Core Complex and Detachment Structure in the Kaiping Sag, Pearl River Mouth Basin and a Discussion on the Dynamics
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摘要: 南海北部大陆边缘珠江口盆地开平凹陷裂陷结构复杂，裂陷结构及其成因机制是制约该地区油气地质认识与勘探进程的主要因素.基于新采集的三维地震反射资料，对开平凹陷结构、边界断层几何学与运动学特征以及岩石圈深部结构等方面进行了研究.结果表明，开平凹陷为核杂岩拆离结构，凹陷中部近EW向展布的穹隆构造是一个标准的核杂岩构造.开平凹陷核杂岩的形成与韧性地壳的隆升有关，其中核杂岩在KP9部位发生剥露去顶.核杂岩隆升背景下的拆离作用控制了开平凹陷的裂陷结构和沉积充填过程.开平凹陷核杂岩的发育过程为标准的滚动枢纽模式，地壳内部上拱的层状地震反射界面代表了地质历史时期的古脆‒韧性转换面，其迁移方向与拆离断层上盘滑动方向一致.开平凹陷主拆离断面呈“勺状”，拆离断面上发育大量不同规模的SN向波瓦状构造，指示了拆离断层上盘自北向南滑动.开平凹陷核杂岩的发育与该区域裂陷初始前岩石圈内部先存的薄弱中地壳层密切相关.该研究成果对开平凹陷的裂陷结构与成因机制进行了全新的解释，推动了对烃源岩与成藏规律的认识，为开平凹陷40年油气勘探的首个商业突破提供了关键指导.Abstract: The Kaiping sag within the Pearl River Mouth basin, northern South China Sea rifted margin, shows a complex rift structure, and the rift structure and dynamics are the main factors that hinder our understanding on the petroleum geology and exploration process in this area. Based on the newly acquired 3D seismic reflection data, this study investigates the structure of the Kaiping sag, the geometric and kinematic characteristics of boundary faults, and the deep lithospheric structure. It is revealed that the Kaiping sag has a detachment structure related to core complex, and the E-W extending dome structure in the central part of the Kaiping sag is a standard oceanic core complex structure. The development of the core complex is related to the uplift of the ductile crust, which is exposed and truncated at the KP9 high. The detachment in the context of core complex uplifting controls the structural styles and sedimentary filling process in the Kaiping sag. The development of the Kaiping core complex shows a standard rolling-hinge model, and the domed layered seismic reflectors within the crust represent the brittle-ductile transition surface in geological history, which has a direction of migration same with the hanging-wall movement. The main detachment fault shows "spoon-shaped", and a large number of SN-trending corrugations on the detachment surface indicate top down to the south slip of the hanging wall. The development of oceanic core complexes in the Kaiping sag is closely related to the existence of a weak middle crust layer within the initial lithosphere in this area. This research offers a new explanation on the rift structure and dynamics of the Kaiping sag, which has promoted the understanding on source rocks and hydrocarbon accumulation and provided important guidance for the first commercial exploration breakthrough in forty years.
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Key words:
- core complex /
- South China Sea /
- Pearl River Mouth basin /
- Kaiping sag /
- rift structure /
- petroleum geology /
- geodynamics
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图 2 南海北部陆缘珠江口盆地构造单元及开平凹陷位置平面
Fig. 2. Structural units of the Pearl River Mouth basin and location of the Kaiping sag in the northern South China Sea margin
图 3 开平凹陷基底顶界面地貌
Fig. 3. High-precision geomorphic map of the top basement of the Kaiping sag
图 7 开平凹陷主拆离断层断面3D图
Fig. 7. 3D structure of main detachment fault section in Kaiping depression
图 8 开平凹陷主拆离断面波瓦状构造与地震横切剖面（剖面位置见图8a）
Fig. 8. Fault corrugations on the main detachment fault of the Kaiping sag and seismic cross section
图 9 变质核杂岩发育的两种运动学模型（据Little et al., 2019; Mizera et al., 2019修改）
Fig. 9. Two kinematic models for the development of metamorphic core complex (modified from Little et al., 2019; Mizera et al., 2019)
图 11 开平凹陷地壳内部层状反射面迁移特征与解释模型
Fig. 11. Migration characteristics and interpretation model of layered reflectors in the crust of the Kaiping sag
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