边缘海盆地断层差异演化成因的数值模拟:以西湖凹陷平北斜坡带为例

Differential evolution of marginal basin fault: A case from numerical simulation of Pingbei Slope, Xihu Sag

  • 摘要: 东海陆架盆地中生代以来的成盆过程中,发育了大量陆倾控盆断裂,其发育模式与形成演化与东亚其他边缘海盆地差异明显。前人关注东海陆架盆地迁移特征,而忽略了断层差异演化的形成机制,对断层发育过程控制因素缺少深入研究。西湖凹陷平北斜坡带北部为海倾断层组成的断阶区,南部为陆倾断层组成的半地堑区,断裂差异演化指示着东海陆架盆地的成盆过程。本文通过离散元数值模拟,模拟陆倾和海倾断层的形成及演化,以探讨断层几何发育特征的控制因素。结果表明,岩性差异对斜坡带断层演化有较大影响,较高抗剪强度岩层破裂易产生陆倾控盆断裂,而低抗剪强度岩石则易形成向海倾断层。应力作用方向是区域差异演化的重要控制因素,岩石强度相同,应力作用方向相反时,断层倾向相反。盆地形成过程中发育众多凹陷斜坡,但坡度不是断层差异演化的主导因素。平北斜坡带和边缘海盆地的差异演化可能是由基底强度差异或应力方向差异导致的。本文利用离散元数值模拟平北斜坡带断裂差异演化过程,为东海盆地构造演化机制及边缘海盆地的形成提供了思路和理论依据。

     

    Abstract: In the East China Sea shelf basin, massive landward-dipping boundary faults were developed since the basin was born in the Mesozoic. Its structural geometry and tectonic process differ from those of other East Asian marginal basins. Previous studies focused on mainly tectonic migration of the East China Sea shelf basin, but differential evolution mechanism and its controlling factors in fault geometry were generally neglected. The northern part of the Pingbei Slope is the fault terrace zone formed by seaward-dipping normal faults, while the southern part is half-grabens controlled by landward-dipping normal faults. Differential evolution of faults in this region reflects the overall formation of the entire basin. To clarify the controlling factors, we investigated the development of landward-dipping and seaward-dipping faults by discrete element modeling. Results show that lithological differences affected the fault development. The landward-dipping normal faults tended to develop in the context of high-strength rocks, while seaward-dipping normal faults usually formed in the region of low strength. The stress orientation was another important factor for regional differential evolution, giving rise to faults development in opposite dipping. Although many slopes were developed during the formation of marginal basins, the topographic slope alone were not able to dominate the fault differential evolution. The differential evolution of the Pingbei Slope and the marginal basins might be controlled by the bedrock strength and geostress orientation. This study provided an insight and theory basis for understanding the mechanism of differential evolution in marginal basins.

     

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