The Ar Differential Release Plot and the use of X-ray micro-Computed Tomography allow to disentangle misleading Ar/Ar spectra of micas from mylonites and to avoid methodological limitation in vorticity estimates, respectively. This new approach allows to unravel the microstructural and petrological complexities linked to the temporal and kinematic evolution of the MCTz. The obtained data indicate a structurally downward shift of deformation, i.e. towards the foreland, across the MCTz, with an increasing pure shear component mirrored to a progressive rejuvenation of the mica age of structures.

The Vinschgau Shear Zone (VSZ) is an E-W striking ductile shear zone developed within the central-eastern Austroalpine domain. Along the VSZ the Ötztal-Stubai nappe with an amphibolite facies Alpine metamorphism overthrusted the Campo nappe (and respectively the Engadine Dolomites) characterized by low-to-medium metamorphic conditions (Schmid and Haas, 1989). In the eastern part the shear zone involves also the Texel Unit at the base of the Ötztal nappe. Foliation gently dips (20°-30°) northward and lineation constantly strikes E-W, with kinematic indicators that point to a top-to-NW sense of shear. The VSZ, deepening to the E, developed mostly under amphibolite to greenschist-facies conditions. A remarkable variation in finite strain along its length can be observed: lenses of protomylonites are bounded by deeply sheared ultramylonites, mylonites and phyllonites, these occurring in the western part of the VSZ.In order to fully assess the evolution of the VSZ a multidisciplinary approach based on structural and petrochronological analyses has been carried out on three representative transects of the shear zone from W to E: Eyrs, Schlanders and Juval transects.Our fieldwork based analyses suggest that the dip of mylonitic foliation increases from W to E and the lineation dips towards the W in Schlanders and Juval, whereas towards the E in Eyrs, probably due to a folding event related to Cenozoic shortening (Pomella et al., 2016).Chemical and microstructural analyses suggest that deformation temperatures of 350-400 °C have been reached during shearing, as testified by Ti content in biotite and by bulging and subgrain rotation as dominant recrystallization mechanisms in quartz.Timing of deformation along the VSZ has been constrained through 40Ar/39Ar dating of syn-shearing micas confirming previous geochronological data of Thöni, 1981, which reveal a Late Cretaceous age of the VSZ mylonites. A systematic younging trend of deformation towards the central part of the shear zone has been observed in the studied area. Vorticity analyses, both through shear bands and rigid porphyroclasts methods, show a clear decrease in simple shear component correlated to the younging direction of mica ages (i.e. towards the core of the shear zone). This evolution is clearly consistent with Type 2 evolution of Fossen and Cavalcante (2017), where strain localizes into the core of the shear zone during deformation.Our data confirm the age of the VSZ only supposed on the base of indirect observations and demonstrate that the Austroalpine domain was severely affected by W-NW verging thrust stacking much time before the Adria-Europe continental collision. ReferencesFossen, H. & Cavalcante, G.C.G., (2017) - Shear zones–A review. Earth-Science Reviews, 171, 434-455.Pomella, H., Flöss, D., Speckbacher, R., Tropper, P., & Fügenschuh, B. (2016) - The western end of the Eoalpine High‐Pressure Belt (Texel unit, South Tyrol/Italy). Terra Nova, 28(1), 60-69.Schmid, S.M., & Haas, R. (1989) - Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps. Tectonics, 8(4), 697-718.Thöni, M. (1981): Degree and evolution of the Alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determinations on micas. - Jb. Geol. B.A. 124, 1: 111-174.

The Meran-Mauls Fault (MMF) connects the North Giudicarie Fault with the Pustertal segment of the Periadriatic Fault System (PFS), which separates the Southern Alps from the Europe-vergent Alpine collisional belt. Our study is based on detailed multiscale analyses carried out within the CARG project (National Italian Cartography) for the survey of the Meran (Bargossi et al., 2010), Sankt Leonhard and Sterzing geological sheets of the 1:50,000 geological map of Italy. In the frame of this project, we performed detailed geological mapping along key-sections, mesoscopic and microscopic structural analyses of fault rocks, vorticity analyses of mylonites, paleostress estimates and radiometric dating of pseudotachylytes. Although several models suggest that the PFS developed in a dextral transpressional regime during the Cenozoic, following the Adria-Europe collision, our analyses suggest that the Meran-Mauls Fault and related structures were active with a marked reverse kinematics (Viola et al., 2001), displaying several switches in the late stages between ductile and brittle deformation. Reverse motion is chiefly suggested by down-dip mineral lineations in early mylonites developed in greenschist-facies conditions within the Austroalpine units exposed along the hanging wall of the fault zone, as well as in the footwall within the Brixen Phyllites and in the Ifinger Permian intrusive, both belonging to the crystalline basement of the Southern Alps. SE-vergent thrusting was followed in time by a dextral reactivation in brittle conditions, which is evident especially in the eastern portion of the MMF starting from the Pennes Pass area. Our reconstruction is corroborated by the analyses of other important fault zones mainly developed in the Austroalpine thrust stack in the hanging wall of the MMF, showing a similar kinematic evolution. Strike-slip to normal faults with different trends ranging from NW-SE to NE-SW followed in time the dextral motion along the MMF, displacing previous structures. Paleostress reconstructions indicate a progressive switch of the main direction of compression from NW-SE to N-S, as previously suggested by us in the Central Alps (Agliardi et al., 2009).ReferencesAgliardi F., Zanchi A., Crosta, G.B. (2009) – Tectonics v. gravitational morphostructures in the central Eastern Alps (Italy): Constraints on the recent evolution of the mountain range. Tectonophysics, 474, 250-270.Bargossi G.M., Bove G., Cucato M., Gregnanin A., Morelli C., Moretti A., Poli S., Zanchetta S. & Zanchi A. (2010) - Note illustrative della Carta Geologica d’Italia a scala 1:50.000 Foglio 013 Merano. CARG. ISPRA, 285 pp.Viola G., Mancktelow N.S., Seward D. (2001) - Late Oligocene-Neogene evolution of Europe-Adria collision: New structural and geochronological evidence from the Giudicarie fault system (Italian Eastern Alps). Tectonics, 20, 999–1020.

Crustal scale low-angle normal faults are typical tectonic features in orogenic post-collisional setting driving the exhumation of deep portions of the orogenic wedge. These extensional structures are commonly active at mid to upper crustal levels within quartz- and feldspar-rich rocks. As deformation localizes along these large-scale shear zones, the understanding of mechanisms controlling their development could provide invaluable insights on the rheology of the continental lithosphere. PT ambient conditions, differential stress, pore fluid pressure and time duration of activity are all factors that could significantly operate on how a shear zones develops in space and time.We investigated by means of a quantitative approach the evolution of the Simplon Fault Zone (Western Alps, N Italy – Switzerland). We took into account: (i) meso- and microstructures distribution across the shear zone, (ii) its time of activity by 40Ar/39Ar dating of syn-shearing micas, (iii) vorticity distribution across the shear zone and its correlation with mylonite ages, (iv) the estimates of magnitude and variation of differential flow stress and strain rates during shear zone evolution obtained through EBSD-assisted quantitative microstructural analysis. All these data have been combined to reconstruct the structural evolution of the shear zone as the result of the rheological response of involved rocks to changing PT and stress conditions.The Simplon Fault Zone formed as an extensional detachment accommodating E-W directed lateral extrusion after the collision between Adria and Europe. Several tens of kilometres of extension were accommodated by this structure, allowing the exhumation of the deepest portions of the Central Alps. The shear zone evolved from epidote-amphibolite to greenschist facies and then brittle conditions during shearing. A decrease of simple shear component from c. 90% to c. 40% towards the top of the shear zone is observed, with mylonites displaying ages within the 12-8 Ma time interval. Calculated differential stress (60-80 MPa) and strain rate (10-11-10-12 s-1) estimates are in agreement with values displayed by several others crustal-scale low-angle normal faults developed at medium to shallow crustal levels.The quantitative approach used at different scales pointed out that the Simplon Fault Zone experienced a complex evolution, with shear strain that was heterogeneously distributed across the fault zone. Despite this heterogeneity, a general decrease of the simple shear component and increase of the differential flow stress toward the top of the shear zone is clearly defined.

Vorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two-dimensional data. We describe a 3D approach based on the use of X-ray micro-computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non-coaxial shearing. 40Ar/39Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow.

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales << 10 µm and almost always altered to various degrees. We describe here 39Ar-40Ar stepheating using the combination of two independent indicators that allow the discrimination of coexisting mica generations from each other and from the ubiquitous retrogression/alteration phases. A necessary first step is electron probe microanalysis to assess both inventory and spatial distribution of the mineral phases that need to be distinguished a posteriori by 39Ar-40Ar systematics. One indicator is based on mica stoichiometry, which can be proxied by the 39Ar concentration in combination with the 37Ar/39Ar and 38Ar/39Ar (i.e. Ca/K and Cl/K) ratios. The other indicator is the furnace temperature, at which a degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on the average bond strength in the crystal structure, it is predicted (and indeed observed) that the temperature of the differential Ar release peak is variable among different minerals. As the Ca/Cl/K signatures of pure micas coincide with the Ar release peak, their combination identifies the isochemical steps that correspond to the degassing of pristine micas. Only these should be used to date the activity of shear zones.This procedure should become routine in analysing polydeformed metamorphic rocks.

The Periadriatic Fault System (PFS) is one of the most important tectonic element in the Alps, separating the Europe-verging collisional wedge from the S-verging Southern Alps. The PFS developed in a dextral transpressional regime during the Cenozoic, following the Adria-Europe collision. The area between the Passeier and the Eisack rivers (Meran, NE Italy) is a key area for the understanding of the interactions among the PFS, the Giudicarie Fault and the fault network here active in the middle to late Cenozoic. Here the elsewhere E-W trending PFS rotates to a NE-SW trend, impliying significant changes in the fault kinematics and evolution.The NE-SW strand of the PFS, known as the Meran-Mauls fault, is connected to the North Giudicarie Fault to the west and to the Pustertal segment of the PFS to the east. A general evolution from the ductile to brittle deformation regime has been recognized on the base of field-based structural analysis and microstructural analysis of fault rocks. Pseudotachylytes occur all along the fault zone, testifying to the seismic activity of the Meran-Mauls fault. 40Ar-39Ar dating of pseudotachylytes provided ages in the 32-22 Ma time interval, indicating that the PFS experienced a prolonged seismic activity during middle Cenozoic times. Several pseudotachylytes veins show a re-activation as cm-thick ductile shear zones, indicating that the plastic-brittle transition was not sharp in time.Combining the structural analysis of the PFS with other adjacent faults connected in space and time (Passeier fault, Faltleis fault, Val Nova fault and other minor faults) we reconstructed a marked reverse dip-slip kinematics of the Meran-Mauls Fault during a progressive transition across the plastic-brittle regime, followed in time by a dextral transpression. Paleostress reconstructions performed on these faults populations indicate a progressive switch of the main direction of compression from NW-SE to N-S. This switch likely occurred when the Meran-Mauls segment of the PFS definitively passed to a brittle deformation regime.

The structural analysis of large intracontinental wrench faults is fundamental for deciphering the long-term evolution of continental crust in complex areas in terms of their geodynamic evolution and large-scale crustal block displacements. In this contribution, we demonstrate a pre-Miocene dextral activity of the present-day left-lateral Great Kavir - Doruneh Fault System (GKDFS, Central Iran), one of the major intracontinental active strike-slip faults extending from the Afghan border to the Nain region between Central Iran and the Sanandaj-Sirjan Zone. We document important dextral shearing recorded along a segment of the GKDFS, the Arusan Fault System (AFS), located east of Jandaq, close to the present-day active trace of the GKDFS. The AFS include several ENE-WSW striking strands exposed for a length of more than 50km, which couple pre-Cretaceous ophiolites and metamorphic basement units with the Cretaceous succession of the Khur basin. The fault shows transpressional structures consistent with a dextral shear including thrusts and en échelon folds affecting the Cretaceous carbonate units. Paleostress reconstruction based on mesoscopic fault analysis and related folds geometry allowed to establish vorticity parameters indicating that deformation occurred close to a total simple shear regime with a calculated Wk between 0.9 and 1. The enormous Meso-Cenozoic dextral displacements occurred along the AFS and along the entire GKDFS are attested by the up to several hundreds of kilometers offset of the Paleotethys suture zone, from NE Iran to the western border of Central Iran.

The exhumation of the Lepontine Dome in the Central Alps was mainly driven by extensional shear zones at its borders. The Simplon Shear Zone (SSZ), formed as a consequence of east-west lateral extrusion perpendicular to north-south convergence between Adria and Europa plates, has been the leading structure in the exhumation of the western sector of the Lepontine Dome where the deepest rocks of Central Alps are nowadays exposed. We present here a multidisciplinary study of the SSZ combining fieldwork, microstructural analyses, vorticity estimates, quartz c-axis fabric analysis, quartz paleopiezometry and 40Ar/39Ar geochronology. The SSZ evolved from epidote-amphibolite to greenschist facies and then brittle conditions during shearing. A decrease of simple shear component from 88% to 37% towards the top of the shear zone is observed, with mylonites displaying ages within the 12-8 Ma time interval. Differential stress (59–78 MPa) and strain rate (10−11-10−12 s−1) estimates are in agreement with values obtained for crustal-scale low-angle normal faults developed at medium to shallow crustal levels. Our multiscale and multidisciplinary approach points out that the SSZ experienced a complex evolution, with shear strain heterogeneously distributed across the shear zone in the frame of a decrease of the simple shear component and increase of the differential flow stress toward the top of the shear zone.

A peculiar feature of the Himalaya is the occurrence of a system of low-angle-normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during syn-convergent tectonics. We describe the STDS-related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body cross-cutting the STDS, occurs. A detailed multidisciplinary study, integrating field-based, microstructural, petrographic and geochronological analyses was carried out on rocks along this valley. We demonstrate how the non-coaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P) – temperature (T) conditions of 0.9-1.1 GPa and ≥ 750°C at ≥ 24 Ma. This migmatite has been reworked structurally upwards leading to the formation of high-T sillimanite-bearing mylonites. Further upward, medium-T shearing deformed the Malari granite and leucogranite dykes, forming medium-T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar-Ar geochronology, between ~20-15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement and deformation along low-angle-normal faults.

Constraining the temporal evolution of a shear zone is a key problem in the study of a collisional belt (Challandes et al. 2003) also because shear zones are characterized by intense deformation and are preferential path for fluid circulation leading to mineral reactions, giving rise to difficulties in the interpretation of geochronological data. In the Himalayan belt, the Main Central Thrust zone (MCTz) separates the Greater Himalayan Sequence in the hanging-wall from the Lesser Himalayan Sequence in the footwall. To define the location of the MCTz different criteria have been proposed and its evolution has been investigated related to tectonic models. In the Bhagirathi valley (NW India), the km-wide MCTz is bounded by two narrow thrusts with a top-to-the SW sense of shear: the Munsiari at the bottom and Vaikrita at the top. In order to constrain the timing of deformations recorded within the Munsiari and Vaikrita Thrust rocks we combined microstructural, chemical and 40Ar/39Ar geochronological investigations. Microstructural observations on two mylonitic orthogneisses from the Munsiari Thrust reveal the occurrence of a main disjunctive foliation defined by biotite and rare muscovite. EPMA on biotite reveal that, although similar, biotites from the two samples have different Mg/(Fe+Mg) ratio. The incorporation of Al follows the substitution: Al(tot) <> V IMe2+ + Si4+ + ; incorporation of Ti is accomplished by: Ti4+ + 2O2 <> V IMe2+ + 2OH. 40Ar/39Ar stepheating coupled with Ca-Cl-K correlation diagrams constrains biotite growth on the main foliation at ca. 5 Ma. The microstructures of one mylonitic micaschist from the Vaikrita Thrust include three different textural generation of micas: a well-preserved relict foliation, a main mylonitic foliation, and a late generation of static muscovite together with static chlorite, the latter forming coronites around garnet porphyroclasts. EPMA reveal variable degrees of phengitic substitution IV Al3+ + V IAl3+ <> V IMe2+ + IV Si4+ in muscovites with no differences between the three generations; strongly chloritized biotite is depleted in K. 40Ar/39Ar dating constrains muscovite growth on the main foliation around 8 Ma, similar to age of micas along the main foliation in Dhauliganga valley (Montemagni et al., 2018). For both Munsiari and Vaikrita Thrust we prepared biotite and muscovite separates with different degrees of purity in order to quantify the bias given by fine-grained intergrowths of impurity phases, which make 100% mica purity unattainable. Argon Differential Release Plots (DRP) discriminate the structure collapse of micas sensu stricto from that of impurities. Combining DRP with Ca-Cl-K signatures identifies the step ages reliably dating deformation. Therefore, we emphasize the importance of a multidisciplinary approach based on detailed meso and microstructural, chemical and geochronological investigations in dealing with rocks coming from shear zones and showing the occurrence of multiple generations of fine-grained foliations. In the study area, the Bhagirathi valley (NW Himalaya), our results demonstrate an in-sequence shearing towards the foreland from Vaikrita to Munsiari Thrust from ca. 8 to ca. 5 Ma. Challandes et al. (2003) Chem. Geol., 197, 3-19. Montemagni et al. (2018) Geol. Soc. London Spec. Pub., 481, doi: 10.1144/SP481.3

In the Garhwal Himalaya, the Malari granite, a small pluton, intrudes the upper portion of the Greater Himalayan Sequence (GHS) and is deformed in the South Tibetan Detachment System (STDS) shearing. In the present work, we undertook a multidisciplinary approach involving microstructural, chemical and geochronological analyses of the samples in order to constrain the timing of shearing along the STDS. Microstructural observations of samples suggest a switch in deformation features with structural position from the structurally upper samples to the structurally lower one. X-ray maps and electron microprobe analyses reveal a quite uniform chemical composition of muscovite in the three selected samples. 40Ar/39Ar muscovite ages constrain the waning stages of ductile deformation: in rocks ductilely deformed by the STDS activity, they record recrystallization at 16.0-16.5 Ma; the age of static muscovite grown after sillimanite at 14.3 Ma post-dates cessation of movement on the STDS and pre-dates the main activity of the Main Central Thrust in the Western Himalaya.