Exhumation of Metamorphic rocks during the Taiwan Orogeny: A Study on the Soufeng Fault

Abstract:

Part of the exhumation history of eastern Taiwan is stored in the shear zone between the Yuli and Tailuko belts, the Daguan shear zone (Yi et al., 2012), which is a part of the Soufong fault named by Yen (1963). Our detailed fieldwork across this fault shows that it is marked by a sharp contact between the Yuli forearc metasediments and the Tailuko belt marbles and schists. As the boundary is approached from the eastern Yuli belt a moderate to strong slaty cleavage becomes phyllitic, with anastomosing S-C fabrics and local boudinage of stronger quartzites, subarkoses and graywackes. These fault rocks are characterized by abundant stretched lenticular quartz veins, locally intercalated with thin elongated meta-conglomerates. Across the fault to the west are massive marbles, layered quartz mica schists, chlorite schists, and meta-chert.

Regional Ar dating of fine fraction of white micas in these rocks yields ages of ~1 to 16 Ma (Tsao, 1996). Ar ages of biotite in mylonitic Tananao schists range from 3.0-4.1 Ma (Wang, 1998). We separated and dated very fine-grained phyllitic folia of the phyllites and phyllonites approaching the fault zone, and find ages even younger than this. Our preliminary step heating experiments show age spectra with reproducible age steps less than 1 Ma. These spectra climb to ages as old as found by others, 12 to 14 Ma, demonstrating multiple age populations. However, our separates concentrating the youngest fabric-forming micas demonstrate that recrystallization during faulting persisted to the Middle Pleistocene. Thus differential movement between these belts probably played a critical role in driving the uplift and exhumation of these rocks.