南海西南海盆构造演化的热模拟研究

Thermal modeling of the tectonic evolution of the southwest subbasin in the South China Sea

南海西南海盆的张裂和海底扩张是白垩纪末至中始新世南海形成过程中最重要的构造事件.本文采用三维有限单元法对该区的热演化过程进行了模拟计算.通过对变形、温度结构的计算,研究了西南海盆张裂变形、海底扩张持续时间、地幔物质上升、地壳岩墙沿扩张中心的挤入扩张活力、岩浆活动等.计算结果表明:由于其深部动力学条件不足,海盆一次扩张持续时间在10～15Ma之间,其后地幔物质的上升活动逐渐停止,地壳失去扩张动力,使得扩张中心成为残留扩张中心的死亡裂谷,而未构成中脊或中隆带.虽然该处地幔物质上升的潜力不足,但伴随局部的断裂,尤其是盆、缘边界的拆离拉张,仍能产生相当强烈的岩浆喷溢活动,导致此区海盆成型之后的海山崛起.

The rifling and seatloor spreading of the southwest subbasin of South China Sea were the most important tectonic events in the forming process of the South China Sea from late Cretaceous to Middle Eocene. We simulate the thermal evolution process of this region with the 3-D finite element method and research the rifling deformation in the southwest subbasin, the duration of seafloor spreading, the crush and spreading energy of the lithosphere dyke along the spreading cent the rising of mantle materials, er, and the magmatism by the computing deformation and temperature structure. The result indicates that the extension duration of the sea basin is 10 to 15Ma once, because of the deep dynamics condition is insufficient. The rising of mantle materials stops gradually and the lithosphere loses the extensional force, and then the spreading center becomes a death rift valley with the residual spreading center but not a ridge or a central uplift belt. Though the rising potential of the mantle materials in this area is insufficient, there are quite a lot of violent magma spewing activities indicating the growing up of seamounts after the molding of the sea basin in the region with the partial rifting, especially the extension and detachment of the boundary between the basin and the margin.