Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity

Li-Wei Kuo,Jiann-Neng Fang,Haibing Li,Sheng-Rong Song,Jialiang Si,Jyh-Rou Huang,En-Chao Yeh

doi:10.3390/min8090393

Abstract

In recent works on the determination of graphitization of carbonaceous materials (CM) within the principal slip zone (PSZ) of the Longmenshan fault (China), we demonstrated that the formation of graphite, resulted from strain and frictional heating, could be evidence of past seismic slip. Here we utilize Raman Spectroscopy of CM (RSCM) on the CM-bearing gouges in the fault zone of the Longmenshan fault belt, at the borehole depth of 760 m (FZ760) from the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1), to quantitatively characterize CM and further retrieve ancient fault deformation information in the active fault. RSCM shows that graphitization of CM is intense in the fault core with respect to the damage zone, with the graphitized carbon resembling those observed on experimentally formed graphite that was frictionally generated. Importantly, compared to the recognized active fault zone of the Longmenshan fault, the RSCM of measured CM-rich gouge shows a higher degree of graphitization, likely derived from high-temperature-perturbation faulting events. It implies that FZ760 accommodated numerous single-event displacement and/or at higher normal stresses and/or in the absence of pore fluid and/or along a more localized slip surface(s). Because graphite is a well-known lubricant, we surmise that the presence of the higher degree graphitized CM within FZ760 will reduce the fault strength and inefficiently accumulate tectonic stress during the seismic cycle at the current depth, and further infer a plausible mechanism for fault propagation at the borehole depth of 590 m during the Mw 7.9 Wenchuan earthquake.

Highlights

During an individual seismic event, fault-zone rocks presumably accommodate most of the deformation [1] and likely preserve the evidence of slip events
The X-ray powder diffraction analysis (XRD) patterns of carbonaceous materials (CM)-bearing gouge show the presence of chlorite, quartz, graphite, and high background before 10◦ 2θ coverage likely derived from the presence of illite/smectite mixed-layer minerals, as it is absent in the surrounding breccia zones
It is notable that the presence of numerously tiny peaks in the XRD patterns of CM-bearing gouge shows the complexity of mineral phases within the fault core of FZ760

Summary

Introduction

During an individual seismic event, fault-zone rocks presumably accommodate most of the deformation [1] and likely preserve the evidence of slip events. Deformation events occurring at typical seismic rates of ~1 m/s, considered as localized frictional sliding, may drive thermally activated physical and chemical processes such as flash heating of asperities [2], frictional melting [3], gelification [4], and decarbonation [5]. Fault pesudotachylytes, derived from rapid solidification of frictional melts, have been naturally and experimentally documented and is widely recognized as the evidence of seismic slip which allows the retrieval of earthquake source parameters [3,8,9,10]. Because of severe alteration on fault rocks during long term fluid-rock interaction [11], few other geological evidence of fault slip have been reported [12,13,14,15,16], and it remains challenging to bridge rock record and fault deformation.

Methods

Results

Conclusion