Magnetic Fabric Analysis of Chelungpu Fault Gouge in Taiwan and its Implications
Abstract:Magnetic minerals within the fault gouge could be deformed or neo-formed syn- and/or post- faulting. During this stage, magnetic fabric of the gouge material might preserve valuable information about the faulting process. In this study, we will present the results of the anisotropy of magnetic susceptibility (AMS) from the Chelungpu fault gouge, FZB1136, which hosts the slip zone of the 1999 Chi-Chi earthquake (Mw 7.6) using Taiwan Chelungpu fault Drilling Project (TCDP) hole-B core sample. AMS results from two Chelungpu fault surface outcrops at Ta-Keng near TCDP drilling site will also be discussed.
Previous magnetic mineral study of the FZB1136 pointed out the appearance inconsistency of the maximum remanence and the maximum magnetic susceptibility, which were recognized as the abundance of neo-formed goethite (post-seismic) and principle slip zone (PSZ), respectively. The AMS result indicated that lowest value of magnetic foliation and degree of anisotropy were found at the PSZ, but highest values of the two parameters appear at the location of the maximum remanence. During faulting, high temperature and least fluid found at the PSZ might be the reason of least AMS. However, arrangement of neo-formed post-seismic magnetic mineral, goethite, appeared at the maximum remanence under thermal fluid effect might be the reason to form highest foliation and degree of anisotropy here. In addition, magnetic lineation within the fault gouge is generally very low.
The AMS results of two surface outcrops, named as TK-1 and TK-2, show that the magnetic lineation drops from the wall rock to the fault gouge, but the magnetic foliation and the degree of anisotropy increased first then decreased. Compare the AMS pattern within the fault gouge, the results of TK-1 seems to be similar to FZB1136, but those of TK-2 are not. The outcrop TK-1 seems to be possible the slip zone of the 1999 Chi-Chi earthquake. Undoubted it needs to be further proved.