In the Himalayas the top-to-the-N South Tibetan Detachment System (STDS) is a wide regional shear zone separating the lower high-grade metamorphic rocks of the Greater Himalayan Sequence (GHS) from the overlying Tethyan Himalayan Sequence (THS). Along many sections of the belt STDS is characterized by a lower ductile shear zone, affecting the upper part of the GHS and the amphibolites facies rocks at the bottom of the THS and by an upper brittle fault, above which the very-low-grade to non metamorphic rocks of the THS crop out. It is commonly assumed that the coeval activity of the STDS and the lower Main Central Thrust (MCT), led to the exhumation of GHS including the High Himalayan granites at the point that their contemporaneous shearing become a paradigm for the Himalayan tectonics. Granites are located in the upper part of the GHS, and in the North Himalayan domes, and intrude mainly in the lower ductile shear zone of the STDS. By the way, a large undeformed leucogranite cross-cutting the STDS and intruding both the GHS and the THS has been recognized in Western Nepal (Carosi et al., 2013). U-Pb ages pinpoint a crystallization age at ~ 23-24 Ma constraining the youngest shearing event between the two tectonic units (GHS and THS). Dykes from the upper portion of the granite intrude the low-grade metamorphic rocks of the THS made by biotitebearing quartzites, impure limestone, metarenites and metapelites, characterized by the metamorphic assemblage of calc + qz + ms + bt ± chl and scapolite. Sedimentary structures are well-preserved and contact metamorphism has been observed within a few meters from the granite contact. To investigate the depositional age of THS intruded by the granite, detrital zircons have been extracted by two samples and dated using a laser-ablation, inductively coupled, plasma mass spectrometry. CL features and radiometric results confirm a detrital origin of the zircon grains. Age spectra point out that the younger data, indicate a depositional age from Upper Jurassic to Lower Cretaceous. These findings confirm that the ~ 23-24 Ma undeformed granite intruded the THS up to the Cretaceous layers and rule out the occurrence of a tectonic discontinuity between GHS and THS active after ~ 23-24 Ma. Being the MCT active between 25 and 17 Ma in Western Nepal, our results limit the contemporaneous activity of the MCT and STDS to a short period of time (~1-2 Ma) and thus argue against generally accepted exhumation models of the GHS requiring prolonged contemporaneous activity of the two shear zones. For western Nepal, we suggest an exhumation model driven by shear zones propagating from N to S allowing the exhumation of different slices starting from the upper portion to the lower one of the GHS.

Relations between South Tibetan Detachment System, Tethyan Sedimentary Sequence and High Himalayan granite: a new perspective

MONTOMOLI, CHIARA;IACCARINO, SALVATORE;
2015-01-01

Abstract

In the Himalayas the top-to-the-N South Tibetan Detachment System (STDS) is a wide regional shear zone separating the lower high-grade metamorphic rocks of the Greater Himalayan Sequence (GHS) from the overlying Tethyan Himalayan Sequence (THS). Along many sections of the belt STDS is characterized by a lower ductile shear zone, affecting the upper part of the GHS and the amphibolites facies rocks at the bottom of the THS and by an upper brittle fault, above which the very-low-grade to non metamorphic rocks of the THS crop out. It is commonly assumed that the coeval activity of the STDS and the lower Main Central Thrust (MCT), led to the exhumation of GHS including the High Himalayan granites at the point that their contemporaneous shearing become a paradigm for the Himalayan tectonics. Granites are located in the upper part of the GHS, and in the North Himalayan domes, and intrude mainly in the lower ductile shear zone of the STDS. By the way, a large undeformed leucogranite cross-cutting the STDS and intruding both the GHS and the THS has been recognized in Western Nepal (Carosi et al., 2013). U-Pb ages pinpoint a crystallization age at ~ 23-24 Ma constraining the youngest shearing event between the two tectonic units (GHS and THS). Dykes from the upper portion of the granite intrude the low-grade metamorphic rocks of the THS made by biotitebearing quartzites, impure limestone, metarenites and metapelites, characterized by the metamorphic assemblage of calc + qz + ms + bt ± chl and scapolite. Sedimentary structures are well-preserved and contact metamorphism has been observed within a few meters from the granite contact. To investigate the depositional age of THS intruded by the granite, detrital zircons have been extracted by two samples and dated using a laser-ablation, inductively coupled, plasma mass spectrometry. CL features and radiometric results confirm a detrital origin of the zircon grains. Age spectra point out that the younger data, indicate a depositional age from Upper Jurassic to Lower Cretaceous. These findings confirm that the ~ 23-24 Ma undeformed granite intruded the THS up to the Cretaceous layers and rule out the occurrence of a tectonic discontinuity between GHS and THS active after ~ 23-24 Ma. Being the MCT active between 25 and 17 Ma in Western Nepal, our results limit the contemporaneous activity of the MCT and STDS to a short period of time (~1-2 Ma) and thus argue against generally accepted exhumation models of the GHS requiring prolonged contemporaneous activity of the two shear zones. For western Nepal, we suggest an exhumation model driven by shear zones propagating from N to S allowing the exhumation of different slices starting from the upper portion to the lower one of the GHS.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/781855
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