Extrusion, Extension and Exhumation in an Emerging Mountain Belt, Taiwan

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This goal of this project is to test the new hypothesis that extension in southern Taiwan initiated 1 to 2 million years ago as the Philippine Sea plate initially collided with a zone of transitional continental crust (5 to 20 km thick) followed by collision with continental crust of more typical thickness (about 30 km). The transition in collision mode initiated along-strike flow or extrusion and local areas of extension, especially in the southern Central Range of Taiwan. Field-based structural geology (brittle and ductile structural analyses) and low-temperature thermochronology (fission track and (U-Th)/He dating) coupled with seismological (focal mechanisms, cluster geometries) and GPS analysis will be carried out to determine: 1) if and when exhumation changed from relatively slow to fast rates; 2) the regional extent of the any change in exhumation rates; 3) if there has been a change from shortening to extension; 4) determine which structures accommodate extension and rock uplift; and 5) document southward propagation of specific structures, structural sequences or exhumation. The overall goal will be to document the current and past kinematics in the area and then tie the interpreted kinematic evolution to the history of uplift and exhumation as revealed by the thermochronologic data.One of the most significant recent breakthroughs in understanding processes that affect the Earth's surface is the discovery that some contractional orogenic belts also experience horizontal extension. The orogenic belt in Taiwan provides an unusual opportunity to better constrain the origin and significance of synorogenic extension. The orogen straddles the boundary between the Eurasian and Philippine Sea plates and is actively propagating as the two plates collide. In the area where the orogen is just emerging above sea level, a dense GPS network shows a broad zone of relatively fast extensional strains. The topography in this area also climbs to the maximum elevation in the collision zone (just under 4 km) and available thermochronological data suggest relatively high uplift rates. Why is this area of well-documented extension also an area of substantial rock and surface uplift? Answering this question would greatly improve understanding of processes that operate during collisonal mountain building episodes.