Seismic Stratigraphy, Tectonic Structure and Extension Model Across the Reed Bank Basin in the South Margin of South China Sea: Evidence from NH973-2 Multichannel Seismic Profile
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摘要: 针对“973”项目中“南海大陆边缘动力学与油气资源潜力”这一研究课题, 对在南海南部陆缘礼乐盆地采集的NH973-2测线进行了研究.对地震剖面的解释共划分出6个层序界面, 将地层划分为4个构造沉积单元.根据地震解释, 对不同时期断层的水平断距进行了测量及分析, 获取了与脆性拉张相关的伸展信息: 研究区的拉张作用可以分为2期, 主要的拉张作用发生在大陆裂谷阶段(古近纪), 形成了一系列的地堑—半地堑以及翘倾断块; 第2期拉张作用的时期为晚渐新世—早中新世, 断层活动强度明显变弱.在南海南部陆缘广泛发育了碳酸盐沉积, 其发育的时代和南海的海底扩张时期一致.对穿越礼乐滩区地震剖面伸展特征的分析表明, 根据断层水平断距获得脆性伸展因子与根据重力反演获得的全地壳伸展因子之间存在差异, 表明研究区的拉张在纵向上并非是均一的, 新生代的拉张经历了深度决定拉张模式.Abstract: One recently acquired regional multi-channel seismic profile (NH973-2) across the Reed Bank basin, the South China Sea (SCS), is interpreted.Based on the interpretations of these profiles, we have worked out its stratigraphic sequences, tectonic structures and extension factors.Four tectonic-stratigraphic units are determined, together with 6 sequence boundaries.Detailed analyses on the extension factors based on the measurement of fault heaves reveal two episodes of continental extension separated by a distinct unconformity.The main extension occurred during the continental rifting (Early Tertiary), and resulted in the formation of half-grabens and rotated blocks, controlled by deeply rooted detachment system.During the second extension phase (drifting period, Late Oligocene-Early Miocene) intensity reduced sharply and focused on the continental-oceanic transition area.A widespread carbonate platform developed across the Reed bank region concurrent with the drifting period of the SCS.Fault-related stretching factor (βf) is found to be comparable and shows discrepancy with the whole crust stretching factors (βc) in the study area.Thus we conclude that the continental crust of the Reed Bank basin experienced depth-dependent extension.
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Key words:
- Reed Bank basin /
- stratigraphy /
- tectonics /
- stretching factors /
- marine geology
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图 1 南海南部陆缘地貌特征及主要构造单元
黑色实线为NH973-2多道地震测线
Fig. 1. Morphological features and major tectonic units in the south margin of South China Sea
图 2 研究区主要层序界面时代、岩性及主要构造事件
Fig. 2. Seismic stratigraphy, lithology and major tectonic events of the study area
图 3 穿越礼乐盆地的NH973-2多道地震剖面
a.原始地震剖面; b.主要地震反射特征; c.地质解释图; 断层水平断矩测量的区域见图 3c
Fig. 3. Multichannel seismic profile NH973-2 across the Reed Bank basin
图 4 NH973-2测线反映伸展结构的典型地震剖面段(a) 及地质解释(b) (位置见图 3a)
Fig. 4. Seismic section (a) and interpretation (b) in the Reed Bank basin showing a consecutively developed half-graben system
图 5 NH973-2测线COT区基底内部反映莫霍面的内部反射(位置见图 3b)
Fig. 5. Blow-ups of the deeper parts of the seismic sections showing possible Moho reflections in the area of the continent-ocean transition (COT)
图 6 NH973-2测线断陷盆地区基底内部反射(内部反射可与其上的控坳边界断层相接, 组成低角度拆离系统.位置见图 3b)
Fig. 6. Internal reflectors beneath the rift grabens along profile NH973-2
图 7 礼乐盆地区不同阶段的拉张系数
Fig. 7. Extension rates for different periods as derived from the estimation of fault heaves
表 1 南沙区域拖网站位获取的碳酸盐岩的岩性及沉积环境(据Kudrass et al., 1986)
Table 1. Lithology and depositional environment of carbonate rocks
拖网站位 岩性 年代 沉积环境 SO23-27 松散富红藻粒状灰岩, 内成碎屑粒泥灰岩 晚渐新世—早中新世 浅海—开阔海相, 泻湖相 SO23-38 多孔含红藻—圆盾虫粒泥灰岩, 生物碎屑粒泥灰岩 晚渐新世—早中中新世 浅海—开阔海相 SO27-16 印模粒泥灰岩, 内成碎屑泥粒灰岩 晚渐新世—中中新世 浅海—开阔海相 SO27-21 层状含石英, 富红藻泥粒灰岩 晚渐新世—早中新世 开阔海相 SO27-25 富红藻粒泥灰岩, 内成碎屑富红藻泥粒灰岩 晚渐新世—早中新世 浅海—开阔海相 SO27-26 多孔富红藻泥粒灰岩, 粒泥灰岩 晚渐新世—早中新世 深海相—开阔海相 SO27-69 富有孔虫泥粒灰岩, 粒泥灰岩 晚始新世—早中中新世 浅海—深开阔海相, 半深海相 
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表 2 礼乐盆地区根据断层水平间距计算的相关伸展参数
Table 2. Extension factors caused by normal faulting in the Reed Bank basin
测线 计算长度(km) 阶段 拉张量(km) 拉张率(%) 总拉张量(km) 总拉张率(%) 伸展因子(βf) NH973-2 171.40 裂谷期 16.15 9.42 23.14 13.50 ~1.2 漂移期 5.80 3.38 后漂移期-1 1.23 0.72 后漂移期-2 0.00 0.00
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