Synorogenic Extension and Normal Faulting in Southern Taiwan

Abstract:

A recent leveling survey (Ching et al., 2011) across the southern Central Range in Taiwan shows a significant gradient in the rate of uplift along the eastern flank of the range, with western sites recording generally higher rates of uplift. To understand the origin of this gradient, we collected brittle fault data along a 32 km section of the range near the village of Lidao and documented more than 150 outcrop-scale faults of which 94 recorded a normal sense of slip, 41 recorded strike-slip and 15 were unknown. We also recognized two styles of deformation: A) fault characterized by calcite or quartz fibers on the fault plane and B) faults characterized by fault gauge or breccia on the fault plane. The highest concentration of faults occurs in the area of the Wulu Gorge, east of Liado and most are NE-dipping normal faults. Paleostress inversion of the faults identified two phases of deformation. Phase 1 (i.e., stress state with the most faults) shows a sub-horizontal extension direction that trends NE. Phase 2 shows a sub-horizontal shortening direction that trends NW. Most of the faults identified in the field as “A” are included in phase 1 and the extension direction is similar to extension directions determined from GPS and earthquake focal mechanisms. Phase 2 contains most of the field-identified “B” faults and the shortening direction is parallel to a previously recognized stretching lineation associated with slaty cleavage development. The phase 1 normal faults are interpreted to be part of a regional-scale east-side-down shear zone based on the leveling and earthquake data and on a topographic lineament that can be traced 11 km to the north near Jia-Ming Lake and the crest of the Central Range.