Restricted Crustal Extrusion in Eastern Tibet Plateau: Insight From Anisotropic Q Tomography of Lg Wave

Crustal rheology is crucial for understanding crustal deformation and block interaction during the eastward expansion of the Tibetan plateau. The presence of significant structural boundaries and fault zones in eastern Tibet complicates the determination of primary factors contributing to high crustal attenuation. It remains unclear whether this attenuation is due to scattering resulting from seismic heterogeneity or intrinsic absorption from anelasticity. In this study, we perform azimuthal anisotropy crustal attenuation tomography using Lg waves to invert for the isotropic Q values (Q_iso), anisotropic Q values (Q_ani), and the high-Q directions in eastern Tibet. The widespread low Q_ani values indicate significant azimuthal-dependent attenuation in the Tibetan Plateau, suggesting strong heterogeneity in the crust. Prominent low Q_iso zones within the Songpan-Ganzi block are constrained by major faults, indicating that crustal faults restrict the crustal extrusion of the plateau. Within the inner zone of the Emeishan-large-igneous-province (ELIP) and along the eastern margin of the plateau, the high Q_iso values indicate low intrinsic attenuation, implying strong crustal rheology. These rigid regimes with strong rheology hinder the southeastward extrusion of the Tibetan Plateau, characterized by weak crustal rheology. Extremely low Q_iso values found in the intermediate zone of the ELIP near the Xiaojiang fault imply weak mechanical strength of the crust. We propose that the presence of the α-β quartz phase transition exacerbates this high crustal attenuation associated with partial melting in the crust.