Study of magma-starved extension in the southern Philippine Sea

帕累斯贝拉盆地是菲律宾海弧后盆地中一个已经灭绝的盆地，一直被认为是一些大洋核杂岩(沿大型滑脱断层在地表附近/表面暴露的下地壳和上地幔物质)发育的地方。该盆地被认为是在岩浆匮乏、构造主导的环境下形成的。其中一个核杂岩是世界上最大的杂岩之一，扩张速度较快，是了解岩浆匮乏扩张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，核心复杂分布的范围和盆地的初步演化史也没有得到解决。在本次研究期间，我们于2005年进行了两次地球物理和岩石学调查，取得了以下成果1。在13-14°N之间发现了另一个大型岩心杂岩，辉长岩就是在那里恢复的。还绘制了其他可能的核心杂岩图，表明盆地规模的、岩浆匮乏的扩张发生在盆地形成的最新阶段。在岩心杂岩面上首次发现了超糜棱岩和糜棱岩，表明存在从地幔岩石圈发育而来的大型滑脱断裂。首次绘制了盆地南端的构造图，并提出了盆地形成的早期模式。在盆地南端发现了指示强烈岩浆活动的独特地形，这可能是弧后开张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.