Abstract:

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.

Key words: multiphase rifting, fault activity, sediment dispersal pattern, Eocene, Lufeng Depression, Pearl River Mouth Basin