Microstructural and Magnetic Investigations of Pseudotachylyte and Ultracataclasite in the Hoping River, Tananao Complex, Eastern Taiwan

Unlike other fault rocks, fault pseudotachylytes form through frictional melting during an earthquake. Therefore, these rocks, regarded as earthquake fossils potentially hold valuable information on seismic deformation. Paradoxically, although Taiwan is a seismically active zone, reports of pseudotachylyte outcrops in Taiwan remain rare. Previous studies reported the Hoping River pseudotachylyte locality is by far the only natural pseudotachylyte outcrop in the metamorphic region of Taiwan. To understand the melting process and generation mechanism of pseudotachylyte, we apply geochemical, microstructural and magnetic approaches to investigate the pseudotachylyte veins in this study. X- ray fluorescence (XRF) geochemical analysis show that the pseudotachylyte melt, formed by incongruent melting, is depleted in SiO2 and N2O, and enriched in Fe2O3,and K2O compared with the result of ultracataclasite and host rock. X-ray diffraction (XRD) analysis suggest the composition of pseudotachylyte and the host rock are dominant by quartz, feldspar, biotite, muscovite, and chlorite. In 4-10 and 20- 30 2-theta degrees, glass humps are prominent in pseudotachylyte, which is an evidence of melt- origin of the pseudotachylyte. The existence of microcrystallite (tiny, new grown crystals) under scanning electron microscope (SEM) also supports its melt- origin. Since iron content of the pseudotachylyte is slightly higher (4 wt. %) than the ultracataclasite and granitic host rock, magnetic hysteresis measurements were performed under high field (up to 1 Tesla) using a vibrating sample magnetometer (VSM) to determine the nature of magnetic minerals. Magnetic hysteresis curves show the pseudotachylyte veins of the Hoping River are dominated by paramagnetic phases, with a very weak saturation isothermal remanent magnetization (SIRM). Magnetite concentration, inferred from these measurements, is on the order of a few ppm only. Submicron iron-oxide (possible) grains are discovered under transmission X-ray microscope (TXM), which may be formed by the breakdown of biotite during melting. The presence of iron oxide grains appears restricted to the melted part of the pseudotachylyte. The pseudotachylyte in the Hoping River are formed by incongruent melting. The enrichment of Fe did not strongly change the nature of magnetic properties, resulting in paramagnetic- dominant of the pseudotachylyte. The generation mechanisms of pseudotachylyte in the Hoping River involve ultracataclasis and extreme limited melting, leading to the coeval formation of ultracataclasite and pseudotachylyte.