Upper-mantle velocity structures and anisotropy under the Mongol-Baikal region

The Mongol-Baikal region is located in the western segment of the Central Asian Orogenic Belt. This area experienced multiple periods of extensive compression starting from the Proterozoic. In the Cenozoic, the study region was modified by neotectonics, featuring large extensional rifting (the Baikal rift zone) and plateau uplift (the Hangai Dome). However, the deep mechanisms of the onset of rifting and doming are still debated. We performed high-resolution 3-D P-wave tomography under the southwestern Baikal and western Mongolia. The images show distinct low-velocity anomalies under the Baikal Rift at ~60 km depth, the Hangai Dome at ~200 km depth, and beneath the Siberian craton. This may indicate that potential mantle flows ascended from the deep Siberian MTZ to shallower levels, influencing the rifting of the Baikal rift zone and the lithospheric process of the Hangai Dome. We then further determined the seismic anisotropy of the upper mantle under western Mongolia using SKS splitting measurements. The study region is dominated by NW-SE trending fast polarization directions (FPD), which indicates consistent compressional and transitional stress among the whole study area. Small delay times in the Hangai Dome spatially coincide with the low-velocity anomalies, supporting remarkable asthenosphere upwelling. However, the local Hangai upwelling did not affect the general anisotropic structures significantly, indicating that the lithospheric process only occurred in a limited area.

References

Wu, H., Huang, Z., 2022. Upper mantle anisotropy and deformation beneath the western Mongolian Plateau revealed by SKS splitting. Tectonophysics 835, 229376. https://doi.org/10.1016/j.tecto.2022.229376

Wu, H., Huang, Z., Zhao, D., 2021. Deep structure beneath the southwestern flank of the Baikal rift zone and adjacent areas. Physics of the Earth and Planetary Interiors 310, 106616. https://doi.org/10.1016/j.pepi.2020.106616