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


Physical 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 2nd-stage NW-SE-striking faults have grown and interacted with the N-S-striking Tusse Fault, noting a series of key end-member styles including faults that are: i) isolated and non-interacting; ii) abutting; iii) cross-cutting; and iv) hybrid. To constrain the nucleation sites, growth histories and diagnostic throw distributions associated with each interaction style, we apply throw-versus-length (T-x), throw-versus-depth (T-z) and 3D strike-projection throw contouring techniques to key faults. Our results show that: i) pre-existing (1st-stage) faults can act as sites of nucleation for 2nd-stage faults; ii) abutting relationships are particularly common and can develop by 2nd-stage faults nucleating either at, or away from pre-existing faults; iii) the throw distribution on reactivated 1st-stage faults will be modified in a predictable manner if they are intersected or influenced by 2nd-stage faults; and iv) fault segment boundaries, and fault kinks or corrugations along 1st-stage faults, can act as preferential nucleation sites for 2nd-stage faults, and facilitate the development of complex cross-cutting relationships. In addition to furthering fundamental understanding of the characteristic geometries, kinematics and throw distributions of normal faults in multiphase rifts, our results also have broader implications for understanding the physiographic and tectono-stratigraphic evolution of multiphase rift basins.