Mantle Transition Zone Structure Beneath Myanmar and Its Geodynamic Implications

Linking the India-Tibet collision to the north and the Andaman oceanic subduction to the south, Myanmar occupies a crucial position in the India-Eurasia convergence system. Various seismological studies have indicated that the Indian plate is obliquely subducted along the Burma arc. However, the depth extent and continuity of the subducted slab remain enigmatic. With seismic recordings collected from 114 recently deployed seismic stations, we map the topographies of the mantle transition zone (MTZ) boundaries, that is, the 410- and 660-km discontinuities, beneath Myanmar using receiver functions. Regional 3-D velocity models were adopted to account for the lateral velocity heterogeneity. The 410-km discontinuity is uplifted by over 15 km within 95 degrees E-97 degrees E and 21 degrees N-24 degrees N beneath Myanmar. This feature correlates well with the east-dipping high-velocity anomaly in the tomographic models, with a velocity increase of 0.9%-1.2% at the 410-km discontinuity depth, suggesting that the subducted slab has reached the MTZ. The uplift of the 410-km discontinuity terminates to the south at similar to 21 degrees N, indicating a distinct change in slab geometry. Our results also reveal a depressed 660-km discontinuity, which is spatially offset to the southwest of the uplifted 410-km discontinuity. We propose that the offset between the 410-km discontinuity uplift and the 660-km discontinuity depression could indicate a slab break-off and tearing beneath Myanmar, which was triggered by the northward motion of the Indian plate during the eastward subduction. We further speculate that the slab tear could mark the transition from oceanic to continental plate subduction.