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

Taiwan has often been used as a classic example of arc-continent collision and of reversal of subduction polarity, but the precise geometry and kinematics of the system in 3D has remained elusive until recently. It is located at a complex plate junction, where two subduction zones come together in a quasi-orthogonal, kinematically stable configuration.

We find that under Taiwan the upper crust of Eurasia is largely decoupled from the rest of the lithosphere by a detachment, which forms also the main subduction interface between Eurasia and Philippine Sea plate (PSP) at depth. This interface is mappable in both seismicity and crustal tomography at shallow depths, and can be followed into the mantle to depths of 450-500 km using global tomography. The dip of the plate interface between Eurasia and PSP in the upper 100 km changes from ~60° in the south, to vertical (south-central Taiwan), to overturned (north-central Taiwan). The Eurasian Moho is similarly folded, but it does not overturn. Shortening across the plate boundary is accomplished by a combination of subduction of Eurasian lithosphere, folding and thrusting in the Eurasian upper crust, and a secondary subduction zone within the PSP.

We hypothesize that: (1) once arc-continent collision occurs, subduction of Eurasian lower crust and upper mantle can continue by lithospheric delamination and by continuity with the much larger Eurasian slab to the south; (2) the upper crust of Eurasia deforms by faulting and folding; (3) the increase in convergence rate (from ~30 to ~90 mm/yr) in the last ~2 Ma is currently being taken up by secondary subduction within the marginal zone of PSP (i.e. Coast Range and Longitudinal Valley); (4) a margin-parallel STEP (Subduction-Transform-Edge-Propagator) fault forms the northern limit of Eurasian subduction, which allows the whole system to propagate self-similarly southwestward.