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白云凹陷是珠江口盆地最深的凹陷,中心处沉积物厚度达12.5km以上,其中30Ma破裂不整合面以上厚度达6.5km以上.按照张裂盆地构造发育的理论模型,如McKenzie的经典理论模型,裂后由于冷却收缩而产生的盆地沉降随时间的变化是岩石圈伸展系数的函数,可以计算出来.白云凹陷裂后期的实测沉降量是否大于应用理论模型计算的热沉降量,即是否存在裂后异常沉降?为了回答这一问题,本文将实测与理论裂后沉降量进行对比.实测裂后沉降可通过回剥去除沉积物荷载等因素引起的沉降而得到,但理论热沉降的计算则需要知道盆地的岩石圈伸展系数,后者是未知的.本文设计了正演模拟和反演回剥相结合的方法解决这一问题,即应用基于挠曲悬臂梁模型的二维模拟方法正演模拟白云凹陷的幕式张裂过程,通过试错达到实测的张裂期剖面形态,从而获得岩石圈的伸展系数,计算出理论的裂后热沉降,并与回剥得到的实测裂后沉降量进行对比.结果不仅表明白云凹陷的确存在裂后异常沉降,而且揭示了异常沉降的时空特征:30Ma以来,快速沉降主要发生在17.5～13.8Ma;凹陷中心的异常沉降量最大,达2km,其次为凹陷南侧的异常沉降量,明显大于凹陷北侧;根据凹陷两端的异常沉降量没有降低的趋势,推测向凹陷北部的陆架区和凹陷南部的洋盆方向依然存在裂后异常沉降.正演灵敏度实验还表明,白云凹陷的张裂期剖面形态只能通过低角度(≤13°)的边界断层的伸展来获得.针对可能引起白云凹陷裂后异常沉降的机制,初步探讨了几种可能机制,认为东亚季风增强导致的沉积物供给量增加以及由此导致的下地壳流动可能是主要原因.
Baiyun sag is the deepest depression in the Pearl River Mouth Basin, and the thickness of the sediments in the center is more than 12.5km, of which the thickness above the 30Ma unconformity surface reaches more than 6.5km.According to the theoretical model of structural development in the Cracked Basin, such as McKenzie’s classical theoretical model, The settlement of the basin after cooling due to cooling and shrinking changes with time as a function of the lithospheric extension coefficient and can be calculated.If the measured settlement at the post-fracture stage of Baiyun Sag is larger than the thermal settlement calculated by using the theoretical model, Abnormal Subsidence In order to answer this question, this paper compares the measured and theoretical subsidence after fracture.The measured subsidence after fracture can be obtained by removing the sedimentation caused by sediment load and other factors, but the calculation of theoretical heat sinking needs to know The latter is unknown.This paper designed a combination of forward modeling and back-stripping to solve this problem, that is, using the two-dimensional simulation method based on flexural cantilever model to simulate the clouds Depression curtain rupture process, through the trial and error to achieve the measured section of the fracture phase morphology, lithospheric extension coefficient obtained, calculated The results show that the Baiyun Depression does not only show the existence of post-fault anomalous subsidence, but also reveal the spatial and temporal characteristics of the anomalous subsidence: Since 30Ma, the rapid subsidence mainly occurs in the 17.5 ~ 13.8Ma. The anomalous subsidence in the center of the depression is the largest, reaching 2km, followed by the anomalous subsidence on the south side of the depression, which is obviously larger than the north side of the depression. According to the tendency of the anomalous subsidence at both ends of the depression not decreasing, The anomalous subsidence still exists in the direction of the ocean basin in the southern part of the depression and the depression.The forward sensitivity experiment also shows that the profile of the fracture in Baiyun depression can only be obtained through the extension of low angle (≤13 °) boundary fault. Some possible mechanisms are discussed preliminarily. The increase of sediment supply due to the increase of the monsoon in East Asia and the consequent lower crustal flow may be the main reason.