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Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity
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Minerals 2018, 8(10), 457; https://doi.org/10.3390/min8100457

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

1
Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
2
Department of Earth Sciences, National Central University, Taoyuan 320, Taiwan
3
Department of Earth Sciences, National Taiwan Normal University, Taipei 106, Taiwan
4
Department of Geosciences, National Taiwan University, Taipei 106, Taiwan
5
Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
6
Boyue Instruments (Shanghai) Co., Ltd., Shanghai 201612, China
7
National Taiwan Museum, Taipei 100, Taiwan
8
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
*
Author to whom correspondence should be addressed.
Received: 22 June 2018 / Revised: 20 September 2018 / Accepted: 21 September 2018 / Published: 16 October 2018
(This article belongs to the Special Issue Mineralogy of Natural Graphite)
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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. View Full-Text
Keywords: carbonaceous materials; trench; Longmenshan fault zone; faulting mechanism carbonaceous materials; trench; Longmenshan fault zone; faulting mechanism
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Si, J.; Li, H.; Kuo, L.-W.; Huang, J.-R.; Song, S.-R.; Pei, J.; Wang, H.; Song, L.; Fang, J.-N.; Sheu, H.-S. Carbonaceous Materials in the Longmenshan Fault Belt Zone: 3. Records of Seismic Slip from the Trench and Implications for Faulting Mechanisms. Minerals 2018, 8, 457.

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