地震层析地质解释原理及其在滇西深部构造研究中的应用

FUNDAMENTALS OF GEOLOGICAL INTERPRETATION FOR SEISMIC TOMOGRAPHY AND ITS APPLICATION TO STUDYING OF WEST YUNNAN’S DEEP STRUCTURE

本文首先简要地介绍了地震层析方法的原理,着重探讨了三维速度异常地质解释的几个基本问题,包括划分构造层的物理基础、确定分辨尺度的依据以及估计地质时限依据的岩石热力学过程。然后介绍了应用地震层析及地质解释方法研究滇西造山带的地壳、下岩石圈及软流圈构造所得到的主要结果,并与人工地震测深及大地电磁测深结果进行了比较。这一工作说明地震层析可以作为岩石圈与软流圈构造研究的一种有效手段,在造山带深部构造的研究中将发挥独特的作用。

Seismic tomography technique,which is mainly used to determine the three-dimensional ve- locity structure,has been rapidly developed in recent years,but the geological meaning of the seis- mic velocity anomalies,i.e.the relationship between the velocity anomalies and geological structu- res,has been not still studies in detail upto now.In this paper the fundamentals of geological in- terpretation of seismic velocity structure are discussed for the first time.Based on their velocity characteristics and seismic velocity distribution,the crust,lower lithosphere,asthenosphere and asthenospheric upwell is identified.The resolution to distinguish different structures is related to seismic ray path,wave length,time service precision and element size divided.In a better con- dition,the resolution is in the ranges of 15—25km.How long tectonic history which may be traced by seismic tomography depends on the thermo-dynamic process in which the remainder of an ancient orogeny would be assimilated with the environment.A large orogen.aged 300— 400 m.y.can be recognized through its remainder structural signatures in the crust,lower lithosphere and asthenosphere.By seismic tomography and its geological interpretation it is di- scovered that the lower lithosphere of Yangze block in the Ailaoshan area has westwards subdu- cted taro the depths of 250—300 km beneath the Simao-Lincang block and an asthenospheric upwell stands in front of the subducting lithosphere.This implies that Ailaoshan belt is a litho- spheric subduction collision orogen subducting westwards instead of subducting eastwards in- fered formerly by surface faults.The results are proved by seismic reflection and electromagne- tic sounding in the same areas.