藏北岩石圈厚度与减薄机制分析

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

天然地震S波和大地电磁测深给出了两种不同的藏北岩石圈厚度模型,两种测量结果的地质含义至今还不十分清楚。通过对地表高程与地壳厚度回归关系的研究,以回归直线的斜率和截距作为地壳和岩石圈地幔平均密度取值的约束,并考虑相变因素对软流圈密度的影响,采用均衡理论对藏北岩石圈厚度进行了计算。计算结果表明,在可能的软流圈温度取值范围内藏北岩石圈的平均厚度约为106～120km,地壳增厚前的岩石圈平均厚度约80km。藏北新生代火山作用和岩浆起源-分凝深度分析表明,藏北现今岩石圈厚度主要受金云母脱水深度所控制。增厚前岩石圈地幔底部温度高于橄榄岩湿固相线温度,并受闪石和金云母高压脱水作用的影响。加厚岩石圈地幔因其底部不断发生脱水低程度熔融而进入软流圈小尺度对流体系,使岩石圈加厚过程中伴随有底部的脉动减薄作用。

Two different models of lithospheric thickness of northern Qinghai-Tibet plateau are defined by seismic Swaves 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 arnphibole and phlogopite. Because of the bottom being melted by slow degrees constantly, the thickened lithospheric rnantle joined the small convection system of asthenosphere leading to a pulsed thinning of the bottom during the thickening process of the lithosphere