THE LONGITUDINAL AND VERTICAL DISTIRBUTION OF BRITTLE DEFORMATION IN THE SOUTHERN CENTRAL RANGE TAIWAN: CONSTRAINTS FROM EARTHQUAKES AND MESOSCALE FAULTS

The Taiwan orogenic belt straddles the boundary between the Philippine Sea and Eurasian plates and is the product of the collision between the north-trending Luzon volcanic arc and the east northeast trending Chinese passive margin. The prevailing paradigm is that progressive collision has resulted in zipperlike mountain building along the oblique margin, although recent thermochronometric data suggest collision may have been simultaneous from north to south. Regardless, the Philippine Sea plate, which hosts the Luzon volcanic arc, is moving at a rate of ~82 mm/year relative to Eurasia. In the Southern Central Range, the metamorphic core of Taiwan displays very high uplift rates, up to 18.5 mm/year based on land-based surveying methods. Based on thermochronometric data the likely exhumation rate in the last ~1-2 My is likely closer to ~2.5-5.0 mm/year, which is still geologically rapid. In January 2014 we conducted fieldwork along the southern cross island highway (SXIH) aimed at understanding the brittle structures that might reflect the high uplift rates. We documented abundant outcrop-scale normal faults in the area of Litao (~121.03°E) and more abundant strike-slip faults to the east near Haiduan (~121.06°E). Strain inversions reveal that all of these faults reflect subhorizontal NE-SW trending maximum principal stretching, at a moderate angle to the topographic grain of the orogenic belt. Inversions of earthquake focal mechanisms for events in the shallow crust reveal similar strain geometries accommodated also by normal slip or strike slip on model nodal planes. The normal slip tends to occur around ~ 120.93°E. Whereas the strike slip dominates around ~121.04° E. This raises questions: In this area of the SXIH where subhorizontal maximum principal stretching appears consistently oriented, what controls where crustal thinning (normal faulting) versus conservative slip (strike slip faulting) is occurring? What controls the vertical distribution of fault types? Why is the stretching direction oblique to the major tectonostratigraphic units and topographic grain of the island? To solve these questions we will strategically pick clusters of seismic events in the shallow crust to better understand the horizontal and vertical distrubution of deformation styles.