INVESTIGATING THE NE-SW STRETHCING DIRECTION AS RESULT OF MESOSCALE NORMAL FAULTS AND SHALLOW CRUSTAL EARTHQUAKES IN SOUTHERN CENTRAL TAIWAN USING THERMOCRHONOMETRIC DATA

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 model 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. In January 2014, we conducted fieldwork in an ~E-W transect along the south cross island highway (SXIH, 23.11°N) to understand the brittle structures that may help account for the high uplift rates (~5-16 mm/yr) documented in this area. We documented abundant outcrop-scale normal faults in the area of Litao (~121.03°E) and abundant strike-slip faults to the east near Haiduan (~121.06°E). Strain inversions assuming a micropolar model of deformation show that these faults reflect subhorizontal NE-SW trending maximum principal stretching, at a moderate angle to the topographic grain and lithologic strike of the orogenic belt. Inversions of earthquake focal mechanisms for shallow crust events reveal similar strain geometries accommodated by normal slip or strike slip. Normal slip tends to occur around ~ 120.93°E, whereas the strike slip occurs around ~121.04° E. Analyses of peak temperature proxies along the SXIH suggest a continuously varying thermal structure with the highest temperatures at approximately at 121.05°E, east of where leveling data suggest the highest short-term uplift rates are. We hypothesize that the strain field in this region reflects 3D complexity that is important to interpreting exhumation histories. Our inversions of earthquake data and mesoscale faults indicate that data parallel to the SXIH reflect non-plane strain, and therefore, it is essential to look in NE-SW profiles to understand the relation between recent deformation and exhumation. If the earthquakes and faults are contributing to the exhumation process, we predict that peak temperature proxies along NE-SW profiles will reveal substantial discontinuities if thinning is focused and accommodated by simple shear, or smaller step like discontinuities if thinning is diffuse and accommodated by pure shear. The best paleothermal tool for this remains an important question.