LITHOSPHERIC STRUCTURE OF TAIWAN FROM SEISMICITY AND CRUSTAL TOMOGRAPHY

Taiwan is a classic example of oblique arc-continent collision, which gives a progressive view of the collision process. Thanks to recent earthquake relocation and crustal tomography we can now define the present 3-D geometry of this collision in much better detail. The upper 10-15 km of crust consists of an accretionary wedge, largely composed of deformed continental margin, which is fully decoupled from the rest of the lithosphere below by the presence of a major detachment. The detachment is clearly visible in both the seismicity and the crustal tomography. This accretionary wedge accommodates plate convergence differently from the lithosphere below the detachment. The wedge shortens by faulting and folding, resulting in a fast-growing orogen with typical thin-skinned tectonics. The lithosphere below the detachment in contrast shortens first by subduction, but then mainly by large-scale lithospheric folding around a single hinge line.

The plate interface between Eurasia and Philippines coincides for most of its extent with the main detachment level of the Eurasian plate. This interface changes from relatively shallow-dipping in the south (Manila trench, where Eurasia subducts below the Philippines plate) to vertical (south-central Taiwan) to overturned (north-central Taiwan). Further north the subduction polarity flips, and the oceanic Philippines plate subducts under continental Eurasia. The Eurasian Moho is similarly folded, but it does not overturn, suggesting the presence of a fault ramp connecting the Moho with the detachment that has been inherited from the old passive continental margin.

We hypothesize that (1) once the continent enters the trench and arc-continent collision occurs, normal subduction shuts down and convergence has to be taken up by lithospheric-scale folding of Eurasia, with the Philippines plate acting mostly as an indenter, and that (2) the flipping of subduction polarity occurs because once the Moho is folded to vertical, lithospheric folding alone can no longer accommodate plate convergence to a sufficient degree. Flipping of subduction polarity results in deactivation of the orogen in the wedge, where no more accretion occurs.