Multiple generations of pseudotachylyte in the Wenchuan fault zone and their implications for coseismic weakening

Pseudotachylytes are generally considered to be remnent products of past earthquakes, conveying significant information, which provide improved insight into the fault behavior and their mechanical properties. Pseudotachylytes and cataclastic rocks are exposed in the Pengguan complex along the Yingxiu-Beichuan fault zone, which ruptured during the 2008 Wenchuan earthquake, at the eastern margin of the Tibetan Plateau in Sichuan province, China. From outcrop investigations, pseudotachylyte veins are gray, dark-gray, brown and black, with thicknesses ranging from several mm to ∼10 cm. Microstructural observations, via optical microscope and SEM, show that distinct textures, such as spherulites, microlites, honeycomb-like vesicle structures, flow textures, irregularly shaped and deep embayed clasts, are present in the pseudotachylytes, suggesting that the pseudotachylytes are indeed melt-origin. Different colored veins and their overprinting relations observed at both macroscopic and microscopic scales, demonstrate that seismic faulting events and the associated generations of pseudotachylyte occurred repeatedly along the Yingxiu-Beichuan fault zone. Friction melt lubrication, thermal pressurization and mechanical lubrication, all can exist during one fault slip motion, indicating that multiple fault weakening mechanisms may work during a single coseismic event.