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

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.

Strain-Induced Graphitization Mechanism of Coal-Based Graphite from Lutang, Hunan Province, China

“…Graphitization is complicated under the effects of multiple factors [4,[33][34][35][36]. An igneous intrusion introduces thermal energy; thus, the degree of coal graphitization gradually decreases with increasing distance from the intrusion.…”

“…One of the main reasons for this is the critical role of tectonic stress during graphitization [9,11,14]. It is well known that deformation caused by stress and the elimination of structural defects could take place at the same time; for instance, the dynamic recrystallization mechanism during the deformation of minerals [34][35][36][37][38][39]. Similarly, for the purpose of extinguishing structural defects during graphitization, it is possible that deformation also coincides with coal graphitization, which is graphitization induced by deformation.…”

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

“…(a) D1/G peak width ratio versus D1/G intensity ratio. The FZ590 and FZ760 results of the WFSD-1 are also included [24,41]. (b) G peak width of the CM-bearing gouge over the average breccia peak width versus the G band peak position.…”

“…(b) G peak width of the CM-bearing gouge over the average breccia peak width versus the G band peak position. The FZ590 and FZ760 results of the WFSD-1 are also included [24,41].…”

Geochemical Alteration and Mineralogy of Coals under the Influence of Fault Motion: A Case Study of Qi’nan Colliery, China

“…Geochemical and geophysical characteristics of fault rocks are primarily controlled by fault zones or fault systems [1][2][3][4][5]. Fault zones in rocks are generally divided into fault core (comprised of ultracataclasite), damage zone and undeformed zone (comprised of protolith) [6,7].…”