A field investigation of fault rocks within the footwall of the active Chishan Fault, southern Taiwan

A January 2008 investigation for sense of shear indicators within the active Chishan thrust fault in southern Taiwan has questioned the kinematics of well-developed shear bands in the footwall of the thrust. Visibly dark, high-angle shear bands (50-70 degree dips) in the Plio-Pleistocene mudstones of the footwall, which are parallel to the main trace of the fault, have been used in past studies to interpret the oblique sense of shear along the Chishan Fault. We have found ductile deformation within several of these bands and one oriented sample that contains a pronounced normal sense of shear. This discovery has questioned our understanding of these shear bands as related to the kinematics of the Chishan Fault and we now investigate the role (if any) of these shear bands in terms of active thrust fault systems. One possible interpretation is that these shear bands are not accommodating movement along the fault but are instead fluid migration pathways in the footwall of an over-pressurized thrust fault system.