CONTRASTING LITHOSPHERIC-SCALE AND UPPER-CRUSTAL-SCALE ARCHITECTURE AND KINEMATICS OF THE TAIWAN ARC-CONTINENT COLLISION

Taiwan is a classic example of oblique arc-continent collision, which gives a progressive view of the collision process. Here two subduction zones come together in a quasi-orthogonal, kinematically stable configuration. By combining geology with recent earthquakes relocation and crustal tomography, we can define the present 3D geometry and kinematics. The upper crust of Eurasia is decoupled from the rest of the lithosphere below by the presence of a major detachment (Taiwan Main Detachment), which at depth forms the interface between Eurasia and Philippine Sea plate. The detachment is clearly visible in both seismicity and crustal tomography at shallow depths and can be followed into the mantle using mantle tomography. The plate interface between Eurasia and Philippine Sea changes from relatively shallow-dipping in the south (Manila trench) to vertical (south-central Taiwan) to overturned (north-central Taiwan). The Eurasian Moho is similarly folded, but it does not overturn, suggesting the presence of a fault ramp connecting the Moho with the detachment, inherited from the old passive continental margin. Shortening across the plate boundary is accomplished by a combination of subduction of Eurasian lower lithosphere, folding and thrusting in the detached Eurasian upper crust, and formation of a secondary subduction zone between Eurasian and Philippine Sea plates. We hypothesize that: (1) Once the continent enters the Manila trench and arc-continent collision occurs, subduction of Eurasian lower crust and upper mantle can continue thanks to decoupling from the upper crust by lithospheric delamination; (2) The upper crust of Eurasia deforms by faulting and folding in the accretionary wedge; (3) An increase in convergence rate took place in the last 1 Ma and is taken up by secondary subduction of the Central Range and Coast Ranges in Taiwan; (4) STEP fault propagation allows the whole system to move self-similarly southwestward.