Geological Response and Forming Mechanisms of 23.8 Ma Tectonic Events in Deepwater Area of the Pearl River Mouth Basin in South China Sea
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摘要: 南海珠江口盆地深水区23.8 Ma发育一次重大构造事件并同时形成了重要的不整合界面.研究表明：(1)23.8 Ma构造事件是珠江口盆地珠二坳陷新生代构造演化的转换面，它是盆地裂陷阶段和裂后阶段的分界面，23.8 Ma之前为断裂控盆，盆地以伸展和走滑断裂系统为主；23.8 Ma之后为断裂和热作用共同控盆，到晚期以热作用为主.(2)23.8 Ma构造事件导致盆地沉积充填的突变，通过微量元素和同位素分析，23.8 Ma以前盆地物源来自东南沿海花岗岩源区，23.8 Ma以后物源来自青藏东侧及云贵高原源区.23.8 Ma界面是南海北部陆架坡折从白云主凹南侧向北侧跳跃的分界面，相应地沉积环境由浅海陆架沉积突变为陆坡深水沉积环境.(3)23.8 Ma构造事件是盆地热体制的转折面，典型钻井热史定量恢复揭示地温梯度在23.8 Ma附近明显存在一个突变面，以23.8 Ma为界，新生代早期到渐新世末显示地温梯度上升，指示盆地经历了拉张过程；23.8 Ma之后显示地温梯度下降，指示盆地经历热沉降过程.(4)23.8 Ma构造事件是火山活动的转化面，23.8 Ma之前显示成分单一的大规模玄武质岩浆喷发作用逐渐增强，而之后玄武质岩浆喷发作用逐渐减弱.总结了23.8 Ma构造事件的动力学模型，提出了软流圈流动主导的地幔热底辟作用是南海破裂成洋的动力来源，南海扩张脊的跃迁及地幔热底辟作用改变是导致23.8 Ma构造事件的主因.Abstract: An important tectonic event and associated unconformity surface occurred in deepwater area of the Pearl River Mouth basin, South China Sea. Research results indicate: (1)The 23.8 Ma tectonic events is the convert face of Cenozoic tectonic evolution in Zhu Ⅱ depression of the basin, which divides the Paleogene syn-rifting stage and Neogene post-rifting stage. In the former, the basin-forming mechanisms are dominated by the stretching and strike-slip faults systems, however, in the later, faulting and geothermal effect (late in geothermal effect primarily) control the basin evolution. (2) The 23.8 Ma tectonic events resulted in great change in basin sedimentary filling. Through the analysis of trace elements and isotope, the sediment provenance comes from the southeast coastal granite materials before the 23.8 Ma, but from the east of the Qinghai-Tibet plateau and the Yunnan-Guizhou plateau after 23.8 Ma. At the same time, the slope-break surface jumps from south to north in the Baiyun depression, and sedimentary facies change also from shallow continental shelf to deepwater slop environments. (3) The 23.8 Ma tectonic events is a transferring surface of basin geothermal system. The quantitative restore of typical drilling thermal history shows geothermal gradient obviously changes at the 23.8 Ma. Through the thermal history inversion in deepwater area of the Pearl River Mouth basin, geothermal gradient rising process occur during the Paleogene, indicating the basin underwent extensional process, and the geothermal gradient falls clearly after 23.8 Ma, indicating the basin underwent a geothermal subsidence process. (4) The 23.8 Ma tectonic events reveals a great change in volcanic activity, the single ingredient of large basaltic magma eruption gradually strengthened before the 23.8 Ma, and the eruption gradually weakened after the surface. A dynamic model of the 23.8 Ma tectonic events is established. The mantle diapir from the asthenosphere flowing is regarded as the power source for the South China Sea spreading. The 23.8 Ma tectonic events resulted from the jumping of the South China Sea spreading and change in mantle diapir possibly related with the deep magma escaping.
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Key words:
- South China Sea /
- Pearl River Mouth basin /
- deepwater area /
- 23.8 Ma tectonic events /
- geochemistry /
- tectonics
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图 1 珠江口盆地深水区及其周边区域构造划分
Fig. 1. Structural units of deepwater and its surrounding area in the Pearl River Mouth basin
图 2 珠江口盆地深水区构造演化剖面
Fig. 2. Structural evolution of deepwater area in the Pearl River Mouth basin
图 3 珠江口盆地深水区新生代盆地构造样式及其分布模式
Fig. 3. Cenozoic structural and distribution pattern of deepwater area in Pearl River Mouth basin
图 5 南海周边及可能源区Nd同位素分布示意
Fig. 5. Distribution of isotope Nd in South China Sea and other possible provenance
图 6 PY33-1-1井(a)及1148站(b)钕及锶同位素演变特征及可能源区
Fig. 6. Evolution of isotope Nd and Sr in PY33-1-1 (a) and 1148 (b), and possible provenance
图 7 珠江口盆地近南北向地震解释剖面及陆架坡折迁移示意
Fig. 7. Interpreted longitudinal seismic profile showing the migration of slope break of the Pearl River Mouth basin
图 10 珠江口盆地及邻区新生代火山岩平面分布
Fig. 10. Distribution of Cenozoic volcanic rocks in Pearl River Mouth basin and its adjacent area
图 11 珠江口盆地火山活动与盆地演化、区域构造关系
Fig. 11. The relationship between volcanic activities, basin evolution, and regional structure in Pearl River Mouth basin
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