Slab tear of subducted Indian lithosphere beneath the Eastern Himalayan Syntaxis region

Abstract

In the southeastern Tibetan Plateau, a series of region-scale dextral strike-slip shear zones play important roles in accommodating the continental collision and continental subduction during India-Asia convergence. This study provides structural, kinematic and geochronological data along the Dulongjiang shear zone, a newly recognized region-scale dextral strike-slip zone around the Eastern Himalayan Syntaxis (EHS) region. The structures and kinematic indicators record dextral lateral shearing within the zone in the Dulongjiang and Nabang regons of western Yunnan, China. The temperature range for dextral ductile shearing is estimated to be between 550 and 450 ℃, based on ductile feldspar deformation and CPO patterns of quartz in the granitic mylonites. Zircon U-Pb dating of syn-shearing leucogranites indicateds a period of dextral strike-slip movement between 30 and 18 Ma. The 40Ar/39Ar dating results from the mica fragments in mylonitic granites suggest rapid cooling since approximately 17-14 Ma. Combining these findings with previously published data on other dextral strike-slip faults/shear zones around the EHS and southeastern Asia, it is concluded that the Dulongjiang shear zone is connected with the Parlung shear zone in Tibet, the Nabang shear zone in western Yunnan and Sagaing Fault in the southeastern Asia. The Parlung-Dulongjiang-Nabang shear zone, along with other dextral strike-slip zones, forms a regional-scale Cenozoic dextral shear system around the EHS, extending into southeastern Asia. In addition, our study, in conjuncation with high wavespeed tomographic anomalies beneath the India-Asia collision zone, emphasizes the distinct evolution at lithospheric scales in  the southeastern and eastern parts of the collision zone. The intracontinental continuous strike-slip shearing indicates a tectonic transformation from extension in Tibet to block rotation around the EHS. From 30 to 18 Ma, the slab tear is associated, spatially and temporally, with a clockwise rotation and dextral strike-slip shearing around the EHS. These characteristics suggest a warmer geodynamic setting during the rotation and the influence of  a hot mantle flow associated with the tear in ongoing India lithosphere subduction. The Oligocene-Miocene dextral strike-slip shearing around the EHS and their linkage southwards with the dextral Sagaing Fault may correspond to the rotation required for the slab to bend, stretch and eventually tear beneath the EHS region.