A Geodynamic Deformation Model of the Chuandian Region of Southeastern
Tibet, with Constraints from GPS Data, Faults and Low-velocity Zones
Abstract
Southeastern Tibet is a key region for studying the tectonic evolution
of Tibetan Plateau. The region is characterized by distributed faults,
localized low-velocity zones (LVZs) in the mid-lower crust, and
prominent clockwise rotation of GPS velocity field. End-member models
have been proposed to explain the origin of this deformation pattern,
including the block extrusion model highlighting the role of faulted
shear zones, and the crustal flow model emphasizing the effect of a weak
lower crust. Here we use a 3D visco-elasto-plastic finite element model
to reproduce the instantaneous horizontal surface velocity and
investigate the effects of active faults and LVZs on that. The results
show that when only faults are included, the residual surface velocities
between modeled and observed values are large at some locations
overlying the LVZs; when only LVZs are included, the residual surface
velocities along the Xianshuihe-Xiaojiang and Lijiang-Xiaojinhe faults,
two major faults with relatively high slip rates, are significant.
However, when both faults and LVZs are considered, the modeled surface
velocities fit well with observed GPS velocities. Our results therefore
demonstrate that a combination of fault-bounding block extrusion and
crustal flow type of continuous deformation is required to explain the
surface deformation. The model yields a high-resolution strain rate map
which provides an improved understanding of Quaternary tectonics and
seismic hazards. In order to reduce the residual velocities, the
viscosity of LVZs is constrained as ~10-10 Pa·s. Our
study also suggests that LVZs are probably partially molten, which
explains the rheology, seismological, and magnetotelluric data.