Search

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. 350-400C 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 97Ma. 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 consis

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper CarboniferousPermian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper CarboniferousPermian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

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.

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 50 km, 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.

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.

L’Himalaya è classicamente considerata una catena orogenica strutturalmente cilindrica per l’impressionante continuità laterale, da ovest ad est, delle principali unità lito-tettoniche e zone di taglio, caratteristica peculiare di questa catena collisionale. L’esumazione del cuore metamorfico della catena, il Greater Himalayan Sequence (GHS), è favorita dall’attività di due zone di taglio regionali a cinematica opposta: la Main Central Thrust zone (MCTz) a cinematica compressiva e il South Tibetan Detachment System (STDS) a cinematica normale, rispettivamente alla base e al tetto del GHS stesso. In questa tesi ho studiato l’evoluzione strutturale e geocronologica del STDS e della MCTz, con particolare focus su quest’ultima, in due transetti nell’Himalaya indiana occidentale: le valli dell’Alaknanda – Dhauli Ganga e la valle del Bhagirathi – Gangotri nella regione del Garhwal. A questo scopo, ho adottato un approccio multidisciplinare che combina studi microstrutturali, chimici e geocronologici e stime di vorticità cinematica. Poiché le miche sono ubiquitarie nelle zone di taglio, il metodo geocronologico 40Ar/39Ar su biotitie e muscovite è stato ampiamente utilizzato in passato e viene utilizzato tutt’oggi per vincolare l’età della deformazione per taglio. Il metodo 40Ar/39Ar step-heating, il più adatto per questo tipo di studi, è un metodo chiave per risolvere complessità petrologiche e chimiche grazie al riconoscimento di età differenti dovute a un differente rilascio dell’Ar caratterizzato da diversi rapporti Cl/K e Ca/K. Ho applicato questo metodo, combinato con una nuova procedura, l’Ar Differential Release Plot (DRP), che permette di identificare chiaramente l’influenza della coesistenza di fillosilicati nel trend di rilascio dell’Ar durante gli step di riscaldamento, permettendo di selezionare gli step ottimali che corrispondono al degassamento delle miche in senso stretto. Questa nuova procedura permette di determinare l’età in modo molto più, The Himalaya is commonly regarded as a cylindrical belt from west to east due to the impressive lateral continuity of the main litho-tectonic units and faults/shear zones, which is a peculiar feature of this mountain range. The exhumation of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), was favored by two regional scale opposite-kinematics ductile to brittle shear zones: the contractional Main Central Thrust zone (MCTz) at the bottom and the normal-sense South Tibetan Detachment System (STDS) at the top of the GHS itself. In this thesis, I investigated the structural and geochronological evolution of the STDS and the MCTz, with a particular focus on the latter, in two transects in the Indian Western Himalaya: the Alaknanda – Dhauli Ganga Valleys and the Bhagirathi – Gangotri Valley in the Garhwal region. To this aim, I used a multidisciplinary approach, which combines microstructural, chemical and geochronological studies, as well as a kinematic estimate. As micas are ubiquitous in strongly deformed shear zones, the 40Ar/39Ar geochronological method on biotite and muscovite has commonly been employed to constrain the ages of mylonitization. The 40Ar/39Ar step-heating approach, the most useful for the present study, is a key procedure to unravel petrological and chemical complexities because of the recognition of different ages due to different steps of Ar release characterized by different Cl/K and Ca/K ratios. I applied this method, combined with a new methodological approach first developed in detail during this Ph.D. thesis work, to rocks coming from the bounding shear zones of the GHS. This new procedure, named Ar Differential Release Plot (DRP), that allows to clearly identify the influence of the co-existence of phyllosilicates on the trend of Ar release during the heating steps, allowing to select the optimal steps corresponding to the degassing of micas sensu stricto, which leads to more reliable age determinations in such metamorph

Constraining the timing of the activity of the Main Central Thrust zone (MCTz) and the South Tibetan Detachment System (STDS) is one of the major tasks to understand Himalayan tectonics. These shear zones are crustal scale ductileto-brittle structures running all along the belt. The MCTz and STDS are the lower and upper boundary, respectively, of the Greater Himalayan Sequence (GHS), the metamorphic core of the orogen. In some areas the MCTz is a km-wide shear zone, whereas in other areas it is bounded by two distinct thrusts. In Garhwal (NW India), the structurally lower and upper MCTz boundaries are the Munsiari and Vaikrita Thrusts (Valdiya, 1980), respectively. We constrain the time of activity of the Vaikrita Thrust by 39Ar-40Ar dating of biotite and muscovite from two garnet-bearing mylonitic micaschists and one garnet-staurolite-bearing quartzite. Microstructural observations reveal at least three different mica growth stages with muscovite being larger and more abundant than biotite. Mica-1 highlights a relict foliation, only locally preserved, at high angle with respect to the main mylonitic one; mica-2 grew along the main mylonitic foliation; its small flakes are often shredded; mica-3 forms coronitic structures around garnet porphyroclasts. EPMA on muscovite shows restricted chemical variation, with Ti, Mg and Fe contents being systematically lower in muscovite-3. Biotite from micaschists shows no significant chemical variations, whereas in quartzite biotite-2 shows two distinct compositional clusters. Rocks were crushed and sieved. Biotite and muscovite were separated by gravimetry and extensively handpicked. Biotite step ages range between 8.6 and > 12 Ma, and muscovite step ages between 3.6 and > 7 Ma. As all samples are from the same 10 m wide outcrop, "cooling ages" should be equal. Instead, their large variations are petrogenetically controlled. Since chloritization is pervasive in all samples, we use the Ca/K ratio to identify Ar released from mica

Deformation and fuid flow in the Munsiari Thrust (NW India): a preliminary fluid inclusion study. A fluid inclusion study was carried out on quartz veins deformed during the activity of the Munsiari Thrust in the Garhwal Himalaya (NW India, Western Himalaya). These veins are hosted in mylonitic phyllites, with a greenschist mineral assemblage made of quartz-white mica-biotite-chlorite-epidote and minor calcite and plagioclase. Within the veins two groups of isolated "early fluid" inclusions and trails of small secondary fluid inclusions have been observed. Two types of inclusions preserved in isolated groups have been petrographycally recognized: (i) a two liquids and a vapor phase (L1+L2+V) that is the most common type and (ii) a less frequent liquid and a vapor phase (L+V). Both types of fluid inclusions were investigated through optical microthermometric experiments. For "early fluid" inclusions, assuming a trapping temperature based on quartz dynamic recrystallization mechanisms and previous P-T estimates, in the range of 500-520°C, a corresponding trapping pressure in the range of c. 0.50-0.53 GPa is estimated. The trapping pressure range evaluated in the present contribution support that these fluid inclusions have been entrapped during the early stages of the activity of the Munsiari Thrust.

Deformation and fuid flow in the Munsiari Thrust (NW India): a preliminary fluid inclusion study. A fluid inclusion study was carried out on quartz veins deformed during the activity of the Munsiari Thrust in the Garhwal Himalaya (NW India, Western Himalaya). These veins are hosted in mylonitic phyllites, with a greenschist mineral assemblage made of quartz-white mica-biotite-chlorite-epidote and minor calcite and plagioclase. Within the veins two groups of isolated "early fluid" inclusions and trails of small secondary fluid inclusions have been observed. Two types of inclusions preserved in isolated groups have been petrographycally recognized: (i) a two liquids and a vapor phase (L1+L2+V) that is the most common type and (ii) a less frequent liquid and a vapor phase (L+V). Both types of fluid inclusions were investigated through optical microthermometric experiments. For "early fluid" inclusions, assuming a trapping temperature based on quartz dynamic recrystallization mechanisms and previous P-T estimates, in the range of 500-520°C, a corresponding trapping pressure in the range of c. 0.50-0.53 GPa is estimated. The trapping pressure range evaluated in the present contribution support that these fluid inclusions have been entrapped during the early stages of the activity of the Munsiari Thrust.