Abstract:

Characteristics of boundary faults are of significance in researches on the evolution and formation mechanism of sedimentary basins. Based on the analysis of geological and geophysical data, we examined the relationship between the boundary faults, i.e. the Xinanzhuang fault (XNZF) and Baigezhuang fault (BGZF) in Nanpu sag. The paleo-stress field evolution and features of boundary faults were studied by analyzing occurrences and activity differential of various segments of the two faults and their spatial relations to depocenters and contemporaneous faults, consequently, a new formation mechanism for Nanpu sag was proposed. Seiscrops, gravity anomaly distributions and seismic sections revealed that the XNZF and BGZF converged successively with no distinct host-slave correlation and mutual overlapping, behaving like the same fault (the Xi-Bai fault for short). However, each segment has a difference:the XNZF with a dip angle of 43° shows a curved strike and bending fractured surface, whereas the BGZF with a dip angle of 57°~69° has a straight strike and planar fractured surface. Besides, the paleo-downthrow calculation of the boundary faults showed that the XNZF was strongly active as a whole in Paleogene, while the BGZF remained inactive except for the Gaoliu structural zone that acted violently in late Eocene to Oligocene. The comparison of spatial distributions of the contemporaneous faults, depocenters and boundary faults demonstrated that the tectonic evolution of Nanpu sag can be divided into three stages:during the Es₃ to Es₂ deposition, controlled by NE-orientated contemporaneous faults, depocenters extended in the NE direction, indicating a NW-orientated extension of the basin; during the Es₁ to Ed deposition, depocenters shifted in both the NE and the NW directions and there occurred an violent NW-orientated left-lateral strike-slip fault in the basin with an NW-orientated extension; and since Ng deposition, depocenters have experienced continuous transtension due to the joint influence of the NEE-and NW-orientated faults. Correspondingly, the paleo-stress field evolution of Nanpu sag can be divided into three episodes, namely the weakly, strongly and stably transtensional episodes. The present study suggested that Nanpu sag was formed by the XNZF extension along the left-lateral strike-slip direction of the BGZF, consequently, a new genetic pattern was proposed that Nanpu sag in Paleogene should be a classic transtensional fault-termination basin.

Key words: boundary fault, tectonic evolution, Paleogene;mechanism of basin formation, Nanpu sag

摘要：

边界断层特征对于盆地演化历史与成盆机制研究具有重要意义。该研究基于大量地质和地球物理资料分析，揭示了南堡凹陷边界西南庄断层（XNZF）和柏各庄断层（BGZF）的关系，通过边界断层不同区段上产状、活动差异性分析以及与沉积中心和同沉积断层的空间匹配关系，揭示盆地古应力场演化和边界断层的性质，提出南堡凹陷的成盆机制。地震水平切片、重力异常分布和地震剖面显示了XNZF和BGZF在交汇处连续过渡，不存在主次分级和相互切割，表现为同一条断层（简称为西—柏断层）。但西—柏断层的XNZF区段断层倾角小，走向曲折，断面弯曲；BGZF则相反，倾角大，走向和断面平直。边界断层古落差的计算表明，XNZF古近纪整体活动强烈，BGZF仅始新世末期—渐新世在高柳构造带活动强烈。对比同沉积断层、沉积中心与边界断层三者的空间匹配关系发现：古近系沙河街组三段（Es₃）—沙河街组二段（Es₂）沉积时期，受NE向同沉积断层控制，沉积中心呈NE向展布，说明盆地呈NW向伸展；古近系沙河街组一段（Es₁）—东营组（Ed）沉积时期，沉积中心展布有NE和NW两个方向，盆地NW向断层左旋走滑作用强烈，并伴随NW向伸展；新近系馆陶组（Ng）沉积以来，沉积中心受NEE和NW断层联合控制，表现为持续的走滑伸展作用。相应地南堡凹陷应力场演化可分为3个阶段：弱转换伸展期、强转换伸展期和稳定转换伸展期。研究认为XNZF沿BGZF左旋走滑方向上的伸展形成了南堡凹陷，提出了南堡凹陷古近纪新成因模式——转换伸展断层终止盆地模式。

关键词: 边界断层, 构造演化, 古近纪;成盆机制, 南堡凹陷