Lithosphere Stretching Model of Deep Water in Qiongdongnan Basin, Northern Continental Margin of South China Sea, and Controlling of the Post-Rift Subsidence
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摘要: 为了揭示盆地深水区演化及裂后期大规模沉降的成因机制, 在琼东南盆地典型的、高品质地震剖面地质构造精细解释基础上, 结合岩石圈变形的挠曲悬臂梁模型和挠曲均衡模型, 应用正演和反演模拟技术, 定量恢复了该盆地所处地区的上地壳、地壳以及岩石圈的伸展程度.结果表明, 琼东南盆地自陆架边缘到深水坳陷区, 岩石圈上地壳的伸展系数较小, β值最大为1.23~1.32;整个地壳的伸展系数变化较大, 盆地边缘隆起区的β值在1.1~1.2之间, 向盆地中部β值逐渐增大到3.14;而对整个岩石圈而言, 其伸展系数β值由陆架到陆坡深水盆地也从1.2逐渐增大到4.2.根据对南海地区的构造及岩石圈和地壳的结构分析认为, 与McKenzie的岩石圈均一伸展以及由热控制的裂后期缓慢沉降过程不同的是, 上述与深度相关的岩石圈伸展减薄是由南海西北次海盆扩张过程中深部物质的离散上涌流动所导致的下地壳的快速而强烈的塑性流动所引起的, 并由此建立了琼东南盆地的形成演化模式, 来解释和探讨深水坳陷区及裂后期快速而大规模沉降的成因机制.Abstract: In order to thoroughly explain the mechanism of significant subsidence in deep waters of Qiongdongnan basin, a rifted continental marginal basin, we analyzed the high resolution seismic reflection profiles in the study area, and conduct a quantitative study on upper-crust, whole crust and whole lithosphere stretching and thinning respectively by incorporating flexural cantilever model and flexural cantilever isostasy model and using both forward and reverse basin modeling techniques Our integrated analysis shows that the extension in brittle upper crust decreases from shelf to deep basin. And the largest amount of stretching factor β is 1.23-1.32; the whole crust stretching factors change from β=1.1-1.2 of uplifts in basin margin to β=3.14 in central basin; And estimates of the whole lithosphere stretching factor change from β=1.2 on continental shelf to β=4.2 in deep basin. Theanalysis of tectonic geology involving lithosphere and crust, depth-dependent stretching of lithosphere in Qiongdongnan basin described above is closely related with sea floor spreading of northwestern South China Sea, which caused the discrete upwelling flow of middle and deep substance, then it resulted in the plastic flow of the lower crust rapidly and intensely. This model is different from the depth-uniform lithosphere stretching model and its slow post-rift subsidence controlled by geothermal isostatic re-equilibration (McKenzie, 1978). The formation and evolution model of Qiongdongnan basin is proposed to interpret the mechanism of rapid and large-amplitude post-rift subsidence in deep water.
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图 1 琼东南盆地区域地质背景及其构造单元划分(据龚再升等, 2004)
1.新生代盆地(E-Q); 2.海底火山弧; 3.海陆边界; 4.盆地中的隆起、低凸起; 5.俯冲及仰冲带; 6.海壳及磁条带、扩张轴; 7.断裂
Fig. 1. Regional geological setting and its tectonic unit division of Qiongdongnan basin
图 2 琼东南盆地地层及盆地演化综合柱状图
Fig. 2. Comprehensive column map of strata and basin evolution of Qiongdongnan basin
图 3 岩石圈内不同深度的伸展计算方法示意图(Davis and Kusznir, 2004)
Fig. 3. Schematic diagram illustrating the three methods used to estimate stretching at depth within the lithosphere
图 4 岩石圈变形的挠曲悬臂梁模型(Kusznir et al., 1991)
Fig. 4. A schematic diagram of the flexural-cantilever model for lithosphere deformation
图 5 琼东南盆地地震剖面及其地质构造解释(位置见图 1)
Fig. 5. Seismic profile and geological structure interpretation across the Qiongdongnan basin
图 6 正演及挠曲回剥反演模拟琼东南盆地A-B剖面形态及其伸展系数β
a.正演模拟的裂陷期剖面; b.裂陷期上地壳伸展系数β剖面; c.挠曲回剥反演恢复的现今剖面形态; d.反演到裂后初期(21 Ma) 时的剖面形态; e.控制热演化的伸展系数β剖面
Fig. 6. The setions and and the stretching factor profile by forward modeling and flexural backstripping reverse modeling ofA-B section in Qiongdongnan basin
图 7 琼东南盆地A-B剖面地壳结构及岩石圈不同深度的伸展和减薄系数
a.琼东南盆地剖面A-B的地壳结构; b.整个岩石圈, 整个地壳以及上地壳的伸展系数β变化剖面; c.整个岩石圈.整个地壳以及上地壳的减薄系数(1-1/β变化剖面
Fig. 7. The crustal structure and the stretching and thinning factors of different depths within the lithosphere of A-B sectionin Qiongdongnan basin
图 8 琼东南盆地A-B剖面裂后阶段不同时期沉降速率曲线(位置见图 1)
Fig. 8. The subsidence rate of different post-rift stages of the A-B section in Qiongdongnan basin
表 1 正演和反演模拟所用的属性参数
Table 1. Property parameters used in the forward modeling and reverse modeling
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