The metamorphic conditions and the age of thermal overprint were determined in meta-pelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kübler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K-Ar and zircon and apatite (U-Th)/He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage (c. 170 °C) and the amphibolite facies (c. 600 °C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c. 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum green-schist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchi- to epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c. 24-22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times (c. 13 Ma). Post-metamorphic cooling below c. 180 °C lasted until Late Miocene and took place at different times
Anisotropy of magnetic susceptibility (AMS) combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the Triassic flysch within the eastern Tethyan Himalaya Thrust Belt in SE Tibet. The attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron-bearing silicates. Two different tectonic domains can be defined: (a) the southern domain is controlled by the Eohimalayan tectonic foliation (S1) recorded in the magnetic foliation which trends east–west and dips to the north; (b) the northern domain is dominated by the Neohimalayan magnetic foliation with WNW–ESE strike and dips to the south opposite to the vergence of the main structures. A slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of S1 and the subtle development of the S2 tectonic foliation. Hinterland propagation of the deformation lead to the Great Counter backthrust generation, pointed out by the SSW steeply plunging magnetic lineation. Furthermore different orientations of magnetic foliation may indicate an Early Miocene c. 20° clockwise vertical-axis rotation.
The SE Tibetan area is a key region to better understand Tibetan Plateau formation. Lateral motion is evident by the alignment of major rivers in Southeast Asia and GPS velocities indicating motion around and away from the Eastern syntaxis. We try to find vertical axes rotations utilizing paleomagnetic studies combined with geological-petrological investigations to clarify the process of uplift of the Himalaya-Tibetan Plateau in the south eastern Tibet.A total of 17 sites with about 10 cores per site were drilled in Lower Cretaceous diorite dykes. These dykes intruded widespread into the Triassic flysch of the Tethyan Himalaya. A total of 11 dykes were drilled in Nagarze area and 6 sites more to the east (south of Tsetang-Gyaca) (Figure 1).Rock magnetic analysis demonstrates the presence of pyrrhotite as the principle carrier of the remanence. IRM saturates from 300 mT to 500 mT and thermal demagnetization of SIRM is mainly achieved around the Curie temperature of pyrrhotite (325°C). Most of the samples show also a decay of the SIRM around 580°C indicating the additional presence of magnetite.The values of the intensity of the natural remanent magnetization varies strongly from 0.5 to 446 mA\m. In about 50% of the samples it is possible to isolate a significant pyrrhotite component, usually unblocking between 250°C and 350°C. Equal area projection mostly shows a scattered distribution of the pyrrhotite components. In three sites the characteristic pyrrhotite remanence components are well grouping and in four sites a small circle distribution can be observed. We relate the origin of the pyrrhotite remanence to last metamorphic cooling in the area (K/Ar ages indicate ~24 Ma). Sulphor for pyrrhotite formation has been likely delivered from the adjacent schists during peak metamorphism. The remanence is therefore probably secondary,
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