ALONG-STRIKE VARIATIONS IN LARGE-SCALE TECTONIC GEOMORPHOLOGY OF THE TAIWAN OROGEN; IMPLICATIONS FOR OROGENIC DEVELOPMENT

The overall morphology of an active orogenic belt generally reflects the first-order tectonic controls on uplift and deformation. Accordingly, examination of the first-order tectonic geomorphology of a mountain belt should yield insights into along- and across-strike variations in underlying tectonism. The western Taiwan orogen displays large-scale along-strike variation in morphology that reflects variations in underlying tectonic processes. The southwestern Central Range and Foothills (north of the Hengcheng Peninsula) are characterized by an orogen-parallel drainage network, such as the upper Tsengwen, Nantzuchien, and Kaoping rivers. The rapidly propagating fold belt in the Southwestern Foothills is manifested by deformed Holocene marine and fluvial terraces, which record extremely rapid uplift (up to 10 mm/yr or more), which provide evidence of rapid and recent propagation of the fold belt. This pattern is interrupted by the Alishan highland, a highland that extends west from the range crest. North of this the drainage network in the western Central Range is characterized by orogen-normal rivers, such as the Choshui and Wu rivers. The Foothills belt is distinct north of the Alishan highland as well: the mountain front here is abrupt and continuous, marked by the Changhua fault, in contrast to the emerging range-front to the south. The fault belt appears to be propagating westward, as the Changhua fault has uplifted late(?) Quaternary deposits that were originally shed across the Chelungpu fault-bounded range front (site of the 1999 Chichi earthquake). The key difference between this portion of the Foothills and that to the south is the greater continuity of surface faults, as reflected in the continuity of the range front and internal range fronts. This implies that fault/fold behavior changes from north to south across the Alishan highland, and further that seismic behavior should be distinctly different as well. The fundamental difference in morphology also implies that southern Taiwan does not provide a space-for-time substitution for understanding evolution of the orogen; southern and central Taiwan have evolved differently, producing different landscapes.