Possible kinematic model for an active blind thrust fault in SW Taiwan: an example of fold-accommodation fault

Taiwan lies at the collision zone between the Eurasian and Philippine Sea plates, leading to rapid crustal deformation in the mountain-building belt and making the region seismically active. The Liuchia Fault is one of the active faults in southwestern Taiwan, trending parallel to the stratal boundaries in the west-dipping forelimb of the Niushan Anticline in the western foothills and located not far from a high-tech industrial park producing semiconductors. Although geological and geophysical field surveys, as well as wellbore data, provide evidences for the existence of the Liuchia Fault, its subsurface structure, which is important for estimating earthquake potential, remains unclear. Several models for the fault-related fold have been proposed, including a detachment fold with or without breakthrough fault thrusting to the west, and a fault-propagation fold with westward vergence.

This study aims to propose a 3D geometry of the Liuchia Fault and the associated fold structure. We reconstruct several geological cross-sections based on surface and well data. On the surface, the Niushan Anticline axis is oriented N-S, with the hinge plunging to the north and south. The strata tilt westward at high angles as they approach the Liuchia Fault. In comparison, the east limb of the anticline, which is also cut off by another eastward-thrusting fault (the "A" fault), is steep but dips at a shallower angle. The linkage between the surface fault trace and the fault plane penetrated by wellbore data indicates that the Liuchia Fault thrusts to the west at an angle of approximately 30° to 40°. On the other hand, reconstruction of the balanced cross-section using surface and subsurface stratal dip angles shows that the subsurface Niushan Anticline is characterized by overturned layers in the forelimb (west limb) in its shallower part. The displacements of 200 meters along the Liuchia Fault and 170 meters along the "A" Fault, as estimated from wellbore data, could not fully explain the formation of the overturned strata. Here, we propose a new kinematic model different from the previous ones. There was once a blind fault beneath the present-day Niushan Anticline. This blind fault could have been a detachment fault, which formed the embryonic Niushan Anticline. The subsequent development of the Niushan Anticline resulted in the formation of the overturned strata, which in turn caused fold-accommodation faults, such as the Liuchia and "A" faults, which could be regarded as breakthrough faults cutting off both developing limbs of the detachment fold.