Estimate of thermal history within clayey characteristics from Hole-A of Taiwan Chelungpu fault Drilling Project (TCDP)

Abstract

To investigate the thermal history in a fault zone during an earthquake, we examine the clayey characteristics in the fault zone and compare those with host rocks closed to the primary slip zone of Chelungpu fault by performing the isothermal heating experiments. The microstructure of natural and experimental sample surface are also observed with SEM to provide the possible thermal history they have exceeded. Three fault zones of Chelungpu fault system are determined at the depth of 1111m (1107.59m - 1112.29m), 1153m (1150.60m - 1155.26m), and 1221m (1219.00m - 1222.30m) (described as FZ1111, FZ1153, and FZ1221 hereafter) of TCDP Hole-A, respectively. We collected clay samples sequentially recovered from three fault zones to reveal the spatial fluctuations in thermal processes during coseismic events. All of these three fault zones have same fault structures such as damaged zone and fault core which are included grey gouge and black gouge, but are asymmetric in fault architecture. The black gouge among three fault zones are thought as primary slip zone and may record the latest thermal history during coseismic events. Three particular and different clay characteristics of black gouge in FZ1111, FZ1153, and FZ1221 are discovered and it implies that these three fault zones went through different frictional heating. The mineral assemblage of FZ1111 shows melting occurrence that initmate high temperature had been achieved, and the temperature is estimated as high as 900°C within the observeation of SEM for in situ fault materials and the experimental ones. Meanwhile, the relative clay mineral assemblage of FZ1153 refer to the lower temperature than FZ1111 has been proceeded, and was estimated about 550°C becaues of dehydration of kaolinite described as thermal decompose. Last but not the least, the clayey characteristics of FZ1221 is similar with ones of host rocks, and hint that there is no or not sufficient friction heat to proceed the kinetic process in this fault zone. These differences of thermal history may be the causes of the contrastive local slip behaviors during the earthquake.