Discovery of the pseudotachylytes in the Qiangtang Rift, Tibet, and their petrological characteristics and tectonic significance

Tectonic pseudotachylytes are produced by rapid sliding and melting, and then solidified fast in faults during earthquakes, which are considered as fossil earthquake. Pseudotachylytes record the physical-chemical processes related to earthquake in fault zone, which are essential materials for understanding the history of fault activity.  Here we focus on the pseudotachylytes and cataclastic rocks in the East Yibug Caka fault, SN-trending normal fault in the Qiangtang terrane, in the hinterland of the Tibetan Plateau. Combined optical microscope, scanning electron microscope, powder X-ray diffraction (XRD) with in situ X-ray fluorescence (XRF) analyses, their microstructures, mineral composition and elemental distribution were analyzed in detail. Field investigation shows that the dark gray to brown in color pseudotachylytes, associated with cataclastic rocks, are occurred as fault veins and injection veins with thickness ranging from a few mm to 1 cm. Microstructural observations show that multiple lines of evidence, such as embayed quartz fragments, honeycomb-like vesciles and locally developed microcrystallines and cluster aggregates, indicate that the pseudotachylytes were the products of frictional melting during the seismic slip. In addition, pseudotachylytes present as clasts in cataclastic rocks and fault breccias, and younger cataclastic rocks contain breccia of earlier cataclastic rocks, these characteristcs indicate that large seismic events occurred repeatedly in this fault zone. Considering the initial active time of the normal faults in this area is 13.5 Ma, the formation depth of the pseudotachylytes and associated cataclastic rocks is 10 km, the exhumation rate of the these fault rocks from deep depth is at least ~0.74 mm/yr. Pseudotachylytes along normal faults are seldom reported, this is the first time that we find melt-origined pseudotachylytes in the SN-trending normal faults in the Qiangtang terrane, and where present they have important implications for learning regional seismic activity and fault evolution process.