Northeastward expansion of the Tibetan Plateau: Seismic anisotropy evidence from shear-wave splitting measurements

The northeastern Tibetan Plateau (NETP) is a key region to clarify the nature and mechanism of the Tibetan Plateau expansion. This region includes the eastern Qilian Orogenic Belt, the Alxa Block, the Helan-Yinchuan Tectonic Belt and the Ordos Block. We make shear-wave splitting measurements using teleseismic waveforms recorded at 17 portable broadband stations that we newly deployed in the study region. Our measurements provide new information on seismic anisotropy and deformation patterns in the lithospheric mantle, which can be used to constrain the extent of the NETP northeastward expansion. The dominant fast polarization direction (FPD) is NW-SE in the eastern Qilian Orogenic Belt, the southern Alxa Block and the Helan-Yinchuan Tectonic Belt, subparallel to the prominent orientation of tectonics in the NETP. The same NW-SE FPD in the three tectonic units indicates NE-SW compression in the lithospheric mantle there caused by the NETP northeastward expansion. NE-SW FPDs are revealed in the interior Alxa Block, which may reflect fossil anisotropy probably formed by the Paleo-Asian oceanic subduction and closure. ENE-WSW FPDs exist in the western Ordos Block, reflecting fossil anisotropy probably caused by convergence of the South China Craton and North China Craton. Combined with the NW-SE FPDs in the southwestern margin of the Ordos Block revealed by previous studies, our results suggest that the NETP northeastward expansion has obviously affected the southern Alxa Block and the southwestern margin of the Ordos Block. However, its recent northeastward expansion mainly occurred along the Helan-Yinchuan Tectonic Belt.