南海北部潮汕坳陷上白垩统盆地原型及其大地构造背景分析

The Upper Cretaceous prototype basin of the Chaoshan depression in the northern South China Sea and its tectonic setting

潮汕坳陷MZ-1井揭示的中生界为深入分析南海北部晚中生代的构造演化提供了关键性的资料.基于MZ-1井的标定,开展了系统的地震剖面构造-地层解释,在中生代地层内识别出Tm30区域性不整合面,同位素定年确定该界面发育于早白垩世末至晚白垩世初,落实了潮汕坳陷上白垩统的分布.此外,在研究区西南部识别出大型的兴宁—东沙逆冲推覆带,主要由多条NW—SE向延伸、西倾的叠瓦状逆冲断层及其伴生的不对称褶皱组成,其明显控制了上白垩统厚度分布.由此可见,上白垩统构造层不具有张裂盆地的典型特征,因此南海北部主动陆缘向被动陆缘的转换不会早于晚白垩世末.研究认为,在南海地区特提斯残留洋盆关闭的总背景下,在约80 Ma时期,南海地块与华南陆块强烈碰撞挤压,在靠近碰撞带处的礼乐滩、潮汕坳陷西南部形成褶皱冲断构造体系,进而控制了潮汕坳陷晚白垩世周缘前陆盆地的发育.

Mesozoic strata of the Chaoshan depression revealed by well MZ-1 provides critical data for the analysis of the Late Mesozoic tectonic evolution of the northern South China Sea.Based on interpreting the seismic profiles calibrated through well log MZ-1,a systematical tectono-stratigraphic analysis has been carried out.A regional unconformity Tm30 was identified,and isotope dating determined that Tm30 formed during late Early Cretaceous to early Late Cretaceous.Accordingly,the distribution of the Upper Cretaceous in the Chaoshan depression was determined.In addition,a large-scale Xingning-Dongsha thrust-fold belt was recognized in the southwest of the research area,which consists of several NW-SE extending,westward dipping imbricate thrust faults and associated asymmetric folds,posing a significant impact during the deposition of Upper Cretaceous.Due to the lack of typical characteristics of a rift basin in the Upper Cretaceous sequence,it can be inferred that the transition of the northern margin of the South China Sea from an active margin to a passive margin postdates the end of the Late Cretaceous.Our research results suggest that in a general tectonic setting of the closure of the remnant Tethys oceanic basin about 80 Ma ago,a strong collision happened between the South China Sea and South China continental blocks.The fold and thrust system could develop near the collision zone,such as in the Reed Bank and southwest of the Chaoshan depression,and further controlled the development of the Late Cretaceous peripheral foreland basin of the Chaoshan depression.