KINEMATIC ANALYSES OF THE HSUEHSHAN RANGE, TAIWAN - A LARGE-SCALE POP-UP STRUCTURE

The Hsuehshan Range, exposed in the northern and central Taiwan slate belt, is a fault-bounded structural high cored by biotite grade slates and metasandstones. Syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern Hsuehshan Range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland. Near the eastern boundary of the Hsuehshan Range, however, pressure shadows record noncoaxial strain histories consistent with a top-to-the-east sense of shear along a steep NW dipping shear zone. This noncoaxiality is attributed to SE directed backthrusting on the Lishan fault, which separates the higher-grade, Eo-Oligocene rocks of the Hsuehshan Range from the lower-grade Miocene rocks of the Backbone Range. Because it is bounded to the east by the SE-vergent Lishan fault and to the west by a series of NW-vergent thrusts (e.g., the Chuchih fault), the Hsuehshan Range is envisaged as a pop-up structure. Strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain. Departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces (e.g., the Lishan fault). The Hsuehshan Range tapers in width to the south; thus the pop-up may be buried or die out to the south where the collision is younger.