Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

Abstract

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along‐strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along‐strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left‐lateral tear fault or transfer fault that offsets two distinct frontal thrust‐and‐fold sheets.