南海北部地壳密度结构:基于约束三维重力反演

地壳结构的揭示是研究陆缘伸展机制的基础.尽管在南海北部陆缘已开展了大量地壳尺度的二维地震探测,但目前还存在许多覆盖空白的地区,这些调查所得到的结果无法提供地壳结构的区域视图.为了揭示南海北部的地壳结构,本研究将水深和沉积物厚度信息作为约束条件,对空间重力异常进行区域三维反演,并利用地震研究的结果来衡量反演结果的质量.沿地震测线的密度切片显示,重力反演结果与地震研究结果具有良好的一致性.本研究通过选择两个与地震研究结果最匹配的密度异常等值面分别作为康拉德面和莫霍面,获取了莫霍面深度和上、下地壳的厚度.根据假设的相应初始地壳厚度,本研究进一步计算了全地壳、上地壳和下地壳的拉张因子.通过与已发表的居里面深度比较,发现南海北部大部分地区的居里面深度均位于莫霍面之下,其中西沙海槽的拉张因子βw大于3.5并且缺乏岩浆活动,是地幔橄榄岩蛇纹石化的潜在区域.南海北部陆缘的地壳拉张因子显示其经历了伸展方向为128°和160°的两组张裂运动,分别对应于神弧运动和珠琼运动一幕及二幕,在张裂过程中应力场发生了顺时针旋转.此外,上、下地壳的拉张因子表明北部陆缘普遍存在正向和反向差异伸展,陆架区域表现为反向差异伸展,洋陆过渡带为正向差异伸展,推测这种正向和反向差异伸展可能是由下地壳流动导致的,由地壳厚度差异引起的横向梯度力、软流圈浮力和沉积物负载共同驱动.

Crustal density structure of the northern South China Sea from constrained 3-D gravity inversion

The imaging of crustal structure is the basis of studying the extension mechanism of the continental margin. Although a large number of crustal-scale 2-D seismic surveys have been carried out on the northern South China Sea, there are still many areas lacking seismic coverage, and the results obtained from these surveys cannot provide a regional view of crustal structure. To resolve the crustal structure across the northern South China Sea, a 3-D regional inversion of the free air gravity data constrained by bathymetric and sediment thickness information was undertaken, and the results from seismic studies were used to assess the quality of the inverted results. Density anomaly slices along the seismic profiles show an excellent agreement between the inverted results and the seismic Moho. We select two density anomaly isosurfaces that match well with seismic constraints to represent the depth of Moho and Conrad and to provide Moho depth and the upper and lower crustal thickness. Then, the crustal stretching factor as a whole and upper and lower crust are computed by assuming the corresponding initial thickness. Comparing the Moho depth with the published Curie depth, the results show that the Curie depths are larger than Moho depths in most of the northern South China Sea. Among them, the Xisha Trough with crustal stretching factor greater than 3.5 and starving post-rift magmatism is a potential area for serpentinization of the uppermost mantle. The crustal stretching factors (β_w) in the northern margin of the South China Sea show that it has experienced two groups of rifting with extension directions of 128° and 160°, corresponding to Shenhu event and the first and second episode of Zhuqiong event, respectively. The stress field rotates clockwise during rifting. In addition, the stretching factors of the upper and lower crust indicate that the positive and inverse extension discrepancy are widespread in the northern continental margin. The shelf regions exhibit inverse extension discrepancy, and the oceanic-continental transition zones display positive extension discrepancy. It is conjectured that the positive and inverse extension discrepancy may be caused by the lower crustal flow, which is driven by a combination of the lateral gradient press arising from the crustal thickness variations, the asthenospheric buoyancy and the sediment loading.