Normal faulting from simple shear rifting in South Tibet, using evidence from passive seismic profiling across the Yadong-Gulu Rift

The Tibetan Plateau is undergoing north–south shortening accompanied by west–east extension, as evidenced by the widespread development of north–south trending normal faults, grabens and rifts. While the mode of the north–south shortening has been the main focus of most international studies, knowledge of the deep structure beneath South Tibet is required for understanding the mechanism of the west–east extension. The onset of the north–south trending normal faulting is commonly taken as an indicator that the Tibetan Plateau was uplifted to a near-maximum elevation before entering a collapsing stage. Here we report on the receiver functions of a seismological experiment across the northern segment of the Yadong-Gulu Rift (YGR), one of the youngest rifts in South Tibet. The migrated receiver function images reveal that the YGR is a high-angle normal fault characterized by a 5-km Moho rise from its western to eastern flank, together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks. This highly asymmetric lithospheric structure suggests whole-crustal extension controlled by a simple/general shear rifting mechanism. This simple/general shear rifting in the YGR is attributed to an eastward (horizontal) shear at the base of the upper crust, as evidenced by the observed Tibetan GPS velocity field and our observation of shear wave splitting discrepancy among the upper crust, lower crust and lithospheric mantle. We propose that in the YGR, simple shear rifting accommodates the northward injection of the Indian lithosphere, which may suggest that the onset of the north–south normal faulting does not indicate gravitational collapse of the Tibetan lithosphere.