Geochemical Variations of the Pseudotachylytes during Their Formation and Subsequent Uplift Processes, in the Longmen Shan Thrust Belt

Abstract

Pseudotachylytes, as the product of the frictional melting during seismic slip, convey important information about the earthquake rupture in the fault zone, and provide crucial material for learning fault zone formation processes and their faulting activity history. As fault rocks drilled from the deep fault zone exempt the supergenetic physical-chemical weathering, their chemical properties should be much similar to those during their formation. In this paper, multiple generations of pseudotachylytes in southern Yingxiu-Beichuan fault were studied. Their chemical properties were analyzed using SEM, µXRF core scanner and M4 TORNADO. Microstructures show that the pseudotachylytes are of melt-origin. Results of geochemical analysis indicate that the melts were formed in a high temperature reductive environment, characterized by high Fe, Ti, K, low Si with high magnetic susceptibility. The chemical properties of the surface pseudotachylytes were significantly changed due to the supergenetic weathering processes. Different elemental features of the multiple generations of pseudotachylytes in the WFSD drilling cores also indicate that the meteoric water (atmospheric water and surface water) had injected into 585 m or greater depth along the fault zone. Therefore, the geochemical variations should be taken into consideration when pseudotachylyte is used to judge the instantaneous sliding environment of seismic slip.