Sedimentary Filling Response to Detachment Structural Deformation in Shallow-Water Continental Shelf of Pearl River Mouth Basin: A Case Study of Enping Sag
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摘要: 珠江口盆地新生代发育跨越浅水区至深水区、多类型的拆离断陷，其中浅水陆架区拆离断陷是探究拆离构造变形与沉积充填响应的重要窗口.基于地震资料和钻井资料详细解析，对恩平凹陷低角度拆离断层特征进行了研究，探讨了拆离构造变形的控制因素、变形过程和沉积充填响应机制.研究表明，恩平凹陷低角度边界正断层为壳间拆离断层，长度约50 km，倾角平均17.5°，断面最深达中下地壳，裂陷期经历了中低角度到低角度的转变.拆离断层形成于中生代先存逆冲断层基础之上，与中下地壳韧性剪切穹隆和边界调节性走滑断裂相伴生，联合控制形成均衡深拆离、前展式宽拆离和迁移型复式拆离3种凹陷结构类型.垂向上可划分出三期裂陷幕，因差异构造变形而具有不同的沉积充填响应：（1）早文昌期均一裂陷幕，快速的裂陷沉降发育于先存逆冲断层弱面，形成具厚层中深湖的均一窄深箕状半地堑；（2）晚文昌期拆离伸展幕，凹陷受差异构造变形而发育成东、西沉积充填差异化的宽浅断陷，西部韧性剪切穹隆弱隆升与边界走滑断层强走滑控制了恩平17洼自迁移型中深湖和大型辫状河三角洲-重力流沉积，而东部韧性剪切穹隆强隆升控制了恩平12-18洼沉积中心跳跃式异迁移的沉积充填，过渡带发育大型走向斜坡扇三角洲；（3）恩平期拆离-断拗联控幕，构造变形由简单剪切向纯剪切转变，沉积向外流水系主导的浅水辫状河三角洲-浅湖相充填转变.低角度拆离断陷具有与高角度脆性断陷不同的构造变形和沉积充填响应，烃源岩与砂岩储层的时空分布非均质性强.研究成果对恩平凹陷的烃源岩和深部储层预测，以及南海北部陆缘同类拆离断陷的构造-沉积充填研究具有良好的指导意义.Abstract: The Pearl River Mouth Basin developed many types of Cenozoic detachment depressions from shallow-water area to deep-water area. Among them, the detachment depression in continental shelf shallow water area is an important window to explore the detachment structural deformation and its sedimentary filling response. Based on the detailed analysis of seismic and drilling data, the characteristics of low-angle detachment fault system in Enping Sag are studied, and the control factors, deformation process and sedimentary filling response mechanism of structural deformation in detachment fault depression are discussed. The research shows that the low-angle boundary normal fault in Enping Sag is an inter-crust detachment fault, with a length of about 50 km and an average dip angle of 17.5°, deepest reaches to the middle-lower crust. During the rifting period, the fault-dip transformed from medium-low angle to low angle. The detachment fault was formed on the basis of the pre-existing thrust fault in the Mesozoic era, associated with the ductile shear dome of the middle-lower crust and the boundary regulating strike slip fault, and jointly controlled the formation of three types of subbasin structures, such as balanced deep detachment, forward spreading wide detachment and migration compound detachment subbasin. It can be divided into three rifting stages with different sedimentary filling responses due to different structural deformation. (1) In the homogeneous rifting stage at Early Wenchang period, the extension stress was concentrated on the weak surface of the pre-existing thrust fault, and the rapid rift formed narrow and deep homogeneous half-grabens, which developed thick medium-deep lakes. (2) In the detachment extensional stage at late Wenchang period, the sag developed into a wide and shallow fault depression with differential sedimentation and filling in the east and west due to differential structural deformation. In the western sag, the weak uplift of the ductile shear dome and the strong strike slip of the boundary strike slip fault controlled self migrating medium-deep lacustrine source rocks and large-scale braided river delta turbidite sedimentary system developed in Enping 17 Subsag. In the western sag, the strong uplift of ductile shear dome controlled the jumping and allochthonous migration of sedimentary center from Enping 12 Subsag to Enping 18 Subsag. A large fan delta was developed in the transition trending slope. (3) In the detachment-depression transition stage at Enping period, the structure changed from simple shear to pure shear deformation, and the sedimentation changed from internally drainage to large shallow braided river delta lacustrine filling dominated by externally drained conditions. The low-angle detachment depression has different structural deformation and sedimentary filling from the high-angle brittle fault depression. The research results have significance guiding value for the prediction of source rocks and deep reservoirs in Enping Sag, and for the study of structural deformation and sedimentary filling of similar detachment fault depressions in the northern continental margin of the South China Sea.
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图 1 珠江口盆地构造区划与断陷分布（改自庞雄等（2021）和李洪博等（2020））
Fig. 1. Distribution of tectonic units and depressions in the Pearl River Mouth Basin(modified from Pang et al. (2021), Li et al. (2020))
图 2 恩平凹陷基底顶面构造地貌与地层柱状图
Fig. 2. Structural geomorphology of basement top and stratigraphic column of the Enping Sag
图 3 恩平凹陷控洼断层基底断距（a）、现今倾角（b）以及早、晚文昌期倾角（c）
Fig. 3. Fault displacement (a), present dip angle (b) and dip angle of Lower and Upper Wenchang Formation depositional period (c) of the depression-controlling faults in the Enping Sag
图 4 恩平凹陷结构剖面（剖面位置见图 2）
Fig. 4. Seismic sections demonstrating the structure of the Enping Sag
图 5 恩平凹陷中下地壳韧性剪切穹隆特征
a. 韧性剪切穹隆顶面深度与T80断裂叠合图；b. 过恩平12-18洼穹隆地震剖面；c. 过恩平17洼地震剖面
Fig. 5. The characteristics of ductile shear dome in the middle-lower crust below Enping Sag
图 6 F3走滑断层构造特征
a. T80层深度构造图，平面位置见图 2；b. F3走滑断层及其派生构造样式
Fig. 6. The characteristics of the F3 strike-slip fault
图 7 恩平凹陷文昌组下段与上段地层等厚图
Fig. 7. The stratigraphic thickness of Lower and Upper Wenchang formations in the Enping Sag
图 8 E17井古生物与单井相综合柱状图
Fig. 8. Comprehensive column of paleontology and single well facies of Well E17
图 9 恩平17洼地震剖面与沉积充填解释（剖面位置见图 2）
Fig. 9. Seismic section and its sedimentary filling interpretation cross Enping 17 Subsag
图 10 恩平12洼和恩平18洼地震剖面与沉积充填解释（剖面位置见图 4d）
Fig. 10. Seismic section and its sedimentary filling interpretation cross Enping 12 and 18 subsags
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