南沙东部海域裂陷结束不整合面时空迁移规律及构造意义

南沙东部海域构造位置特殊,油气资源丰富,其构造过程对认识南海形成演化意义重大.由于地质条件复杂、资料分布不均,其地层系统和主要构造变革面属性认识尚不清楚,尤其具有断坳转换性质的裂陷结束不整合面地质时代和意义存在诸多争议.本文在综合国内外地层划分方案的基础上,运用连井剖面对比、地震相类比方法,对该海域多道地震资料进行系统的构造-地层综合解释,重点厘定了T7、T6和T5三个与裂陷结束相关的不整合面.研究表明,裂陷结束不整合面具有抬升剥蚀、断块掀斜、挠曲变形、拆离滑脱、沉积相突变等特征,且具有东北早西南晚,南侧靠陆早、北侧近洋晚的带状时空迁移规律.结合南海构造演化、地层岩性和沉积环境变化分析裂陷结束不整合面的构造含义,认为该界面是南海岩石圈破裂、洋脊跃迁、挠曲前隆、陆陆碰撞、扩张停止等一系列构造事件在不同构造位置单次或多次叠加的构造-沉积响应.

Temporal and spatial migration of rift-terminate unconformity surfaces and their tectonic significance in the eastern part of Nansha Block

The timing, naming, and distribution of the Cenozoic unconformities affecting the sediment rocks and have a great significance to understand the tectonic evolution of the South China Sea. Due to the complexity of geological processes and the different data coverage, it is complicated and controversial to divide and date the major unconformities in the eastern part of Nansha Block, southeastern of the South China Sea. In this work, all kinds of stratigraphic systems proposed in the literature were compared and summarized, and nearly ten thousands km 2D seismic data were interpreted, and the seismic reflection characteristics about key tectonic unconformities were identified. Three major rift-terminate unconformity surfaces were divided and dated through biostratigraphy analysis and well-profile comparison, and they include T7, T6, and T5. On the seismic profile, the rift-terminate unconformity surfaces are characterized by uplift erosion, tilted fault block, forebulge development, detachment slippage, cessation of rifting, and sediment environment mutation. The spatial distribution of these unconformities is band-like, and they formed early in the northeast and late in the southwest. Based on analyzing the tectonic evolution history, stratigraphic lithology, and sedimentary environment, the different causal mechanisms of the rift-terminate unconformity surfaces were considered to lithospheric breakup, oceanic ridge jumps, over-thrusted, continental collisions, and the stopping of seafloor spreading in the South China Sea. The rift-terminate unconformity surfaces were related to a single or multiple tectonic-stratigraphic responses of the tectonic events at different tectonic locations.