Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 1. Signatures within the Deep Wenchuan Earthquake Fault Zone and Their Implications

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

doi:10.3390/min8090385

Abstract

Graphitization of carbonaceous materials (CM) has been experimentally demonstrated as potential evidence of seismic slip within a fault gouge. The southern segment of the Longmenshan fault, a CM-rich-gouge fault, accommodated coseismic slip during the 2008 Mw 7.9 Wenchuan earthquake and potentially preserves a record of processes that occurred on the fault during the slip event. Here, we present a multi-technique characterization of CM within the active fault zone of the Longmenshan fault from the Wenchuan earthquake Fault Scientific Drilling-1. By contrast with field observations, graphite is pervasively and only distributed in the gouge zone, while heterogeneously crystallized CM are present in the surrounding breccia. The composite dataset that is presented, which includes the localized graphite layer along the 2008 Wenchuan earthquake principal slip zone, demonstrates that graphite is widely distributed within the active fault zone. The widespread occurrence of graphite, a seismic slip indicator, reveals that surface rupturing events commonly occur along the Longmenshan fault and are characteristic of this tectonically active region.

Highlights

Graphitization is a progressive and irreversible transformation from disordered carbonaceous materials (CM) into stable ordered graphite and can occur under a wide variety of conditions (e.g., [1]).Current studies propose that graphitization of CM is facilitated with confining pressure, burial time, temperature, fluid, and shear stress/strain [2,3,4,5,6]
Mw 7.9 earthquake near Wenchuan in SE Tibet, producing a 240 km-long Yingxiu–Beichuan surface rupture and an 80 km-long Guanxian–Anxian surface rupture, respectively (Figure 1a) [10]. Both field observations and the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1) along the Yingxiu–Beichuan surface rupture reported the presence of CM within the active fault zone of the southern Longmenshan fault, presumably derived from the Triassic Xujiahe formation sedimentary rocks [11,12,13,14,15]
Kuo et al [16], conducted rock friction experiments on the CM-rich fault gouge of the WFSD-1 and characterized experimentally deformed products, suggesting that gouge graphitization processes likely occurred within the active fault zone of the Longmenshan fault during the 2008 Mw 7.9 Wenchuan earthquake

Summary

Introduction

Graphitization is a progressive and irreversible transformation from disordered carbonaceous materials (CM) into stable ordered graphite and can occur under a wide variety of conditions (e.g., [1]). Both field observations and the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1) along the Yingxiu–Beichuan surface rupture reported the presence of CM within the active fault zone of the southern Longmenshan fault, presumably derived from the Triassic Xujiahe formation sedimentary rocks (coal-bearing sandstone) [11,12,13,14,15] This provides a unique opportunity to link seismic fault motions and the relevant gouge graphitization by integrating the evidence of CM from the field and laboratory. Kuo et al [16], conducted rock friction experiments on the CM-rich fault gouge of the WFSD-1 (at a slip rate of 3 m/s and normal stresses of 5–25 MPa) and characterized experimentally deformed products, suggesting that gouge graphitization processes likely occurred within the active fault zone of the Longmenshan fault during the 2008 Mw 7.9 Wenchuan earthquake.

Methods

Results

Microstructures

Discussion and Conclusions