初始地壳厚度和伸展系数的计算方法及其在南海北部深水区的应用

Stretching factor, indicating the amount of stretching of passive continental margins, on which the heat flow and hydrocarbon maturation depend, is calculated either by the definition (the ratio between initial crustal thickness and thinned crustal thickness) or by the observed tectonic subsidence assuming the initial crustal thickness is constant which can not be measured directly. Actually, the initial crustal thickness of passive margins may change violently spatially, especially on the northern margin of South China Sea. Researches have proved the crust of Baiyun deepwater region of Pearl River Mouth Basin was characterized of thinned initial crustal thickness before rifting. So, it is not necessarily correct to calculate the stretching factor using the methods above assuming a constant initial crustal thickness. A new method is developed to simulate the initial crustal thickness and the stretching factor controlled combined by the present crustal thickness and observed syn-rift tectonic subsidence through the model of strain rate on the selected seismic surveys covered deepwater regions on the northern margin of South China Sea taking the anomalous post-rift subsidence into account. The modeling results are helpful to recover the paleogeography and paleotectonic background of deepwater regions to the north of South China Sea and have a great significance in understanding the dynamic mechanism and evaluating the hydrocarbon potential of deepwater basins.

伸展系数是量化被动陆缘盆地伸展减薄程度的重要参数，也是热史和生烃史计算的基础参数，但由于初始地壳厚度无法通过测量获得，地质学家们主要通过假设初始地壳厚度按照定义（初始地壳厚度与现今地壳厚度之比）或根据实测构造沉降反演等方法获得。然而，被动陆缘初始地壳厚度在空间上可能变化很大,尤其是南海北部陆缘,已有研究表明珠江口盆地白云深水区在伸展前为减薄的地壳，前述方法对初始地壳厚度的估算可能偏差很大，使伸展系数的计算不一定准确。针对上述问题，本申请拟选取地震测网覆盖较好的南海北部陆缘深水区，考虑到其裂后异常沉降的存在，将在现今地壳厚度和裂陷期构造沉降双重实测数据的共同约束下，通过应变速率反演方法模拟初始地壳厚度和伸展系数，模拟结果不仅为认识南海北部陆缘深水区在伸展前的古地理、古构造背景提供帮助，而且对分析深水盆地在新生代的形成机制和油气资源评价有重要的理论和实际意义。

初始地壳厚度是影响伸展系数计算最重要的参数。由于初始地壳厚度无法通过直接测量获得，而通过假设初始地壳厚度来计算伸展系数则会带来很大的误差。本课题提出根据现今地壳厚度和裂陷期构造沉降由一维有限拉伸方法来反演初始地壳厚度，并编程和应用到南海北部的琼东南盆地和珠江口盆地。结果表明南海北部陆缘的初始地壳厚度很不均匀，琼东南盆地初始地壳厚度由北部的~26 km向南逐渐增加到~34 km，但珠江口盆地则由北部的~30 km向南逐渐减小至~15 km。推测琼东南盆地在张裂前处于弧后的构造位置，而珠江口盆地白云凹陷则可能位于弧前的构造位置。此外，根据初始地壳厚度计算的伸展系数要比假设初始地壳厚度为32 km计算的伸展系数小。南海北部陆缘新生代平均应变速率的数量级约为10-15，且裂后存在有快速构造沉降。