Gravity Inversion on Crust Structures of the Shikoku Basin, Philippine Sea, and Its Implication to the Evolution Process

The Shikoku basin is a back-arc basin in the Philippine Sea which experienced arc rifting in Cenozoic. Study on deep structures plays an important role in understanding the break-up of the Izu-Bonin-Mariana arc and back-arc rifting history. With constraints of multi-channel seismic data and refraction seismic data, as well as the magnetic data, we carry out gravity inversion on two profiles crossing the whole Shikoku basin to study the density changes along the lines, and to reconstruct the deep structures. Results show that the oceanic crustin creases gradually from west to east, and the Moho depth increases sharply in the residual spreading ridge. According to the crustal density and thickness, we separate the Shikoku basin into three domains:the thin nedoceanic domain, the thickened oceanic domain, and the domain affected by post-spreading magmatism, which corresponds to th einitial slow spreading stage, single-wing fast spreading stage, and the symmetrical spreading stage, respectively. The spreading rate calculated from the different tectonic domains fits well with that from the magnetic omalies. High-density body exists under the oceanic crust with in thinned oceanic domain, which is correspondence with high-velocity layer in the refraction seismic velocity model. We suggest that this high-density body was formed by mantle serpentinization caused by active detachments during slow-spreading stage.