Structural Evolution and Magmatism of Fault Depression in Baiyun Sag, Northern Margin of South China Sea
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摘要: 断陷盆地的结构-构造主要受控于边界断层的构造作用，但是在盆地发育过程中有不同程度的岩浆作用发生时，断陷结构会受到显著的改造和影响.通过对位于南海北部陆缘超伸展区的珠江口盆地白云凹陷断陷结构差异演化特征的描述和分析，探讨陆缘伸展过程中岩浆作用的参与程度对洼陷结构样式和断陷构造-地层-沉积演化的影响.研究表明，白云凹陷主洼强烈伸展期间，岩浆作用不明显，上地壳发生脆性断裂，中下地壳则发生韧性伸展薄化，产生了壳幔拆离断层控制的宽深断陷，构造作用即地壳伸展拆离薄化作用是断陷发育的主要机制；然而，白云凹陷东部洼陷在经历了早期的脆性破裂之后，随即发生了显著的岩浆上涌作用，改变了上地壳的结构强度，脆-韧性转换面上移，产生了上地壳拆离断层控制的宽浅断陷，断陷结构受岩浆上涌作用的改造而表现出坡坪式拆离断层控制的半地堑系，沉积中心发生规律性迁移，构造-岩浆作用是断陷发育的机制.白云凹陷主洼与白云凹陷东部洼陷中岩浆作用参与程度的不同，不仅导致了洼陷结构样式及其演变过程的不同，而且断陷中的沉积充填体系也表现出显著的差异.构成白云凹陷主洼的宽深断陷中发育了巨厚的中晚始新世上文昌组-恩平组，在北部缓坡和南部深洼区，这套地层依次由大型三角洲体系和深湖相沉积体系构成；白云凹陷的东部洼陷则受岩浆上涌改造，发育多隆凹结构的宽浅断陷，形成多个小物源供源的小型三角洲-浅湖沉积体系，而且沉积物中富含火山碎屑.研究成果对于白云凹陷成盆机制的研究意义重大，同时对该凹陷的油气勘探亦具有重要的实际应用价值.Abstract: The structure of faulted basins is mainly controlled by the tectonic action of boundary faults, but when magmatism occurs in different degrees during the development of basins, the structure of faulted basins will be significantly transformed and influenced. Based on the description and analysis of the differential evolution characteristics of fault depression structure in Baiyun Sag, Pearl River Mouth Basin, which is located in the northern margin of the South China Sea, in this paper it discusses the influence of the participation of magmatism on the structural style of the sag and the evolution of fault depression structure-stratum-sediment in the process of continental margin extension. The research shows that during the intense extension of the main depression in Baiyun Sag, magmatism was not obvious, brittle fracture occurred in the upper crust, and ductile extension thinning occurred in the middle and lower crust, resulting in the broad and deep fault depression controlled by crust-mantle detachment fault, and tectonism, namely, crustal extension detachment thinning, was the main mechanism of fault depression development. However, in the eastern depression of Baiyun Sag, after the early brittle fracture, a remarkable magmatic upwelling took place, which changed the structural strength of the upper crust, and the brittle-ductile transition plane moved upward, resulting in a broad and shallow fault depression controlled by the upper crust detachment fault. The fault depression structure was transformed by magmatic upwelling, showing a semi-graben system controlled by the slope-flat detachment fault, and the sedimentary center moved regularly. Tetconism and magmatism were the mechanism of fault depression development. The different degrees of magmatism participation between the main depression of Baiyun Sag and the eastern depression of Baiyun Sag not only led to the different structural styles and evolution process of the depression, but also the significant differences in the sedimentary filling system in the fault depression. In the broad and deep fault depressions that constitute the main depression of Baiyun sag, there were extremely thick strata of Upper Wenchang Formation-Enping Formation in the Middle and Late Eocene. In the gentle slope in the north and deep depression in the south, this stratum is composed of large delta system and deep lacustrine sedimentary system in turn. The eastern depression of Baiyun Sag was transformed by magma upwelling, which developed a broad-shallow fault depression with many uplifts and depressions, forming a small delta-shallow lake sedimentary system with several small sources, and the sediments were rich in pyroclastic rocks. The results are not only of great significance for the study of basin-forming mechanism in Baiyun Sag, and but also of important practical application value for oil and gas exploration in this sag.
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图 2 南海北部陆缘白云凹陷结晶地壳厚度图（a）和断陷期地层厚度、控洼断裂分布图（b）
Fig. 2. Crustal thickness map (a) and stratum thickness, boundary fault map in rifting stage (b) of the Baiyun Sag in the northern continental margin of South China Sea
图 3 白云凹陷主洼断陷结构（NE向）
Fig. 3. The profile of depression architecture in the main depression of the Baiyun Sag
图 4 白云凹陷主洼断陷结构(SE向)
Fig. 4. The profile of depression architecture in the main depression of the Baiyun Sag
图 5 白云凹陷L4井（a）和L6井（b）年代地层综合图
Fig. 5. Comprehensive biochronostratigraphic map of Well L4 (a) and Well L6 (b) in the Baiyun Sag
图 6 白云凹陷东部断陷结构剖面
Fig. 6. The profile of depression architecture in the eastern depression of the Baiyun Sag
图 7 白云凹陷东部断陷结构（据庞雄等，2021修改）
Fig. 7. The profile of depression architecture in the eastern depression of the Baiyun Sag (modified from Pang et al., 2021)
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