FOOTWALL GEOMETRY AND TOPOGRAPHY IN THE TAIWAN ARC-CONTINENT COLLISION

Previous interpretations of the arc-continent collision in Taiwan have treated the colliding passive margin as relatively homogeneous and used the approximately constant width of the orogen along strike to argue for a steady state in the regional-scale topography. More detailed analyses, however, suggest significant anomalies in both the topography and the geometry of the colliding continental margin. We present a new analysis of the topography and focus on a region in central Taiwan where the orogen is anomalously narrow and steep, and displays a relatively low ridge crest height; the cross-sectional area of the orogen in this area is therefore less than it is to the north and south. Using a 40-DEM, 61 basins and sub-basins were recognized and extracted for detailed study. The results show higher hypsometric integrals, higher first order stream gradients and higher stream length-gradient indices on both flanks of the Central Range with the highest values in the center of the range where it is anomalously low and narrow. This area also correlates with the location of an inferred continental prong or indenter in the underthrusting continental margin. We propose that the rigidity of the continental indenter and colliding arc constrain the orogen to be relatively narrow in this area, which limits the height of the orogen. The orogenic systems north and south of the indenter appear to have advanced and widened westward, thus allowing for topographically higher orogenic wedges in these areas. This is also consistent with a slight westward deflection in the ridge crest in the northern and southern regions. The area between the indenter and the arc may have also experienced more internal shortening, steepening the backstop and leading to eastward migration of the ridge crest. The higher geomorphic indices in this area may reflect the steeper flanks of the orogen in this area, higher uplift rates, or both. In either case, the presence of an irregular continental footwall appears to have resulted in an irregular orogenic system rather than a uniform orogen with a steady size and shape.