Study of magma-starved extension in the southern Philippine Sea

Parece Vela盆地是菲律宾海的一个已灭绝的弧后盆地，是一些海洋核杂岩（下地壳和上地幔物质暴露在地表附近/沿大型拆离断层）发育的地方。盆地被认为是在岩浆匮乏、构造主导的环境下形成的。其中一个岩心杂岩是世界上最大的杂岩，扩展速度较快，因此是了解岩浆匮乏扩展和滑脱断裂发育的重要目标。盆地南部尚未进行系统的调查，对岩心杂岩的分布范围和盆地的初始演化史仍没有定论。在研究过程中，我们于2005年进行了两次地球物理和岩石学巡查，取得了以下成果：在13 ~ 14°N之间发现了大型岩心杂岩，并发现了辉长岩。其他可能的岩心杂岩也被绘制出来，表明在盆地形成的最后阶段发生了盆地尺度的岩浆饥渴扩张。在岩心复杂表面首次发现了超长、糜棱岩，表明存在从地幔岩石圈发育而来的大型滑脱断裂。首次圈定了盆地南端构造，提出了盆地早期形成模式。盆地南端独特的地形特征表明岩浆活动强烈，可能是弧后裂口之前形成的较老地壳的一部分。

The Parece Vela Basin is an extinct backarc basin in the Philippine Sea and has been know as a place where some oceanic core complexes (lower crust and upper mantle material exposure near/on the surface along a large detachment fault) develop. The basin is considered to have formed under magma-starved, tectonic dominant environment. One of the core complexes is the world-largest one and the spreading rate was relatively fast, so it is an important target to understand the magma-starved spreading and the development of detachment faults. The southern part of the basin has not been systematically surveyed and the extent of the core complex distribution and an initial evolutional history of the basin have remained unsolved. During this study, we conducted two geophysical and petrological survey cruises in 2005, and obtained the following results.1. Another large core complex was found between 13-14°N, where the gabrroic rocks were recovered. Other possible core complexes were also mapped, suggesting the basin-scale, magma-starved spreading occurred in the latest stage of the basin formation.2. Ultramylonite and mylonite were first reported on the core complex surfaces, suggesting the existence of large detachment fault developing from mantle lithosphere.3. The structure of the southern tip of the basin was first mapped and a model of early stage of basin formation was proposed. Unique topography indicating robust magmatism was found in the southern tip of the basin, which may be a part of older crust formed before the backarc opening.