Structural and stratigraphic expression of multiphase extension in rift basins
Creators: name: Claringbould, Johan
Publisher: name: Imperial College London
Year: 2015

description: The structural geometry and stratigraphic development of continental rifts provides an important insight on the underlying lithospheric processes, seismic hazard assessment, and hydro-carbon prospectivety. Many continental rift basins have been exposed to multiple phases of extension and have been studied extensively in the field, subsurface and using analogue models. However, parameters such as the impact of pre-existing structures, rheology heterogeneity, and thermal perturbation size on the rift geometry evolution are not yet well understood. Furthermore, due to the limited dimension of the data available and the methods applied, the temporal growth character of the fault segments and systems that make up a rift margin covering fault array is not yet well-constrained. The East Shetland Basin is located on the western margin of the north Viking Graben, northern North Sea, and comprises a full fault array, and it is generally assumed that this basin developed in response to two phases of extension, separated by a period of tectonic quiescence. This study uses recently available, high resolution subsurface data that covers the full extent of the basin, and in contrast with previous work, this work proposes that the fault array in the East Shetland Basin gradually developed during a single-phase extension event. Highly detailed qualitative and quantitative seismic analyses show a continuous, diachronous development of the fault array, not only between individual faults but also between the fault segments along strike of the large basin- covering faults from pre-Triassic-to-Late Jurassic. This diachroneity is also observed in the basin-wide, well analyses of chronostratigraphically constrained depositional units. Moreover, a regional assessment of the tectono-stratigraphic relationship implies that the fault activity rate was equal or less than the sedimentation rate during Late Triassic-to-Late Jurassic. Pre-existing normal faults show limited impact on the structural geometry development of the basin, as most pre-Triassic faults get buried or cross-cut during later fault activity and fault reactivation is limited. This study suggests that a changing geometry of the underlying thermal perturbation as predicted by previous numerical models, affected basin- wide strain distribution over time and, therefore, the structural evolution of the fault array in the East Shetland Basin. The diachronous development of a fault array complicates the basin- wide usage of the conventional discrete tectonic stages (i.e., pre- syn- and post-rift) and the assumption of synchronous fault array growth mainly during the syn-rift stage. This work suggests the usage of rift activity maxima and minima when describing fault array development during a first-order syn-rift stage. The work presented here implies that the investigation of fault populations is scale dependent: besides crustal-scale parameters, at fault array scale, lithospheric-scale variables also affect the geometry and temporal development. The results yielded by this study are at the resolution that allow direct comparison to predictions by current numerical models, however, caution is required for the application of predictions from analogue rift growth models, which do not include the role of lithospheric thermal and rheological evolution.