How fast is the middle lower crust flowing in eastern Tibet? A constraint from geodetic observations
Various geophysical observations, including seismological and magnetotelluric imaging, have implied that the deep crust beneath eastern Tibet may be partially melted and flowing faster than the brittle upper crust. However, it is still unclear how much faster the deep crust is flowing. Geodetic observations, which are more sensitive to dynamic changes, provide constraints on the flow rate of the middle and lower crust (MLC) Three dimensional GPS velocities show that deformation within the brittle upper crust contributes little (0.02 0.02 mm/yr) to the overall surface uplift (2.7 0.3 mm/yr) Therefore, two plausible models for the surface uplift are discussed, which are consistent with the geodetic constraints. In the deep crustal flow model, crustal thickening requires the horizontal flow rate of the MLC to be 330% 710% of the rate of motion of the upper crust, and the deepening of the Moho is only up to 35% of that required to maintain isostatic balance; isostasy may not be maintained over the geodetic timescale. In the hybrid model of deep crustal flow and convective lithospheric detachment, the Moho is uprising and only weak or moderate (130% 250% deep crustal flow is required, which results in moderate present day crustal thickening beneath eastern Tibet. This result improves our understanding of the plateau construction and dynamics, and also offers advice for numerical simulations.