渤海湾盆地裂后期快速沉降机制的构造-热演化模拟研究

The Bohai Bay Basin is the largest Cenozoic rifted basin of the North China Craton and an important petroliferous basin with active seismicity. The post-rift tectonic subsidence rate of the basin has significantly increased since the Pliocene, which is obviously inconsistent with the traditional exponential decay model. The basin had also experienced intensive tectonic reactivation during the post-rift stage. However, the mechanism of the rapid post-rift subsidence is still controversial, and conflicts among evidence exist. It can provide direct evidence for the mechanism of the rapid post-rift subsidence through clarifying the anomalous tectonic subsidence independent of thermal decay, and testing the possible geodynamic models. This project intends to firstly simulate the syn-rift stretching factor and the post-rift thermal subsidence using a multi-phase extension model, and determine the starting time and distribution of the anomalous tectonic subsidence. Then, we plan to conduct tectono-thermal modeling of the possible mechanisms accounting for the anomalous tectonic subsidence, including the rifting, compression-induced downwarping and deep thermal anomaly models. These models can be tested through comparing the calculated basin thermal history, present-day thermal state and Moho depth with those obtained from previous studies. The research results would have significant theoretical and practical value for understanding the complete geodynamic process of basin evolution through the Cenozoic, the mechanism of hydrocarbon accumulation and the dynamics of earthquake in the Bohai Bay Basin.

渤海湾盆地是华北克拉通最大的新生代裂陷盆地及重要的含油气盆地，也是地震活动频繁的地区。其裂后期的构造沉降速率自上新世以来显著增加，与传统的指数衰减模型明显相悖，且伴随强烈的构造再活动。目前学术界对该盆地裂后期快速沉降的成因机制争议很大，证据之间的冲突也多存在。厘清盆地裂后期与热沉降无关的异常沉降量，并对可能的动力学模式进行建模、检验，可为剖析盆地裂后期快速沉降机制提供直接证据。本项目拟采用多幕非瞬时拉张模型计算盆地裂陷期的拉张系数及其后的热沉降量，确定异常构造沉降的起始时间及分布规律；在此基础上，对造成裂后期异常沉降的可能机制（拉张、挤压引起的岩石圈下挠、深部热异常）进行构造-热演化模拟；依据各模式在盆地热历史、现今热状态及Moho面深度等方面的不同响应，结合已有观测资料，分析其可能性。研究成果对完整认识渤海湾盆地新生代演化的动力学过程、油气成藏机理及地震动力学都具有重要的理论和应用价值。

渤海湾盆地是中国东部最大的新生代裂陷盆地，其裂后期的快速沉降与理论上热沉降速率的指数衰减趋势明显偏离，但对这一现象的成因机制尚无定论。本项目通过多幕有限时拉张模型对覆盖渤海湾盆地的18条剖面及48口钻井开展构造-热演化模拟，在实际构造沉降史、现今大地热流及地壳厚度的共同约束下，重建裂陷前的地壳及岩石圈厚度，反演盆地各裂陷幕的拉张系数，恢复盆地基底热流的演化史，厘定裂后期与前期拉张有关的热沉降量，进而从观测值中分离出异常构造沉降量。研究结果显示，盆地裂陷前的地壳及岩石圈厚度分别为30-42km及75-110km。盆地新生代期间的总拉张系数为1.02-2.39，平均1.21；拉张峰期出现在始新世中、晚期。盆地基底热流在古近纪中、晚期达到峰值71-100mW/m2，之后逐渐降低至现今；盆地西部热流峰期出现的时间早于东部。由拉张系数和基底热流的结果得出，盆地新生代的拉张有着自西向东，自南向北迁移。将裂后期由岩石圈冷却造成的热沉降量从实际构造沉降量中去除后，获得盆地异常沉降的时空分布。渤海湾盆地的异常沉降主要发生于12Ma之后。异常构造沉降速率自晚中新世时的19m/Ma增加至第四纪时的75m/Ma。盆地总异常沉降量主要集中在200-600m，平均400m。结合前人的研究成果，认为新一期裂陷、深部热异常、郯庐断裂的走滑及岩石圈挠曲可能并非是异常沉降的主要机制。全盆地范围的异常沉降可能与俯冲相关的深部地幔流动引起的动力地形有关，但现今异常沉降比预测的动力地形幅值低200-1100m。原因可能是动力地形的计算模型中往往忽略225-300km深度之上的浅部地幔。渤海湾盆地之下120-200km范围的软流圈温度异常可造成350-650m的地形抬升。因此，浅部与深部地幔活动可能共同影响了盆地裂后期的构造沉降。该项目深入探索了深部动力学过程的浅部构造响应，为完整认识渤海湾盆地新生代演化的动力学机制提供了帮助。