Modulation of Crustal and Mantle Flow Systems by a Cratonic Root in NE Tibet and Western North China Craton: Evidence From Seismic Azimuthal Anisotropy Analysis

Abstract

The northeastern Tibetan Plateau (TP) and the western North China Craton (NCC) provide a natural laboratory for studying the interaction of tectonic and mantle processes, contrasting the active plateau with the stable craton. In contrast to many prior studies that relied on station‐averaged shear‐wave splitting (SWS) measurements and sparsely distributed stations, this study explores seismic azimuthal anisotropy between the dynamic TP and the stable NCC using 6,409 high‐resolution SWS measurements from 465 broadband seismic stations at intervals of less than 50 km. Some stations in the central study area show splitting parameters with a π/2 periodicity based on azimuthal variations, indicating a complex double‐layer horizontal anisotropy structure. The anisotropy of the upper layer is linked to ductile flow in the middle‐to‐lower crust originating from the TP. This flow encounters the rigid lithosphere of the Alxa Block, leading to a bifurcation into northeastward and southeastward directions. The anisotropy in the lower layer, exhibiting fast orientations in a NW‐SE direction, is consistent with the observed one‐layered anisotropy and aligns with the absolute plate motion of the Eurasian plate. Analysis of the spatial coherence of splitting parameters suggests that asthenospheric flow is the dominant source of anisotropy. This mantle flow is southeastward beneath the Alxa Block and shifts to an approximately eastward direction along the thinner lithospheric passage between the Ordos and Sichuan cratonic keels.