The subducted Indian crust and its geodynamic implications for collision between the Indian and Eurasian plates in Myanmar: Insights from local earthquake tomography

Myanmar is located at the convergent boundary between the Indian plate and the Eurasian plate, and the Indian plate is obliquely descending eastward beneath the Eurasian plate along the Burma arc. Previous tomographic results have revealed an east-dipping, high velocity structure extending far beyond regional seismicity into the upper mantle beneath the Indo-Burma Ranges. However, the regional-scale seismic velocity structure beneath Myanmar remains enigmatic mainly due to the lack of local and regional seismic observations. From June 2016 to February 2018, we have conducted the China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO) and deployed the first portable seismic array in Myanmar. This seismic array was consisted of 71 seismic stations, which transected the main geological units of Myanmar, i.e., the Indo-Burma Ranges, the Central Lowland and the Shan Plateau.

Based on the new seismic data, we conducted studies on the crust and uppermost mantle structures of Myanmar using the double-difference seismic tomography method. The preliminary results display a significant eastward low velocity anomaly with a dip of approximate 20° between latitudes from ~22 °N to ~24°N, and its thickness gradually increases from south to north. This low velocity structure in the upper mantle is consistent with the relocated seismicity. We interpret this low velocity zone beneath Myanmar as the subducted Indian continental crust mainly based on its morphology and velocity structural features (thickness ~≥30km; v_P ~≤7.2 km/s). Synthetic tests further demonstrated that the imaged crust is thicker than a typical oceanic crust, but slightly thinner than the entire Indian continental crust. This study will provide important constrains on the transition from oceanic to continental subduction in this area.

This study is supported by the National Natural Science Foundation of China (grants 41490612, 91755214).