Microstructural and Magnetic Investigations of Pseudotachylyte and Ultracataclasite in the Hoping River, Tananao Complex, Eastern Taiwan
Abstract
Unlike other fault rocks, pseudotachylytes form through friction related melting during an earthquake, regarded as earthquake fossils potentially hold valuable information on seismic deformation. Here we investigate seismic rupture processes through the microstructural and magnetic study of pseudotachylyte and ultracataclasite from the Hoping River area, eastern Taiwan. Previous studies reported the first pseudotachylyte outcrop in the Hoping River from which the magnitude, direction and sense of seismic slip were subsequently determined. In this study, we apply new microstructural and magnetic approaches to investigate the pseudotachylyte veins. X-ray fluorescence (XRF) geochemical analyses show that the pseudotachylyte is depleted in SiO2, Al2O3, Na2O and enriched in Fe2O3, K2O compared with the ultracataclasite and host rock, suggest the pseudotachylyte in Hoping River area are incongruent melting. Scanning electron microscopy (SEM) and transmission X-ray microscopy (TXM) supports the melt origin of the pseudotachylyte, although melting occurred only in small regions, manifested by a few microcrystalline aggregates, with low melt percentage (≈10%). The TXM results suggest these small, dark colored grains are probably neoformed iron compounds in the pseudotachylyte. The presence of iron oxide grains appears restricted to the pseudotachylyte. Since the iron content of the pseudotachylyte is slightly higher (4 wt. %) than the ultracataclasite and granitic host rock, magnetic hysteresis curves show the pseudotachylyte veins of the Hoping River are dominated by paramagnetic phases, with a very weak saturation isothermal remanent magnetization. Magnetite concentration, inferred from these measurements, is on the order of a few ppm only. The paramagnetic-dominant properties of Hoping River pseudotachylyte in the Hoping River suggest the restricted melting proportion and oxidation conditions during its formation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T21D2857C
- Keywords:
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- 8012 High strain deformation zones;
- STRUCTURAL GEOLOGYDE: 8030 Microstructures;
- STRUCTURAL GEOLOGYDE: 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS