Extensional Thinning Mechanism of the Western Continental Margin of the Pearl River Mouth Basin
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摘要: 为揭示珠江口盆地西部陆缘伸展-减薄过程，进行盆地断裂构造样式识别、断层活动速率和一维空盆构造沉降定量计算和综合分析.珠江口盆地西部以铲式断层和拆离断层为主并继承性发育.张裂一幕断层活动和构造沉降集中于开平凹陷，最大速率分别达到239 m/myr和108.6 m/myr.张裂二幕断层活动和构造沉降向洋盆迁移，最大速率分别达到192 m/myr和210.7 m/myr.张裂一幕岩石圈减薄集中在开平凹陷，以地壳脆性薄化为主.张裂二幕减薄中心向洋盆迁移，岩石圈地幔可能发生了局部薄化和软流圈上涌，导致陆架和上陆坡区凹陷内部构造沉降减弱；洋陆过渡带处上地壳快速减薄，且薄化速度比下地壳快.对比西北次海盆南侧上地壳较厚及下地壳较薄或缺失的情况，推测西北次海盆在破裂前发生了不对称的单剪薄化.Abstract: To understand the thinning process of the western margin of the Pearl River Mouth basin, structure analysis, quantitative calculation and comparison between fault growth rate and one-dimensional unloaded tectonic subsidence were carried out to study the structural deformation and migration characteristics. It is found that extensional structures controlled by listric and detachment faults are dominant and inherited. The fault activity and tectonic subsidence were concentrated in Kaiping sag in the first rifting stage, and the maximum rates reached 239 m/myr and 108.6 m/myr, respectively. The fault activity and tectonic subsidence migrated to the ocean basin in the second rifting stage, and the maximum rates reached 192 m/myr and 210.7 m/myr, respectively. In the first rifting stage, lithospheric thinning was dominated by brittle crust thinning. In the second rifting stage, lithospheric thinning center migrated oceanward, regional thinning and asthenosphere upwelling mitigated the subsidence in sagson shelf and slope, at the same time the lithosphere in the ocean basin area thinned rapidly till break-up happened. The upper crust thins faster than lower crust in the northern COT of the Southwest Sub sea basin, compared with the situation in its southern conjugate margin, where the upper crust is thick and the lower crust is thin or even absent, we suggest that the northwest sub-basin may have undergone asymmetric simple shear thinning before break-up happened.
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Key words:
- Pearl River Mouth basin /
- tectonic style /
- fault activity /
- tectonic subsidence /
- lithospheric break-up /
- tectonics
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图 1 珠江口盆地构造单元、测线位置和井位分布
Fig. 1. The basemap of the Pearl River Mouth basin indicating the structural units, well locations, 2D seismic (black line) and OBS(red line) profiles
图 2 珠江口盆地地层划分及构造演化阶段
地层界面年龄据中国海洋石油公司和Sun et al.(2014)；构造运动据李平鲁(1993)和Pang et al. (2018)
Fig. 2. Stratigraphic sequences and tectonic evolution of the PRMB
图 3 1410反射地震剖面（a）及解释（b）
a.原始地震剖面及钻井，大写字母标的实线方框为构造样式的发育位置，数字标识的是长剖面的拼接部分，小写字母标识的虚线框是主要凹陷的断裂结构图；b.地震解释剖面、岩石圈结构及断层编号
Fig. 3. The 1410 reflection seismic section (a) and its interpretation (b)
图 5 盆地西部陆架-陆坡断裂结构图
红色实线为铲式断层；红色虚线为板式断层；黑色虚线为气烟囱.a.恩平凹陷；b.开平凹陷；c.南部隆起；d.鹤山凹陷（位置见3）
Fig. 5. The faults and structures formed in the shelf- slope of the western PRMB
图 6 珠江口盆地西部不同时期断层活动速率图
Fig. 6. Rate of fault activity in different periods of the western PRMB
图 7 珠江口盆地西部不同时期空盆构造沉降速率图
Fig. 7. Unloaded tectonic subsidence rate in different periods of the western PRMB
图 8 断陷期断层活动速率与沉降速率对比图
Fig. 8. Comparison of fault activity rate and subsidence rate during the rift stage
图 10 珠江口盆地西部陆缘伸展-破裂过程模式
a.陆缘张裂一幕（Tg-T80）；b.陆缘张裂二幕（T80-T70）
Fig. 10. The evolution model of the western PRMB margin from rift-break-up
表 1 岩性参数
Table 1. Lithologic parameters
岩性 砂岩 粉砂岩 泥岩 初始孔隙度 0.45 0.55 0.60 压实系数（1/km） 0.27 0.41 0.51 基质密度（g/cm3） 2.64 2.64 2.60 
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表 2 沉积相反映的古水深
Table 2. The paleowater depth estimated from the sedimentary facies
沉积相 沉积亚相 水深（m） 剥蚀区 隆起 -5 陆相 河流-平原相 0~5 滨湖-浅湖相 0~30 中深湖相 30~100 深湖相 > 100 河流三角洲相 0~20 海陆过渡相 三角洲平原 0 三角洲前缘 0~10 前三角洲 10~20 海相 潮坪相 0~10 滨海相 20~50 浅海相 50~200 半深海相 200~2 000 深海相 > 2 000 陆架边缘三角洲 200~500
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