NORMAL FAULT GROWTH AND STRUCTURAL EVOLUTION DURING MULTIPHASE RIFTING: OSEBERG FAULT BLOCK, NORTHERN NORTH SEA

In rift basins where extension has occurred over multiple phases, it may be challenging to determine how pre-existing faults influence those formed in later rift phases. Using 3D seismic reflection and borehole data from the Oseberg fault block, Northern North Sea, we investigate the geometry and evolution of faults over two separate rift phases, the ?Permian-Early Triassic and the Middle Jurassic-Early Cretaceous. In particular, we aim to assess the control of first-phase normal faults and stress field on the evolution of the second-phase fault network by analyzing structure and time-thickness maps, cross-sectional geometry, fault throw variations and fault curvature. Phase 1 is characterized by N-S- and NW-SE-striking faults which mainly dip to the east and bound west-expanding growth package in the hanging wall. During Phase 2, Phase 1 faults are reactivated and propagate upward through the Middle Triassic-Middle Jurassic cover. In addition, new Phase 2 faults nucleate and grow and offset Phase 1 faults. These new Phase 2 faults strike N-S, NW-SE and NE-SW and dominantly dip to the west, forming conjugate fault set with Phase 1 fault reactivated in Phase 2. Phase 2 faults control east-expanding growth packages in the hanging wall associated with the development of the North Viking Graben. The development of faults with different orientations produces three types of fault interaction: crosscutting, abutting and linking. Phase 2 NE-SW-striking faults crosscut Phase 1 N-S-striking faults reactivated in Phase 2 and inhibit their lateral growth, indicated by the rapid decrease of fault throw where they meet. In addition, the Oseberg fault, which originates from Phase 1 and forms by the linkage of N-S- and NW-SE-striking segments has a west slip direction because the axis of curvature has a W-vergence. In contrast, the Brage fault which initiates in Phase 2 and consists of NW-, N- and NE-striking segments displays an overall northwest slip direction owing to a NW-vergence of the axis of curvature.These evidences suggest that the regional or local stress field changes from E-W to NW-SE in Phase 2. Our study shows that normal fault growth and structural evolution of the North Sea Rift system during multiphase rifting could be controlled not only by the pre-existing faults, but also by a variation in the stress field.