Analysis of the lithospheric thickness of the Northern Qinghai-Tibet plateau and its thinning mechanism

Two different models of lithospheric thickness of northern Qinghai-Tibet plateau are defined by seismic S-waves and magnetotelluric sounding(MT),respectively,and we have been unable to determine their relationship.Through analysis of the regressive relationship between elevation and crustal thickness we have calculated the lithospheric thickness of the northern plateau by adopting the equilibrium theory.In this calculation we take the slope and intercept of the regressive line as the constraints on the average density of crust and lithospheric mantle,and consider the effect of the phase change on the asthenospheric density.Our results indicate(1)that the average lithospheric thickness of the northern Qinghai-Tibet plateau is approximately 106-120 km under a reasonable range of asthenospheric temperature,and(2)that the average lithospheric thickness was approximately 80 km before the thickening of the crust.Our analyses of Cenozoic volcanism and magma segregation depth indicate that the lithospheric thickness of the present northern Qinghai-Tibet plateau is mainly constrained by the dehydration depth of phlogopite.The temperature at the bottom of the lithospheric mantle was higher than the wet solidus temperature of peridotite before the thickening of the lithosphere,and was affected by the thickening-related dehydration of amphibole and phlogopite.Because of the bottom being melted by slow degrees constantly,the thickened lithospheric mantle joined the small convection system of asthenosphere leading to a pulsed thinning of the bottom during the thickening process of the lithosphere.