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

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation fr ont is investigated in terms of combined morphostructural analysis, based o n imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic a nd well data was also carried out, resulting in improved mapping of the Neo gene 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 t hrust units; its kinematic evolution is principally controlled by the geome try of the foreland Peikang High, behaving as a buttress for the west vergi ng thrust sheets. A preliminary analytical model of the oblique thrusting a t 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 bas ement boundary (the hinge fault of the Peikang High). This analytical model ing accounts for the occurrence of local extension at the intersection of o blique ramps in the southern Pakuashan. Numerous complementary structural a nd tectonic evidences led us to establish a complete deformation model, inv olving 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 extensio n in the transverse zone, regional wrench deformation, absence of major reo rientation of local stress inside the transverse zone, along-strike variati on of structural styles coupled with low to high uplift rate from the North ern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruc tion of the Pakua Transfer Fault Zone evolution is proposed, as a typical e xample of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left-lateral tear fault or transfer fa ult that offsets two distinct frontal thrust-and-fold sheets.