The Growth and Interaction of Faults in Multiphase Rifts: Horda Platform, Norwegian North Sea

Abstract:

Analogue models predict that multiphase rifts which have experienced a change in extension direction between stretching phases will typically develop non-colinear normal fault sets and hence will display a greater frequency and range of styles of fault interactions than single-phase rifts. We test these model-based predictions by studying a natural fault network in the northern Horda Platform, northern North Sea using an integrated 3D seismic reflection and borehole dataset. We focus on the >60 km long, N-S-striking Tusse fault that has over 500 m of throw and was active in the Permian-Triassic and again in the Late Jurassic-to-Early Cretaceous. The Tusse Fault forms part of a non-colinear fault network that also comprises numerous smaller (2-10 km long), lower throw (<100 m) and predominantly NW-SE-striking faults that were only active during the Late Jurassic to Early Cretaceous. We examine how the second-stage NW-SE-striking faults have grown and interacted with the N-S-striking Tusse Fault, noting a range of end-member styles of interaction including: i) no interaction; ii) hanging-wall abutting; iii) footwall abutting; iv) cross-cutting; and v) hybrid. To constrain the nucleation and growth history of each of these interaction styles, we systematically apply throw-versus-length (T-x) plots, throw-versus-depth plots (T-z) and 3D throw contouring. This quantitative 3D analysis of the fault network demonstrates: i) pre-existing (first-stage) faults can act as sites of nucleation for second-stage faults; ii) abutting relationships are common and can develop by second-stage faults nucleating either at, or away from pre-existing faults; iii) the throw distribution on reactivated first-stage faults will be modified in a predictable manner if they are intersected or influenced by second-stage faults; and iv) fault segment boundaries, and fault kinks or corrugations along first-stage faults, can act as preferential nucleation sites for second-stage faults, and facilitate the development of complex cross-cutting relationships. In addition to furthering our fundamental understanding of the geometric and kinematic evolution of rift-related normal faults, our results also have broader implications for understanding the physiographic and tectono-stratigraphic evolution of multiphase rift basins.