Crust-mantle boundaries in the Taiwan-Luzon arc-continent collision system determined from local earthquake tomography and 1D models: Implications for the mode of subduction polarity reversal

摘要

In order to better understand the mode of subduction polarity reversal in the Taiwan-Luzon arc-continent collision zone, we mapped its crust-mantle boundaries using local earthquake tomography. By contouring surfaces of constant Vp=7.5kms~(-1), we identified three Moho discontinuities and the plate interface that juxtaposes Eurasian lower crust against mantle lithosphere of the Philippine Sea plate. The plate interface dips to the east under southeastern Taiwan and steepens progressively towards north until it becomes vertical at 23.7°N. From there it continues northward in a vertical orientation, until the limit of the tomographic model inhibited further mapping. For the Moho, additional depth constraints were derived from 1D models using P-wave arrivals of local earthquakes. The Mohos of the Eurasian and Philippine Sea plates are disconnected across the plate interface. Beneath southern Taiwan, the Eurasian Moho dips to the east at 50-60°, following the orientation of the plate boundary and continuous with the Benioff zone. Towards north, the Eurasian Moho steepens into subvertical, again together with the plate boundary. The Philippine Sea plate Moho exhibits a synform-like crustal root, interpreted as the base of the magmatic Luzon arc. Towards the north, this root deepens from 30 to 70km underneath the Ryukyu trench. In northernmost Taiwan, the hinge of the vertically subducting Eurasian slab steps westward out of the thrust belt, leaving the deformation front to the east inactive and giving way to the north-dipping Philippine Sea plate. A subhorizontal Moho at 30-35km depth overlies the north-dipping Philippine Sea slab and is interpreted as a newly formed Moho, established after westward rollback and delamination of the subducting Eurasian slab. In combination, these data support a model of progressive subduction polarity reversal, in which a tear within the Eurasian lithosphere propagated southwestward, deactivating the deformation front.