Abstract
Multiphase rifts tend to produce fault populations that evolve by the formation of new faults and reactivation of earlier faults. The resulting fault patterns tend to be complex and difficult to decipher. In this work we use seismic reflection data to examine the evolution of a normal fault network in the Oseberg Fault Block in the northern North Sea Rift System – a rift system that experienced Permian – Early Triassic and Middle Jurassic – Early Cretaceous rifting and exhibits N-S, NW-SE and NE-SW oriented faults.Both N-S- and NW-SE-striking faults were established during the Permian – Early Triassic rifting, as indicated by Triassic growth packages in their hanging walls. In contrast, the NE-SW-striking faults are younger, as they show no evidence of Permian – Early Triassic growth, and offset several N-S- and NW-SE-striking faults. Structural analysis show that a new population of NW-SE-striking faults formed in the Lower – Middle Jurassic (inter-rift period) together with reactivation of N-S-striking Permian – Early Triassic faults, indicating a NE-SW inter-rift extension direction.During the Middle Jurassic – Early Cretaceous rifting, faults of all orientations (N-S, NW-SE and NE-SW) were active. However, faults initiated during the Middle Jurassic – Early Cretaceous rifting show mainly N-S orientation, indicating E-W extension during this phase. These observations suggest a reorientation of the stress field from E-W during the Permian – Early Triassic rift phase to NE-SW during inter-rift fault growth and back to E-W during the Middle Jurassic – Early Cretaceous rift phase in the Oseberg area. Hence, the current study demonstrates that rift activity between established rift phases can locally develop faults with new orientations that add to the geometric and kinematic complexity of the final fault population.
Highlights
Many rift basins are known to have developed through multiphase rifting that comprises two or more rift phases, each of which is separated by a period of tectonic quiescence, such as the North Sea rift system (Badley et al, 1988; Færseth, 1996; Odinsen et al, 2000a; Whipp et al, 2014), the Gulf of Aden (Lepvrier et al, 2002), the East African Rift System (Korme et al, 2004) and the North West Shelf, Australia (Frankowicz and McClay, 2010)
The general increase in fault throw with depth and the notable hanging-wall growth packages of the T1-T2 intervals indicate that the Brage East Fault formed during the Permian – Early Triassic rift phase, during which it was the largest fault in the study area (Figs. 1, 5)
The evolution of fault array in the Oseberg area can be divided into four stages: (1) the Permian – Early Triassic syn-rift period characterized by N-Sand NW-SE-striking, mainly E-dipping faults; (2) early stages of inter-rift evolution, controlled by thermal subsidence and differential compaction; (3) a late inter-rift stage featuring reactivation of Triassic faults and formation of NW-SE-striking faults; (4) the Middle Jurassic – Early Cretaceous syn-rift period resulting in the development of N-S, NW-SE- and NE-SW-striking faults that are mainly W-dipping
Summary
Many rift basins are known to have developed through multiphase rifting that comprises two or more rift phases, each of which is separated by a period of tectonic quiescence, such as the North Sea rift system (Badley et al, 1988; Færseth, 1996; Odinsen et al, 2000a; Whipp et al, 2014), the Gulf of Aden (Lepvrier et al, 2002), the East African Rift System (Korme et al, 2004) and the North West Shelf, Australia (Frankowicz and McClay, 2010). Fault evolution during the two rift phases is not clearly understood, and not the way that Permian – Early Triassic faults reactivated and influenced fault growth during the second phase of rifting In this contribution we use 2D and 3D seismic reflection and borehole data to investigate the geometry and evolution of normal faults that evolved over the entire history of rifting in the Oseberg area in the northern North Sea. The Oseberg area is located at the boundary between the Permian – Early Triassic and Middle Jurassic – Early Cretaceous rift axis (Badley et al, 1984; Færseth et al, 1997; Færseth and Ravnas, 1998; Tomasso et al, 2008), making it possible to study the faults related to both rift phases. The main goal is to understand the fault evolution and interaction between the two phases of rift-related faulting, and to explore what controls fault evolution in the Oseberg area and in multiphase rift basins in general
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