珠江口盆地西部陆缘伸展-减薄机制

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.

Extensional Thinning Mechanism of the Western Continental Margin of the Pearl River Mouth Basin

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.