The Seismic Tomographic Study For The Western Margin Of The Philippine Sea Plate

The Philippine Sea Plate, located among the Eurasian Plate, Pacific Plate andIndo-Australian Plate, is surrounded by subduction zones. Because of the subduction,there are abundant of seismic events and stations which provide sufficient data forseismic tomography. The western border of the Philippine Sea Plate is the ManilaTrench, along which the Eurasian Plate is subducting beneath the Philippine Sea Plate.However, from the Manila Trench northward and southward, the Philippine Sea Plateis subducting beneath the Eurasian Plate along the Ryukyu Trench and PhilippineTrench, respectively. Therefore, it is helpful for understanding the tectonic evolutionof the area to study the morphology of the subducted Eurasian Plate (South China Seaslab) at the Manila subduction zone.The data utilized in the paper is P wave arrival times from InternationalSeismological Centre (1960~2008), comprising regional and teleseismic data. Theseismic tomographical method is applied to invert the regional and regional+teleseismic data to obtain the velocity perturbations in the study area. Based on theinversion results, we have an insight to understand the evolution of the Manilasubduction zone.The results from the regional data reveal that the subducting angle of the SCSslab decreases northward from14°to18°N. Especially at16°N and17°N, it is lack ofthe high velocity anomalies representing the subducting South China Sea slab. And at18°N, the high velocity anomalies are discontinuous at about60km depth from120.5°to121°E. At the map views of60km depth, there are three high velocity anomalyzone in the west, then at80and100km depth, only two separate high velocityanomaly zone can be seen, and the northern part shifts eastward relative to thesouthern part. These suggest the existence of slab tear along the axis of the fossiloceanic ridge of the South China Sea, which could explain the seismicity andvolcanic gap, geochemical difference between the extinct Miocene and Quaternaryvolcanoes in the Luzon Arc, the abnormally much higher heat flow at around16°Nand the adakites and the related porphyry Cu-Au deposits in the Luzon area. The slab tear can also interpreted by the results from regional+teleseismic data, which showthat the subducting angle at17°N is very low, whereas the angles at other latitudes aremuch higher.Then we calculate the length and time of the subducted slab along14°N-18°N,which suggest that the beginning time of the South China Sea decreased northwardfrom14°N, and the size of the original South China Sea was about twice its presentsize. A simple tectonic evolutional model is proposed based on the above analysis.The model suggests that before8Ma, the movement direction of the Philippine SeaPlate was NNW, and a large shear zone connected between the Philippine Sea Plateand the Eurasian Plate. The Manila Trench had formed before15Ma, and propagatednorthward along the shear zone pushed by the Philippine Sea Plate. At8Ma, themovement direction of the Philippine Sea Plate changed to NW, and the shear zonetranslated to the Manila subduction zone. The convergent rate between the PhilippineSea Plate and the Eurasian Plate increased. At about6Ma, the collision between thePhilippine Mobile belt and the Palawan Microcontinent ceased, which caused theLuzon Island rotated counterclockwise. The slab tear of the South China Sea alongthe fossil ridge occurred because of the Western Luzon Oceanic Plateau and thenorthwestward movement of the Philippine Sea Plate.