Characteristics of clay minerals in the principal slip zone of an active fault: A case study from the Taiwan Chelungpu fault Drilling Project

Direct observations of physical structures and chemical composition of the seismogenic zones of active faults are rare, due to the difficulty in reaching the fault zone at depth. Thus, the physico-chemical processes, triggered by energies released from coseismic faulting, remain largely unknown. The Chelungpu thrust fault was northward ruptured as a result of the Mw 7.6 Chi-Chi earthquake which struck central Taiwan on 21st September 1999. To fill the knowledge gaps raised by the 1999 Chi-Chi earthquake, such as what the absolute stress levels was on the fault during the earthquake, how the stresses recover afterward to prepare for the next event, and how the material properties of the Chelungpu fault affect its propensity to catastrophically slide rather than creep, the Taiwan Chelungpu fault Drilling Project (TCDP) was conducted in 2005 and drilled to a depth of 2,003 m. On the basis of continuous coring, a suite of geophysical measurements, and microstructural observation, three fault zones of the Chelungpu fault were identified at the depth of 1,111 m, 1,153 m, and 1,222 m (described as FZ1111, FZ1153, and FZ1222 hereafter). The active fault zone of the Chelungpu fault corresponding to the 1999 Chi-Chi earthquake was recognized at the depth of 1,111 m (FZ1111). By characterizing clay mineral assemblages with continuous sampling from fresh materials of the Chelungpu fault, we aim at achieving the objectives of (1) characterization of clay mineralogy of the Chelungpu fault zones, (2) recognition of the PSZ within the active fault zone(s) corresponding to the 1999 Chi-Chi earthquake, (3) determination of the physico-chemical processes occurred during seismic slip and plausible slip weakening mechanism operated at seismic rates, and (4) estimation of earthquake source parameters (e.g., estimate of temperature for calculating associated frictional energy and thickness of the PSZ for calculating associated surface fracture energy). Distinct clay mineral assemblage within the principal slip zone (PSZ) of FZ1111 show that (1) a decrease in clay content; and (2) significant decline of illite, disappearance of chlorite and kaolinite, and a spike in smectite. Meanwhile, the microstructural observation provides the evidence of melting, and the temperature within the fresh black gouge is estimated to be from 900°C to 1100°C by comparing the SEM images of in-situ natural samples with those of heated materials. The interval of clay anomaly within the PSZ resulted from frictional melting was determined by state-of-art in-situ synchrotron X-ray diffraction analysis and was estimated to be 1 mm. Thus, the characteristics of clay within the PSZ suggest that pseudotachylyte were possibly produced during the 1999 Chi-Chi earthquake and promptly altered to smectite. In addition, based on the estimation of the thickness of the PSZ, the contribution of surface fracture energy to earthquake breakdown work is quantified to be 1.9%. The huge remaining part of the breakdown work would be turned into chemical work (mineral transformation) and mechanic work associated with several processes to lubricate the Chelungpu fault such as thermal pressurization, elastohydrodynamic lubrication, and melting lubrication.