PETROGRAPHIC CHARACTERIZATION OF DEFORMATION MECHANISMS AND KINEMATICS IN POST-CLEAVAGE FAULTS ACCOMODATING DIFFERENTIAL UPLIFT OF THE HSUEHSHAN RANGE: TAIWAN

Much remains unknown about how continental margin architecture, built from a prior tectonic rifting event, can affect past and ongoing collisional orogenisis. West-central Taiwan can help us to understand the fundamentals of this problem because it is thought to be the site of a relic passive margin fracture zone that is controlling the contemporary uplift patterns of the Hsuehshan Range. We focus on microstructures within an oriented sample to identify the primary deformation mechanisms and kinematics. We found an abundance of clasts showing cataclasis while observing the deformation at grain scale and lesser clasts showing signs of pressure solution and calcite veining. We found that our observations of the asymmetric microstructures are also consistent with the kinematics of the system observed in the field.

The focal point of our study is a single fault within a suite of recently recognized northwest-striking faults cutting Pliocene-Miocene sandstone sequences just southwest of the Hsuehshan Range. The sampled fault material consists of fault rock and cohesive fault rock taken from a meter-scale thick fault zone east of the Anmashan Anticline near Kukuan Taiwan. Thin sections were made normal to the fault plane and both parallel and perpendicular to the slip direction.

The bulk of the faults are north-northeast dipping oblique thrust faults and strain inversions suggests the maximum shortening direction to be southwest and northeast. These post-cleavage faults are north-northwest trending nearly normal to a northeast trending regional magnetic high that is believed to mark the edge of full-thickness continental crust northwest of the high. This offset nearly conforms with the topographic break that separates the higher Hsuehshan Range to the northeast from the lowlands of the Puli Basin to the southwest. We infer it to be the northeast facing margin of what appears to be a promontory in the lower plate pointing to the east. This promontory of continental crust in the footwall is now acting as a deformation guide as the trench-fill sediments making up the orogen move northwest in response to collision with the Luzon arc.