Dynamic Mechanism of Anomalous Post-Rift Subsidence in the Yinggehai Basin
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摘要: 为了理解莺歌海盆地形成与演化的动力过程, 用回剥法和应变速率反演方法对该区的钻井和地层剖面资料进行了研究.研究结果表明莺歌海盆地观测得到的裂后沉降和模拟预测的理论裂后沉降结果存在较大差异, 其中在西北部为300~500 m, 中部和东南部为900~1200 m, 其异常裂后沉降明显呈现向东南和向海方向递大的趋势.地幔对流模型预测的结果表明, 20 Ma以来南海北部边缘的动力地貌沉降量为300 m, 因此, 莺歌海盆地裂后异常沉降在300 m左右的地区可以用动力地貌沉降机理来解释, 但是盆地中部和东南部的巨厚的异常沉降远大于动力地貌沉降量, 它是自晚中新世以来盆缘断层的右旋走滑作用、裂后热回沉和动力地貌沉降共同作用的结果.Abstract: In order to understand the dynamic mechanism of formation and evolution of the Yinggehai basin, the backstripping and strain rate inversion methods are used to study the stratigraphic sections and boreholes by analyzing the relevant data.The results show a discrepancy between observed post-rift subsidence and theoretically predicted post-rift subsidence based on a multiple rift model.Our modeling results of two seismic sections show clearly that the anomalous subsidence during the post-rift stage in the northwestern part of the basin, between 300 m and 500 m, is much less than that in the middle and southeastern parts, which ranges from 900 to 1 200 m.We compare these results with those of time-dependent dynamic topography subsidence based on the global mantle flow model.The computed dynamic topography decreases by about 300 m from 20 Ma ago to the present in the northern South China Sea area.Observed anomalous subsidence within a range of less than 300 m during the post-rift stage can be interpreted as the effect of dynamic topography.However, the huge anomalous subsidence in the middle and southeastern areas may originate from a combination of the dextral strike-slip movements of marginal faults since the Late Miocene, rapid thermal subsidence after rifting and the response of dynamic topography.
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图 1 莺歌海盆地构造及垂向序列图和图示研究所用剖面和钻井位置
Fig. 1. Structure and vertical sequence of the Yinggehai basin showing the position of selected cross sections and exploration wells used in this study
图 2 莺歌海盆地西北部二维A-B剖面构造沉降模拟(位置参见图 1)
a.地层剖面; b.构造沉降量, 实线表示裂陷期沉降, 短横虚线代表 21~10.5 Ma的裂后沉降量, 点虚线代表 10.5 Ma以来的区域沉降量; c.3个模拟点的应变速率曲线及异常裂后沉降量, 图框中深灰色阴影带表示裂陷期, 浅灰色阴影带表示裂后沉降期, 白色带表示区域沉降期, 黑色三角带为异常裂后沉降量
Fig. 2. Two dimensional A-B section used for calculating tectonic subsidence in the northwestern Yinggehai basin and the related modeling results
图 3 莺歌海盆地中部二维C-D剖面构造沉降模拟(位置参见图 1)
a.地层剖面; b.构造沉降量, 实线表示裂陷期沉降, 短横虚线代表 21~10.5 Ma的裂后沉降量, 点虚线代表 10.5 Ma以来的区域沉降量; c.3个模拟点的应变速率曲线及异常裂后沉降量, 图框中深灰色阴影带表示裂陷期, 浅灰色阴影带表示裂后沉降期, 白色带表示区域沉降期, 黑色三角带为异常裂后沉降量
Fig. 3. Two dimensional C-D section used for calculating tectonic subsidence in the central Yinggehai basin and the related modeling results
图 4 莫霍面和破裂不整合深度对比(Xie et al., 2006)
Fig. 4. Comparison of the Moho depth (a) and isobath of the breakup uncomformities (b)
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