SUBDUCTED LITHOSPHERE UNDER TAIWAN: TOMOGRAPHIC EVIDENCE FOR PROGRESSIVE FLIPPING OF SUBDUCTION AND CONTINENTAL DELAMINATION

Improvement in global and local tomography allow for testing of models of the oblique arc-continent collision and reversal of subduction polarity near Taiwan. Furthermore they reveal previously unpredicted phenomena, including the delamination of continental mantle lithosphere with lateral inflow of hot upper mantle.

Classic models recognize that the oblique collision of the N-S trending Luzon arc of the Philippine Sea plate with the NE-SW trending China continental margin in Taiwan requires either [1] an obliteration of a trench-trench transform or [2] a progressive flipping of subduction polarity involving progressive tearing of the Eurasian plate along the continental margin associated with progressive termination of collision. In addition 2D models of arc-continent collision require slab breakoff. These and other more complex models make unique predictions of subducted slab geometries.

Modern global tomography (MITP08, 70km grid) augmented with local tomography near Taiwan provides good imaging of the subducted Eurasian and Philippine-Sea lithosphere in the upper mantle and images the predicted torn subducted edge of the Eurasian slab. Slab geometries are in close agreement with the progressive tearing model [2] and in strong disagreement with the transform obliteration model [1]. No slab breakoff is observed, in contrast with 2D models of arc-continent collision. Furthermore, the subducted western edge of the Philippine Sea plate continues northward ~1000km, nearly to Shanghai.

However, it is surprising that the location of the Eurasian tear is not at the edge of the continent but ~250km inboard, under the Eurasian continental shelf. The subducted continent-ocean boundary is clearly shown in the seismic velocities within the subducted Eurasian slab, with lower velocities in the continental part. Furthermore the tear does not propagate through the overlying continental shelf, therefore the continental mantle under the final collisional mountain belt subducts by delamination attached to the oceanic part of the Eurasian plate. This delamination process occurs instantaneously in the zone of flipping and involves the progressive lateral intrusion of hot mantle into this migrating zone from the Okinawa-Ryukyu mantle wedge, which lies above the subducting Philippine-Sea plate northeast of Taiwan.