Mega‐scale Moho relief and the structure of the lithosphere on the eastern flank of the Viking Graben, offshore southwestern Norway

Abstract

The International Lithosphere Project deep reflection seismic survey in the Norwegian sector of the North Sea has been reprocessed, particularly focusing on the deep crust, the reflection Moho, and the upper mantle. The data display shifting reflection patterns of the crust and the upper mantle parallel to the eastern margin of the Viking Graben. In the upper crust, which is mainly seismically transparent by the processing techniques utilized here, large-scale structural features like detachment shear zones and master faults can be identified. Several of the major onshore faults and shear zones match seismic features in the seismic lines. Many of these structures acted as extensional shear zones in the Devonian. The middle crust is of variable reflectivity, whereas the lower crust is generally strongly reflective and is particularly so in the southern domain. The reflection Moho is identified throughout the study area but is of variable character. The presence of a S(E) dipping structure (Hardanger Moho Offset) that displaces the Moho by approximately 10 km, extends deep into the mantle (below the 50 km line depth), is positioned where the shallower Hardangerfjord Shear Zone, which flattens on the level of the middle crust, is situated. The Hardangerfjord Shear Zone/Hardanger Moho Offset-system coincides with change of the crustal thickness (depth to Moho), a change that also coincides with the transition from thin-to thick-skinned Caledonian deformation. Intramantle reflections are common in the study area, some of which are interpreted as shear zones, whereas others most likely represent magmatic intrusions.

The Influence of Structural Inheritance and Multiphase Extension on Rift Development, the NorthernNorth Sea

“…In the northern North Sea, the southern extension of the otherwise N‐S striking Øygarden Fault rotates to a NE‐SW orientation and locally aligns with the NE‐SW striking Hardangerfjord Shear Zone, suggesting a local NE‐SW oriented stress field associated with the shear zone (Figures and ). The Hardangerfjord shear zone represents a major structure across the northern North Sea rift and is associated with a Moho offset at depth (Gabrielsen et al, ; Maystrenko et al, ). Similarly, faults defining the western margin of the Stord Basin follow the underlying Utsira Shear Zone in plan‐view, rotating from N‐S in the south to NE‐SW further north.…”

“…This structure has an enhanced influence throughout the multiphase evolution of the rift compared to other interpreted Devonian shear zones. One possibility is that the Lomre Shear Zone extends to greater (i.e., midcrustal) depths, or that it reactivates a Caledonian or earlier structure, both of which have been proposed for the Hardangerfjord Shear Zone further south, which also exerts a different influence over rift physiography, being associated with a Moho offset at depth and controlling the location and geometry of the rift‐bounding faults in the Ling Depression (Fossen et al, ; Fossen & Hurich, ; Gabrielsen et al, ; Maystrenko et al, ). The Lomre Shear Zone has been proposed to represent the southern extension of the Nordfjord‐Sogn Detachment by Færseth et al (), although it does not appear to correlate with the structure onshore (Figure ).…”

“…Well data are typically collected at relatively shallow (2–3 km), more economic depths, with few wells penetrating deeper areas, particularly in the hanging walls of major faults. Previously, imaging of basement structures was confined to regional seismic sections, often limited to 2‐D and at the expense of resolving shallow structure (BIRPS and ECORS, ; Fossen et al, ; Gabrielsen et al, ; Klemperer & Hobbs, ). However, more recently basement structures have been resolved beneath the northern North Sea rift, particularly where they are situated at relatively shallow depths on the rift margins (Bird et al, ; Fazlikhani et al, ; Lenhart et al, ; Patruno et al, ; Phillips et al, ; Reeve et al, ).…”

Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time

“…cit.). It continues into the North Sea where it reaches lower crustal levels (Gabrielsen et al, 2015). Based on the geometry and kinematics of faults that are crosscut by Permian and Triassic dikes in the Sunnhordland region, a pre-dike extension phase oriented NW-SE has been constrained by Valle et al (2002).…”

Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea

“…In late Palaeozoic and early Triassic times, tensional forces resulted in the supercontinent break up and opening of the North Atlantic during the Tertiary, creating the Viking Graben. The N-S striking faults in the Horda platform area became reactivated during the Jurassic-Cretaceous, leading to further development of the Viking Graben (Nøttvedt et al 1995;Gabrielsen et al 2015). The Utsira High is an intrabasinal structural high between the Viking Graben and the Stord Basin ( Fig.…”