Fluid accumulation beneath the detachment fault in the Central Range of Taiwan

Hideki Masago, Kazuaki Okamoto, Yu-Chang Chan, Tzeng-Fu Yui, Hao-Tsu Chu, Yoshiyuki Iizuka, Katsuyoshi Michibayashi, Yumiko Harigane
2005 Chishitsugaku zasshi  
Hypocenter of the Chi-Chi earthquake is located on the detachment fault distributed approximately 10 km of depth, near the boundary between continental basement and overlying continental shelf sediments. Exhumed section of the basement rocks underwent greenschist facies 350-450 metamorphism. Overlying Eocene-Miocene sediments termed as Slate Belt also underwent sub-greenschist facies 350 metamorphism. We found an outcrop-scale duplex structure exposed in the Slate Belt, suggesting an outcrop of
more » ... sting an outcrop of the detachment fault. The outcrop consists of alternation of sandstone and slate layers. It is notable that pegmatitic, V-shaped quartz veins in the sandstone concentrate beneath the detachment fault, suggesting cracks filled with fluid by hydrofaulting. At this time, overlying slate layers play a major role as an impermeable cap-rock. If the stored fluid beneath the detachment was expelled to the pre-existing link thrust, cracks in the fault zone would propagate and facilitate the rupture process of the earthquake. Fig. .a Simplified geologic map of Taiwan. Studied sample was collected from the roadcut of the Central Cross Island Highway, near Tayulin, in the Central Range shown in the lower section . b Geologic cross-section along A-B in a . Modified after Lee et al. 2002 . Fig. . Polished surface of the meta-sandstone sample and element distribution maps showing the vein structure normal to foliation and parallel to striation . At least two types of veins are developed cutting through the foliation: quartz clear-looking vein and calcite muddy-looking vein. Cross-cutting relation observed at the arrowed point suggests the calcite vein predates the quartz vein. Scanning X-ray analytical microscope Horiba XGT-2700 at National Research Institute of Far Seas Fisheries Research Agency: NRIFSF images correspond to the square area marked by red line on the polished surface show their sharp relationship. Si and Ca images have compensating distribution patterns for each other, and they corresponds to quartz and calcite, respectively. Fig. . Outcrop of the sampling location. The sample was taken from just beneath the detachment fault, which is parallel to the foliation. Duplex structured underplated slate is above the detachment, and coherent meta-sandstone underlies it. Numerous veins are developed in meta-sandstone. Note that few veins are developed in the less permeabile overlying slate layer across the detachment. III
doi:10.5575/geosoc.111.2.iii_iv fatcat:yvd4jjzwabfanbp3rg7mxc3q7m