Carbonaceous Materials in the Longmenshan Fault Belt Zone: 3. Records of Seismic Slip from the Trench and Implications for Faulting Mechanisms

Jialiang Si,Li-Wei Kuo,Jiann-Neng Fang,Hwo-Shuenn Sheu,Haibing Li,Huan Wang,Jyh-Rou Huang,Junling Pei,Lei Song,Sheng-Rong Song

doi:10.3390/min8100457

Abstract

In recent studies on the recognition of graphitized gouges within the principal slip zone (PSZ) of the Longmenshan fault in China, we proposed that the presence of graphite might be evidence of fault slip. Here, we characterized the clay- and carbonaceous-rich gouges of the active fault zone of the Longmenshan fault belt using samples collected from the trench at Jiulong, which was deformed during the 2008 MW-7.9 Wenchuan earthquake, to determine if graphite is present and study both the processes influencing fault behavior and the associated faulting mechanism. Mineralogical and geochemical analyses of the Jiulong trench sample show the presence of a hydrothermal mineral (i.e., dickite) integrated with dramatic relative chemical enrichment and relative depletion within a yellowish zone, suggesting the presence of vigorous high-temperature fluid–rock interactions, which are likely the fingerprint of thermal pressurization. This is further supported by the absence of carbonaceous materials (CMs) given the spectrometric data obtained. Interestingly, the Raman parameters measured near the carbonaceous-rich gouge fall within the recognized range of graphitization in the mature fault zone, implying the origin of a mature fault, as shown in the companion paper. According to both the sharp boundary within the very recent coseismic rupture zone of the 2008 MW-7.9 Wenchuan earthquake and the presence of kinetically unstable dickite, it is strongly implied that the yellow/altered gouge likely formed from a recent coseismic event as aconsequence of hydrothermal fluid penetration. We further surmise that the CM characteristics varied according to several driving reactions, e.g., transient hydrothermal heating versus long-term geological metamorphism and sedimentation.

Highlights

There is a close relationship between fault rock properties and the slip of major faults
This paper presents the results of an integrated fieldgeochemical and mineralogical study on the deformation of the Anxian-Guanxian fault, the frontal Longmenshan fault zone, and the south-eastern margin of the Tibetan plateau in China
Without neutral neutral density filters; during this process, we focused on the 1100–1800-cm 1 region of the Raman density filters; during this process, we focused on the 1100–1800-cm−1 region of the Raman spectrum, spectrum, which includes all first-order bands of carbonaceous materials (CMs)

Summary

Introduction

There is a close relationship between fault rock properties and the slip of major faults. Minerals 2018, 8, 457 while fluid circulation in faults can affect fault mechanical behavior by influencing the distributions of temperature and pressure and the composition of fault-related rocks, which in turn influence the dominant deformation mechanisms and fault rheology. It has been recognized in various major fault zones that elevated pore fluid pressure could reduce effective confining stress and induce frictional slip under low fault stress [3,4,5,6]. Fluids can alter mineral assemblages and the rates of chemical and mechanical processes during and after deformation, potentially leading to the weakening of fault-related rocks and the localization of slip in fault zones, affecting various faulting mechanisms, such as reaction weakening, thermal pressurization hydrolytic weakening and even healing [4,5,8,15,16,17,18]

Methods

Results

Conclusion