大陆岩石圈的热-流变底边界——以Kaapvaal、Fennoscandia和Slave克拉通为例

Thermal-rheological bottom boundary of continental lithosphere:case studies on cratons of Kaapvaal,Fennoscandia,and Slave

板块构造理论的一个基本假设是相对刚性的岩石圈板块漂浮在相对软弱的软流圈之上作欧拉运动.岩石圈与软流圈间存在边界面称为岩石圈-软流圈边界（LAB）.提出一种半解析方法来确定大陆LAB：定义其为有效黏度取极小值所在的深度.该方法得到的LAB,综合考虑了大陆岩石圈地幔介质的热学和流变学性质,故称其为热-流变底部边界.对3个著名克拉通（Kaapvaal,Fennoscandia和Slave）的研究表明：3个克拉通的热-流变底部边界在~250km,与大地电磁测深等其他地球物理方法得到的结果一致.因为热-流变底部边界是从力学的角度来定义的,且提供了岩石圈板块浮于软流圈上运动的极大可能性,因而与板块构造理论的定义更为接近.在该边界及附近,主导的流变机制是湿橄榄石的扩散蠕变,有效黏度极小,差应力小,应变率相对较大.

A fundamental premise of the plate tectonics is that a relatively rigid lithosphere moves over a weaker asthenosphere. Howerer, the nature of the lithosphere-asthenosphere boundary （LAB） remains poorly understood, especially that beneath the continental lithosphere. At the present there are several definitions for the LAB based on different measurements. By defining the LAB as the depth where the effective viscosity has a weak minimum, a semi-analytical method to determine this bottom boundary is presented. This boundary is called thermal-rheological bottom boundary because it combines the thermal and rheological properties of the lithospheric mantle.Studies on the three famous cratons （Kaapvaal, Fennoscandia, Slave） show that the thermal- rheological LAB is at the depth of -250 km, which is consistent with the results based on other geophysical methods （for example, magnetotelluric method）. The thermal-rheological LAB is closer to the LAB in the plate tectonics, because it provides the possibility for the mechanical plates moving over the asthenosphere. At the thermal-rheological LAB and in its adjacent area, the deformation mechanism is predominant by diffuse creep, and the differential stress level is low while the strain rate is high.