下地壳流对南海北缘白云凹陷地壳伸展变形的影响

THE EFFECT OF LOWER CRUSTAL FLOW ON THE CRUSTAL EXTENSION OF THE BAIYUN SAG ON THE NORTHERN MARGIN OF SOUTH CHINA SEA

  • 摘要: 下地壳流经常被用来解释地壳随深度的差异拉张现象,但下地壳流对地壳伸展变形的定量研究却不多见。以白云凹陷的岩石圈伸展变形为研究对象,根据普拉特-海福特重力均衡模式,假设凹陷区原始地壳厚度为32 km,即均衡深度,利用现今的基底厚度、沉积层厚度和水深数据恢复到白云凹陷变形前的原始地壳厚度,发现其值大于32 km,在32.599~33.774 km之间,并且从白云凹陷陆架区向海洋方向递增。我们认为造成这种情况的原因可能是由于下地壳流失导致地幔物质上涌量增多,而且根据数据递增的变化认为下地壳流失从海洋向白云凹陷陆架区方向。在凹陷区和接近海盆地区,由于下地壳的流失,导致全地壳拉张因子减小。

     

    Abstract: Lower crust flow is often used as the key to explain the phenomenon of depth-dependent stretching. However, quantitative studies of the flow and its effects on crustal deformation are rare in lietrature. In this paper, the lithospheric extensional deformation of the Baiyun Sag is selected as a case of study using the Pratt-Hayford isostatic model. Assuming that the original crustal thickness (i.e. equilibrium depth) of the Baiyun Sag is 32 km, the original crustal thickness estimated from the observed thicknesses of the basement and sedimentary cover, and the water depth will be larger than this figure and within the range of 32.599-33.774 km. It becomes thicker from the continental shelf to the ocean basin due to the increase in mantle material upwelling caused by the lower crustal loss. According to the calculation, the lower crustal loss increases from the ocean towards the shelf direction. The loss of lower-crustal flow results in the decrease in the whole crustal extension factor in the Baiyun Sag up to the value of the ocean basin.

     

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