Abstract
This paper establishes a reference data set of carbonaceous materials (CMs) from the active fault zone of the Longmen Shan fault belt that ruptured in the 2008 Mw7.9 Wenchuan earthquake and presents an application of these data for studies of both other exhumed carbonaceous-rich fault zones and deep-drilling cores. The CMs distributed in the active fault zone are found as narrow veins and located along the slip surfaces. Microstructural observation shows that the carbonaceous material veins (CMVs) are located along slip surfaces in the fault gouge zones. Some CMVs have a cataclastic fabric, and their branches intrude into voids around the slip surfaces. Raman spectra of the CMVs show a wide (full width at half maximum >200 cm−1) D-peak at ~1345 cm−1 (defect peak), which is much lower than the O-peak at ~1595 cm−1 (ordered peak), indicating a metamorphic temperature of zeolite facies or lower than 250 °C. In addition, the stable carbon isotopic compositions (δ13C values) of the CMVs, ranging from −23.4 to −26.4‰, are very similar to that of the kerogen collected from the Late Triassic Xujiahe Formation in Sichuan Basin. Given the data at which it may be formed, the Xujiahe Formation is the most likely origin of CMs for the CMVs, and it seems that some CMVs in the fault zone were crushed and intruded into the voids during coseismic events, possibly driven by an enhanced pore fluid pressure. Since graphitization is suggested as an indicator of transient frictional heating in this area, our study providing a reference data set of CMs would help future CM-rich fault-zone research to retrieve seismic signatures presumably occurring in the Longmen Shan fault zone belt.
Highlights
Tectonic veins in a fault zone always yield important information on the (1) stress, strain, pressure, temperature, fluid composition, and origin as well as on the (2) processes activated
Structures Black and shiny carbonaceous material vein (CMV) are very common on the walls of the trench, in fault gouges (Fig. 2)
Most of the CMVs parallel with each other extend along the fault planes (Fig. 2b–d), and they are often offset by subsidiary faults
Summary
Tectonic veins in a fault zone always yield important information on the (1) stress, strain, pressure, temperature, fluid composition, and origin as well as on the (2) processes activated. Pseudotachylite is referred to as “fossil earthquakes” (Lin 2008), as it indicates a melting process induced by coseismic frictional heating (Sibson 1975). Ultracataclastic veins found in a seismic fault zone suggest crushing and the rapid injection of fluidized fine-grained materials associated with seismic faulting (Lin 2011). We report narrow veins of carbonaceous materials (CMs) present in the Longmen Shan fault belt that ruptured during the 2008 Mw 7.9 Wenchuan earthquake. Since CM could be irreversibly graphitized owing to coseismic frictional heating, the graphite present in a fault zone could remarkably lower the strength of a fault (Oohashi et al 2011; Kuo et al 2014). CMVs are inferred to record information about the seismic faulting processes in the Longmen Shan fault belt
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