Vertical fault mapping within the Gutingkeng Formation of southern Taiwan: implications for sub-aerial mud diapir tectonics

Vertical faults were mapped around the periphery of the mudstone-rich Plio-Pleistocene Gutingkeng Formation (Gtk) in southern Taiwan. The faults are manifested as black colored, penetrating shear bands that intersect bedding planes of the Gtk (shear band dips range from 60 - 90 degrees; shear band thicknesses range from several centimeters to several meters). Thin (1-2cm), friable calcite veins were found locally within the shear bands and often contain weakly developed slickensides. Where available, the sense of shear of these faults show consistent interior upward motion of the Gtk. The Chishan Fault is the southeastern bounding structure of the Gtk and where exposed, near-vertical shear bands were observed with a normal sense of shear (west-side-up). In addition, three exploratory cores drilled in 2010 along the western edge of the hanging wall of the Chishan Fault did not intersect the fault or the Gtk after nearly 200 meters of drilling. This suggests that the main Chishan Fault may not be a typical top-to-the-west reverse fault as previously mapped and does not fit the typical fold and thrust geometry that is prevalent in the western foothills of Taiwan. Instead, we interpret the collective field evidence to suggest that the Chishan Fault is a near vertical structure that is accommodating uplift of the Gtk, similar to mechanism by which sub-aqueous mud diapirs grow. A chain of mud diapirs is located off the southeastern coast of Taiwan and the Gtk may be the onshore extension of this chain. The sub-aerial mud diapir hypothesis is further supported by recent leveling data collected across the Gtk that shows the largest vertical uplift in the region to be centered within the Gtk. Finally, observations at the western end of the Highway 3 tunnel through the Chishan fault and overlying Wushan Formation indicate that there is recent west side (footwall) uplift along the Chishan Fault. The vertical faults mapped within the Gtk are significant for they may provide evidence for a long-term history of vertical tectonics in this region that has shaped the structural geometries and geologic patterns of southwestern Taiwan.