多幕裂陷盆地构造-沉积响应及陆丰凹陷实例分析

裂陷盆地常常经历了多层次、多周期的幕式沉降过程。若盆地的演化包括了两幕或以上的裂陷旋回,则称之为多幕裂陷盆地。多幕裂陷盆地构造演化特别是复杂的断裂发育特征及其活动方式引起国际地质学家广泛的关注。多幕次裂陷作用下断裂活动方式的转型,必然会导致物源水系、沉积物入口位置及砂体分散方式的系统性差异。前人研究表明,在第一幕裂陷或者单幕裂陷盆地中断裂经常会发生分段联接。在盆地演化的早期各个洼陷较为分散和孤立,主要以陡坡带和缓坡带水系供源为主,而在断裂发生连锁之后,轴向水系供源体系则开始占据主导。相比较第一幕裂陷,第二幕裂陷作用下盆内断裂活动方式多变化,其沉积响应十分复杂。珠江口盆地陆丰凹陷始新世发育两幕裂陷(Ⅰ幕和Ⅱ幕),区域构造应力场方向偏转可能诱导了主干断层的差异活动,致使裂陷中心从南向北发生迁移。研究表明陆丰凹陷两幕次裂陷旋回的构造-沉积响应差异明显:(1)裂陷Ⅰ幕初始断陷阶段断层位移量相对较小,湖盆表现为“浅盆”特征,而裂陷Ⅱ幕早期阶段快速形成规模大、活动性强的边界断层,湖盆则快速进入深湖环境;(2)裂陷Ⅰ幕初始断陷阶段厚度中心小且较为分散,而裂陷Ⅱ幕早期阶段厚度中心规模大,较为统一;(3)裂陷Ⅰ幕初始-强烈断陷阶段以陡坡带、转换带和缓坡带侧向水系供源为主,而裂陷Ⅱ幕早期轴向水系占据主导。在伸展应力方向偏转的多幕裂谷盆地,晚期裂陷幕往往以特定的边界断层迅速生长和复活为特征。近似垂直于新的应力场方向的先存断层则优先活动,在该地区的位移量快速达到最大,这促进了大型轴向供源体系的形成;而未复活断层控制的地区构造沉降弱,继承了早期裂陷结束时长轴水系供源的格局,具有整体“富砂”的沉积特征。以上认识对珠江口盆地及其他多幕裂陷盆地演化分析及砂体预测具有重要的理论价值。

Tectono-sedimentary development of multiphase rift basins:An example of the Lufeng Depression

Numerous rift basins worldwide were shown to experience two or more distinct extensional phases, and are referred to as multiphase rift basins. Natural observations and physical results revealed distinctive fault behaviors between the first (RP1) and second (RP2) rift phase in multiphase rifts. The transition between fault growth and reactivation in separate rift phases would inevitably lead to changes in drainage catchments, sediment entrance and sandstones dispersal pathways. The evolution of multiphase rift basins, especially the fault array evolution and displacement patterns, have attracted the attentions from numerous geologists worldwide. Typically, RP1 or single rift cycle involves progressive evolution, featuring isolated faults, fault interaction and linkage, and through-going faulting during the climax stages, followed by a final fault-death stage. In proposed genetic models, the basin is mostly relatively scatted and isolated in the early stage dominated by transverse-sourced systems, then axially-source system becomes facilitated in the fault linkage stage and towards the end of the rifting process. Compared to RP1, the rift-related faulting and basin-infill patterns during RP2 are unclear and currently an international hot topic. Our case study was in the Lufeng Depression located in the northern Pearl River Mouth basin of the South China Sea. The Lufeng Depression is a two-phase rift basin. The directional rotation of the regional tectonic stress field might have induced the differential activities of rift-related faults, resulted in northward migration of the subsidence center. The depositional responses to the tectonic activities during RP2 were in marked contrast to those during RP1. Firstly, the initial stage of RP1 was commonly low-displacement with a shallow lake setting, while the rapidly reactivation of selected master faults resulted in a high tectonic subsidence rate associated with a dominant deep lake setting during the early stage of RP2. Secondly, the initial basins of RP1 were usually small-sized with isolated depocenters, but the depocenters were generally large and uniform during the early stage of RP2. Thirdly, the initial and climax rift stages of RP1 were dominated by transverse drainages, whereas the rapid reactivation of selected faults tended to establish the large axially-sourced drainage catchments during RP2. Consequently, a tectono-sedimentary motif for the multiphase rift basin is proposed. It suggests that those pre-existing faults that are nearly perpendicular to the new extensional direction are highly susceptible to immediate reactivation and can propagate rapidly to their full displacements. The RP2 basins therefore can be considered as 1) sustained axially-sourced depositional systems in the inactive fault controlled areas, or 2) facilitating newly captured axially-dominated systems in the rapidly reactivating fault areas. This study may have significant implications for fault depositional evaluation and sandstone prediction in the Pearl River Mouth basin or other multiphase rifts worldwide.