Structural, Sedimentary and Magmatic Records during Continental Breakup at Southwest Sub-Basin of South China Sea
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摘要: 基于跨南海西南次海盆V型尖端区共轭被动大陆边缘的1 000 km长的深反射地震剖面的解释和分析，深入研究南海临界破裂区地壳结构、盆地构造地层格架和岩浆作用特征，阐明地壳岩石圈的伸展破裂过程.结果表明，在南海西南次海盆共轭陆缘可以识别出3条一级界面，即海底、基底（Tg/Tb）和Moho面，根据这些界面可将共轭边缘划分为箱型域、细颈域、楔型域和薄箱型域等多个地壳基底结构构造特征显著不同的构造单元；在西南次海盆V型尖端陆缘盆地充填序列中识别出岩石圈裂解分离的响应界面，即裂后不整合界面T50/Tm，该界面与基底面Tg/Tb限定了陆缘盆地的同裂陷沉积充填序列，其内发育了T70、T60两条重要的幕式构造响应界面，控制了早期高角度正断层控制的断陷盆地系（Tg-T70）、中期拆离断层控制的拆离盆地系（T70-T60）和晚期断坳转换作用控制的坳陷盆地（T60-T50）；西南次海盆V型尖端薄箱型域属于原洋洋壳域，为岩浆型地壳，代表了岩石圈临近裂解分离、但支撑稳定态海底扩张的地幔对流系统还未完全建立起来之前的“临界破裂”状态，发育于23.5~15.5 Ma（T60-Tm）期间.综合考虑地壳初始厚度、断裂活动性和不同时期的盆地原型等，运用平衡剖面技术重建了西南次海盆V型尖端共轭边缘的发育演化过程，建立了南海西南次海盆临界破裂区构造-地层-岩浆相互作用模式，揭示了南海陆缘岩石圈伸展破裂机制.本研究具有重要的理论意义，并对南海的深水油气勘探具有重大的实际应用价值.Abstract: Based on the interpretation and analysis of 1 000 km long deep reflection seismic sections across the conjugate passive continental margins in the V-shaped tip of the Southwest sub-basin of the South China Sea, in this paper it presents a study on the basin structure, tectonic style and the extension progress of continental lithosphere in the final breakup area of South China Sea. Three first order surfaces can be recognized in the SW South China Sea: seafloor, Tg/Tm and Moho. The relationship between these three surfaces can be used to define the box-shape domain, necking domain, wedge shape domain and thin box-shape domain. In the V-shaped SW sub-basin, T50/Tm corresponds to the lithosphere breakup surface, which is also named as the breakup unconformity. This interface and the basement surface (Tg/Tm) define the syn-rifting deposition filling sequence of the continental margin basin. Two important episodic structures, T70 and T60, have developed within it. The surfaces of Tg and T70 constrained the formation of early faulted basin system which was controlled by high-angle normal faults. The surfaces of T70 and T60 controlled the development of detachment basin. The surfaces of T60 and T50 constrained the formation of sag basins. In this paper it shows that the thin box-shape domain belongs to the proto-oceanic domain. The nature of this area consists of magmatic ocean, representing the final breakup process before the formation of mantle convection system in which the lithosphere was close to disintegration and separation, but the steady-state seafloor spreading had not been fully established. The thin box-shape domain developed between the T60 and Tm. Considering the initial crustal thickness, fault activity, and basin prototypes during different periods, and applying the balanced profile technology, the development progress of V-shaped tip at the SW sub-basin has been reconstructed. The structure-stratigraphic-magma interaction model of the final breakup domain in the SW sub-basin of the South China Sea has been established, revealing the extension and breakup mechanism of the continental margin lithosphere in the South China Sea. This mechanism has important theoretical significance, and has great practical application value for deep-water oil and gas exploration in the South China Sea.
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