珠江口盆地白云凹陷新生代构造演化动力学

白云凹陷构造演化史的研究对在白云凹陷开展油气勘探和深水沉积研究具有重要的意义。通过对断裂与沉积结构平面和剖面特点的分析,结合岩浆活动特点,文中提出白云凹陷是一个复式地堑,推测这种结构特点与凹陷下地壳的强烈韧性减薄和颈缩变形有关,表现为热岩石圈的伸展。其发育机制推测与白云凹陷位于构造转换带上有关,特殊的构造位置使白云凹陷成为强烈构造变形区,岩石圈地壳强烈减薄,伴随伸展过程和地幔上涌,脆性地壳或上地幔中部分熔融物质的出现导致岩石圈强度的急剧降低,在区域伸展应力场下以韧性流变方式减薄。岩浆在构造转换带下聚集并发育主岩浆房,由于白云凹陷南北边缘没有发育正断裂系统,岩浆主要沿垂直伸展的方向运移,因此在珠琼运动一幕和二幕南南东向伸展应力作用下,岩浆向白云凹陷的东部和西部运移至北西向基底深大断裂处,那里由于北西向断裂表现为左行张剪性质而成为压力较低的地区,从而成为岩浆上涌和侵位的地方。在岩浆聚集的地区,活动岩浆体附近的脆性变形被分散的韧性变形所取代,因此在凹陷的东北和西南两个角上,发育了张性和张剪性小断裂群,由于热岩石圈弹性较差,白云凹陷长期持续沉降。白云凹陷的断裂活动和沉积演化史还受到南海海盆扩张活动的影响。

Dynamics of Tertiary tectonic evolution of the Baiyun Sag in the Pearl River Mouth Basin

Research on the Tertiary tectonic history of the Baiyun Sag is important to exploration in the sag and to research on deep-water basins. Through analyzing faults and sedimentary structures and combining that analysis with a study of magmatism, we suggest that the Baiyun Sag is a composite rift. It appears to be caused by the ductile thinning and necking of the crust in response to hot lithospheric stretching. Its special deformation is probably due to its location at the transfer zone of the Pearl River Mouth Basin, which led the Baiyun Sag to deform strongly through lithosphere thinning. Together with lithosphere stretching and mantle upwelling, the intrusion of magma into the brittle crust and mantle drastically reduced lithosphere strength, and contributed to ductile thinning during regional extension. Magma collected in the transfer zone, where a major magma chamber developed. Since no normal faults developed along the northern and southern boundaries of the sag, the magma migrated mainly in a direction orthogonal to the extension vector towards the NE and SW corners of the sag. Here, deep-rooted, NW-striking transtensional faults controlled the upwelling and emplacement of magma as low pressure sites. Above such magmatic bodies, brittle deformation was replaced by diffuse ductile deformation; in addition, small fault swarms developed along the NE and SW margins of the sag. Since hot lithosphere has low elasticity, Baiyun Sag kept subsiding since the rifting stage. The faulting and the sedimentary history of the Baiyun Sag were also influenced by spreading in the South China Sea Basin.