High viscosity mid crust in central Tibet derived from studies of paleoshorelines around Lake Nam Co

Abstract

Many lakes in Tibet have shorelines developed around them. When water level decreases, a buoyancy force tends to cause uplift. For uplift to occur, flow in the mid crust is required. The time scale for the flow is related to the basin geometry and mid crustal viscosity. The basin shape is known, so the viscosity can be calculated from the amount of uplift and the time taken. Uplift is should be maximum at the lake center and shorelines at different distances from the center along their length will develop a slope as a result of uplift. Heights of shorelines were determined with kinematic GPS around L. Nam Co, one of the largest lakes in Tibet. These were combined with 14C ages of tufa deposits attached to the shorelines to give an estimate of >10^20 Pa s for the viscosity of the mid crust of the area. This value is much higher than that generally considered and suggests that the idea of a weak partially molten mid crust in central Tibet needs revising.