Slab Tear of Subducted Indian Lithosphere Beneath the Eastern Himalayan Syntaxis Region

Abstract

In the southeastern Tibetan Plateau, region-scale dextral strike-slip shear zones, crucial for India-Asia convergence, were investigated along the Dulongjiang shear zone near the Eastern Himalayan Syntaxis (EHS). Structural, kinematic, and geochronological data from Dulongjiang and Nabang regions in western Yunnan, China, reveal dextral strike-slip shearing between 30 and 15 Ma. Various rocks were affected by moderate-temperature shear deformation (∼450–550°C), inferred from microstructures and quartz CPO patterns, during dextral strike-slip and exhumation of the shear zone. Combined with structures of pre-, syn-, and post-shearing leucogranites, zircon U-Pb dating indicates that the dextral shear along the shear zone began in the Early Oligocene (30–29 Ma) subsequent to the India-Asia collision. Micas in mylonitic granites yield ⁴⁰Ar/³⁹Ar ages, suggesting that the principal dextral shear deformation occurred approximately between 18 and 15 Ma. The Dulongjiang shear zone is linked to the Parlung, Nabang shear zone, and Sagaing Fault, forming a regional Cenozoic dextral shear system around the EHS. The study, combined with tomographic anomalies beneath the India-Asia collision zone, highlights distinct lithospheric-scale evolution in southeastern and eastern Tibet. Continuous intracontinental strike-slip shearing indicates a tectonic shift from Tibetan extension to block rotation around the EHS. From 30 to 15 Ma, slab tear, accompanied by clockwise rotation and dextral strike-slip shearing, suggests a warmer geodynamic setting influenced by hot mantle flow associated with ongoing subduction of the Indian lithosphere. Oligocene-Miocene dextral strike-slip shearing around the EHS, linking southwards with the Sagaing Fault, may correspond to the rotation necessary for slab to bend, stretch, and eventually tear beneath the region.