ACTIVE DEFORMATION OF A TRISHEAR FOLD IN THE SLATE BELT OF SOUTHERN TAIWAN

The Taiwan orogeny is located in the convergent boundary between the Philippine Sea Plate and the Eurasian Plate and has grown during the last few million years as the Philippine Sea Plate collided with the continental margin of China. The resulting collision zone is composed of several tectonostratigraphic units separated by major structural boundaries. To better understand the kinematics of one of these major boundaries, we completed a 3-week field program along the Laonung River in southern Taiwan and integrated available geodetic data and crustal tomography data. In this area, the geometry of penetrative slaty cleavage and associated stretching lineations indicate a regional-scale, SSW-plunging antiform that verges to the NW. The absence of a crenulation cleavage in outcrops and thin sections and the abundance of late-stage faults and brittle structures suggest that the fold is a post-metamorphic structure. The vertical displacement rate data provided by leveling and GPS surveys from the past decade and the dated strath terraces along the Laonung River show higher uplift rates in the crest of antiform, suggesting that the anticline is an active structure. We then use the geometry of the folded cleavage and a trishear folding mechanism to model the development of this regional-scale fold. The results suggest that the dominate structure is a high angle reverse fault, which agrees with the relatively linear trace of the inferred fault zone, locally recognized as the Tulungwan fault. Recent tomography data show overturned velocity contours beneath the Central Range suggesting the present of a crustal-scale, west-verging thrust. We therefor propose that the Tulungwan Fault is a branch a crustal-scale detachment in the southern Central Range.