Active Subhorizontal NE-SW Stretching Across the Eastern Margin of the Southern Central Range of Taiwan at ~23.1°N

Abstract:

Kinematic data for outcrop-scale faults and strain inversions for shallow earthquakes across the eastern slope of the southern Central Range of Taiwan suggest subhorizontal NE-SW stretching plays a role in ongoing exhumation. We document oblique strike-slip near the ridge crest and normal faulting on the eastern flank of the range, kinematics rarely emphasized to date. We invert shallow earthquake focal mechanisms for strain (cosserat model) and show that stretching is accommodated on nodal planes whose geometries vary systematically. Close to the crest of the Central Range from north to south we find a transition from oblique normal faulting on moderately N- and NE-dipping nodal planes to strike-slip faulting on steeply W-dipping and N-dipping nodal planes. On the eastern flank of the Central Range ~13 km south of the Southern Cross Island Highway (SXIH), earthquakes reveal normal faulting accommodated largely on moderately SW- and NE-dipping nodal planes. We also invert brittle faults exposed along the SXIH for strain. Forty-one of the 44 faults record normal or oblique normal displacement with the majority of faults dipping steeply NE and W. The best-fitting strain tensor for these faults indicates subhorizontal NE-SW maximum stretching, similar to the strain tensor for the earthquakes to the south. Some of the faults dip only ~20° suggesting surface processes may be important (e.g., mass movement). Nineteen of the normal faults, however, dip >60° suggesting that they may be linked with faults at seismogenic depths that reflect tectonic strain. Most of these faults dip steeply to the NE with geometries similar to many of the preferred nodal planes from our strain inversions for earthquakes to the south. If the strain geometries we document are representative of processes operating at 10⁵–10⁶ years, then two important conclusions can be made. (1) Normal faults play a role in the exhumation of rocks on the eastern flank of the orogen. These faults largely record SW-side up motion that may contribute to rock uplift along the range crest. (2) Subhorizontal stretching for all areas investigated is at a moderate angle to the orogen suggesting that neither the mapped lithologic contacts nor the topographic grain reflect ongoing deformation in a simple way. This highlights a possible role for deeper, as yet unknown processes.