Western Pacific marginal basin plate tectonics: overview, questions, and insights from mantle structure

Abstract

<p>The western Pacific marginal basins are a collage of plates and marginal seas that primarily formed in the Cenozoic and occupy the complex tectonic area between the converging major Eurasian, Indo-Australian and Pacific plates.  Strongly contrasting plate tectonic reconstructions continue to be proposed for this region.  Here we review current plate models, key questions, and highlight recent insights from mantle structure studies. </p><p>The Philippine Sea plate (PSP) is one of the largest marginal seas in the region.  The plate is currently moving WNW, in a similar orientation to the Pacific and Caroline plates.  Multiple studies show that PSP plate tectonics are key to unravelling the history of the region, for several reasons: (1) the PSP is far-travelled; paleomagnetism indicates relatively rapid (5.5 cm/yr) PSP northward motions since the Eocene from a near-equatorial latitude; (2) PSP tectonic histories imply interactions with many surrounding plates and regions, including the South China Sea (SCS), Taiwan, Japan, the western Pacific, and other smaller plates or fragments (i.e. Luzon or Celebes Sea); (3) the pre-subduction size of the PSP is not well-established because most of the plate is surrounded by subduction zones.  We review recent studies that attempt to ‘unsubduct’ the PSP from tomography and discuss their regional implications.</p><p>Another key but highly controversial topic is South China Sea (PSCS) formation and associated proto-South China Sea (PSCS) disappearance, if such a plate had existed.  The most popular proto-South China Sea plate model invokes southward subduction of the proto-South China Sea beneath NW Borneo during the Cenozoic.  Although southward PSCS subduction is most consistent with our current geological understanding, the regional mantle structure does not easily fit with this model; other scenarios may be possible.  In particular, it has not been straightforward for the southward PSCS model to explain the origin of sub-horizontal slabs under the present SCS at relatively shallow (500 to 800 km) depths.  We show an alternative ‘double-sided’ PSCS subduction model that produces testable hypotheses for future NW Borneo studies. </p><p>Finally, the Caroline Sea plate has an enigmatic history but is important for understanding the southern Marianas, Yap and Palau trenches.  We first review previous evidence for a ‘Caroline hotspot track’ and overlapping LIPs that traverse the northern Caroline plate.  We link these features to an imaged lower mantle plume and show their implications for Caroline-Pacific plate motions since the latest Eocene. </p>