Zijie Zhu

The Cenozoic collision and convergence between the Indian and Eurasian plates drove north-south crustal shortening, uplift, and outward growth of the Tibetan Plateau. Since the Late Miocene-Early Pliocene, the plateau has expanded eastward and southeastward, while lateral extrusion, crustal thickening, and surface uplift have jointly shaped its eastern and southeastern margins. Here we collect continuous waveform data from dense seismic arrays in the eastern and southeastern Tibetan Plateau to construct three-dimensional S-wave velocity and radial anisotropy models. We first apply the frequency-Bessel transform (F-J) method to extract multimodal surface wave dispersion curves from ambient noise data, and then invert these measurements using multimodal ambient noise tomography.Our models delineate the detailed three-dimensional geometry of the mid- lower crustal low-velocity zone (LVZ). The spatial distribution of the LVZ shows a close correspondence with regions of pronounced lateral extrusion and crustal deformation. The LVZ also exhibits strong positive radial anisotropy, which provides seismic constraints on possible ductile crustal flow pathways. In addition, variations in LVZ thickness show clear spatial correlations with surface elevation, Moho depth, and present-day GPS velocities. These observations suggest that weak mid-lower crustal materials may play an important role in surface uplift, crustal thickening, and ongoing deformation across the eastern and southeastern Tibetan Plateau. Based on these results, we propose a three-dimensional geodynamic model that explains two contrasting geomorphic end members: the steep eastern margin and the gently sloping southeastern margin of the Tibetan Plateau. This model provides a framework for understanding the tectonic evolution and outward growth of eastern and southeastern Tibet since the late Cenozoic.

:The collision and convergence of the Indian and Eurasian plates during the Cenozoic caused the Tibetan plateau to shorten in the north-south direction. As the plateau grew, lateral extrusion contributed to its rapid uplift and further crustal thickening. The Tibetan plateau has experienced eastward to southeastward growth from the Late Miocene to the Early Pliocene. We collected the continuous waveform data by dense arrays in the eastern and southeastern Tibetan plateau to obtain high-resolution 3-D S-wave velocity and radial anisotropy model by the Frequency-Bessel transform method. The high resolution 3D S wave velocity model reveals the detailed top and bottom boundaries of the Low Velocity Zone (LVZ) within the mid-lower crust (depth range 12~36 km). LVZ exhibits strong positive radial anisotropy, providing direct evidence for the ductile crustal flow paths. The LVZ distribution coincides with regions of pronounced lateral extrusion and deformation in eastern and southeastern Tibetan plateau.The high resolution model reveals strong correlations between variations in LVZ thickness, with surface topography, the depth of Moho boundary. and even with the current GPS velocities. These evidences indicate that ductile crustal flow acts as an active mechanical layer dominating uplift, crustal thickening, and ongoing deformation in the Tibetan plateau. Based on the ductile crustal flow model, this study explains two distinct geomorphic end-member cases, the steep eastern margin and the gently sloping southeastern margin of the Tibetan plateau. These findings establish the dynamic framework for the tectonic evolution in the eastern and southeastern Tibetan plateau since the late Cenozoic.