Abstract
Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen. In the Gyirong area in south Tibet, deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex (GHC) constitute south-vergent asymmetric folds. The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes: an earlier top-to-north extension and a later top-to-south thrusting, implying a tectonic transformation from N-S extension to N-S shortening. Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma. The data suggest that the tectonic transformation occurred after 18.7 Ma. The chronological data of South Tibet detachment system (STDS) and North-South trending rift (NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma. The transformation of tectonic regime was probably controlled by the India-Asia convergence rate. An increase in the convergence rate resulted in N-S shortening of the orogen, thrusting and folding, with coeval formation of the NSTR in Tibet. A decrease in the convergence rate led to N-S extension and reactivation of the STDS.
Highlights
Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen
The chronological data of South Tibet detachment system (STDS) and North-South trending rift (NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19–13 Ma
The Himalayan orogen lies between the Yarlung Zangbo suture (YLZBS) to the north and the Main frontal Thrust (MFT) to the south, and comprises, from north to south, the Tethyan Himalaya sequence (THS), Greater Himalayan crystalline complex (GHC), Lesser Himalaya sequence (LHS) and Siwalik foreland basin sediments, separated by the south Tibet detachment system (STDS), Main Central Thrust (MCT), Main Boundary Thrust (MBT) and the MFT respectively (Figure 1) [1,2,3]
Summary
The Himalayan orogen in Gyirong area can be divided into five units from south to north: the GHC, the STDS, the THS, the late Cenozoic sedimentary basins and the NHGD (Figure 2) [1]. The STDS in Gyirong area represents a large-scale ductile shear zone with a width of over 10 km. It consists of mylonitic granitic gneiss and foliated leucogranite [22]. Exposed to the north of Gyirong basin are Peiku Co and Malashan domes These two domes are cored by two-mica granite/leucogranite and mylonitic granitic gneiss [23,24], mantled by garnet two-mica schist in the middle, and overlaid by the slate and phyllite of THS. Some of the plagioclase grains show alternation to fine muscovite (Figure 3(d))
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