THE SECOND HALF OF PLATE TECTONICS: FINDING THE LAST ~200MA OF SUBDUCTED LITHOSPHERE AND INCORPORATING IT INTO PLATE RECONSTRUCTION, WITH THE EXAMPLE OF EAST ASIA/PHILIPPINE SEA

Precise plate-tectonic reconstruction of the Earth has been constrained largely by the seafloor magnetic-anomaly record of the present oceans formed during the dispersal of the last supercontinent since ~200Ma. The corresponding world that was lost to subduction has been has been only sketchily known. We have developed methodologies to map in 3D these subducted slabs of lithosphere in seismic tomography and unfold them to the Earth surface, constraining their size, shapes, locations and times of subduction. Mapping to date suggests that a relatively complete and decipherable record of lithosphere subducted over the last ~200Ma exists in the mantle today, providing a storehouse for new tectonic discoveries.

We illustrate this endeavor with a study of the Philippine Sea plate, whose motions and tectonic history have been the least known of the major plates because it has been isolated from the global plate and hotspot circuit by trenches. We map and unfold 28 subducted slabs in the mantle under East Asia and Australia/Oceania, with a total subducted area of ~25% of global oceanic lithosphere, and incorporate them as constraints into new globally-consistent plate reconstructions of the Philippine Sea and surrounding East Asia since a time of major plate reorganization ~52Ma. These subducted-slab constrains, when combined with surface geologic and paleomagnetic constraints, lead to a number of new insights, including: [1] discovery of a major (8000 km x 2500 km) set of vanished oceans that we call the East Asia Seas that existed between the Pacific and Indian Oceans, now represented by a set of flat slabs in the lower mantle under present-day Philippine Sea, eastern Sundaland and northeastern Australian plates, [2] the East Asian Sea subducted during the northward motion of the Philippine Sea and Australian plates, [3] the Philippine Sea plate collided with the Ryukyu/SW Japan continental margin of Eurasia in arc-arc collision ~20-14 Ma, and [4] the Philippine Sea nucleated as a small trench back-arc system along the East Asian Sea/Pacific boundary, adjacent to the Manus plume, somewhat analogous to the recent nucleation of the Bismark Sea.