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Protracted Fault Growth and Reactivation in Multiphase Rift Basins: Structural Evolution of the East Shetland Basin, Northern North Sea

Abstract

Many continental rifts have experienced multiple phases of extension. However, the interaction of faults associated with the different rift phases is not fully understood due to structural overprinting and/or deep burial leading to poor data quality. Multiphase rift basins have previously been studied using physical analogue or numerical modelling, and field and subsurface analyses. These studies typically identify multiple, discrete tectonic stages: pre-, syn-, and post-rift. This simplified approach, however, does not consider the relative evolution of fault systems within a basin as a result of diachronous fault growth, both laterally and between faults. This study focuses on the East Shetland Basin (ESB), a multiphase rift basin located on the western margin of the North Viking Graben, northern North Sea. At least two extensional phases are typically recognised in the basin (Permian-Triassic and Mid-Late Jurassic), with the overall geometry of the latter rift being the result of selective reactivation of faults associated with the former rift. Between and within fault blocks, intra-rift strata gradually thicken eastwards and this geometry has led to two different interpretations: (i) this wedge documents Early-Middle Jurassic, differential thermal subsidence after the first rift event; or (ii) the wedge documents Triassic syn-depositional activity on west-dipping faults. Analysis of 2D and 3D seismic reflection and well data allow us to re-evaluate the structural evolution of the ESB. In the NW of the ESB, Permian-Triassic syn-rift deposits are observed along large (>20 km length) NE-SW striking faults, while elsewhere in the basin, Permian syn-rift deposits are capped by gradually eastward-thickening strata, suggesting a Triassic post-rift sag. Subsequent Early-to-Middle Jurassic deposits thicken across large N-S striking faults eastward, suggesting fault growth during this time and thus the onset of another rift phase. Our results challenge previous interpretations of the structural evolution of the ESB: the timing of initiation, and duration of activity of large fault systems are not consistent throughout the basin, with faults showing polyphase activity, cross-cutting relationships, and protracted growth. The results of this work highlight the complex structural evolution of multiphase rifts and demonstrate that the conventional rift package nomenclature of pre-, syn-, and post-rift does not necessarily apply to multiphase rift basins.