Controls on strain localisation in the Middle to Late Jurassic North Sea rift system
Gill, Caroline E
Extensional fault propagation and linkage play an important role in the structural and sedimentological development of rift basins. In this study, use of 85,000km² 3D seismic data provides a new and unique opportunity to evaluate and quantify the processes of fault growth within an evolving rift system. The study uses a comprehensive dataset from the East Shetland Basin and North Viking Graben, northern North Sea to document the evidence for, and effects of fault growth during the Mid to Late Jurassic (Bajocian to Ryazanian) rift episode. Integration of excellent biostratigraphic control with these seismic data enables enhanced temporal control than that which can be achieved in the field. The data allows the development of a new 4-dimensional holistic model in which strain localisation can be shown to be the dominant control on the spatial and temporal evolution of structure in an evolving rift province. The significance of my approach is the integration of seismic stratigraphic analysis combined with facies analysis and biostratigraphy in the East Shetland Basin to demonstrate that during the Mid to Late Jurassic rift episode the locus of extension migrated from west to east towards the Viking Graben. Systematic documentation of the variation in age of syn-rift deposits across the basin shows that motion on faults ended progressively throughout the Kimmeridgian and Portlandian with the latest motion occurring on the Visund-Gullfaks- Alwyn fault array during the Volgian. The effect of strain localisation towards the basin centre with time caused passive rotation of earlier (more westerly) structures (e.g. Snorre) and their hangingwall depocentres. This is the first study to challenge the traditional view that the Mid to Late Jurassic structures in the East Shetland Basin were active synchronously throughout the rift episode. Detailed interpretation of the syn-rift architecture associated with four major faults situated on a 100km long, east-west transect across the basin allows the development of a detailed model in which the timing of activity on a suite of faults can be accurately quantified. Initially, during the Upper Bajocian, extension was characterised by a large number of small faults of both dip directions which grew by radial tip propagation and subsequent segment linkage. Strain was initially focussed onto a small number of throughgoing fault arrays at the expense of less optimally positioned structures. The focussing of strain onto these structures resulted in an increased slip rate and upward bowing of the footwalls, leading to the development of release faults formed as a result of differential vertical displacements along the length of the fault. These small (less than 10km long) structures form perpendicular to the main fault and show maximum displacement of c. 300m at their proximal end decreasing away from the main fault. As the rift episode progressed, strain was gradually localised towards the rift axis. It culminated in the Ryazanian with localisation of strain onto the rift axis. At this time activity was primarily confined to a single thoroughgoing fault, the Visund-Gullfaks Fault, defining the western edge of the North Viking Graben. As such the Visund-Gullfaks Fault developed to be the largest fault in the basin both in terms of length (125km) and maximum accumulated displacement (>5km) as a result of the basinward (eastward) migration of the locus of extension.