Late Jurassic to Late Cretaceous canyons on the Måløy Slope: Source to sink fingerprints on the northernmost North Sea rift margin, Norway

The Måløy Slope is a key area for studying the connection between onshore and offshore geology of South Norway. It has functioned as an area of bypass, erosion and deposition between the Norwegian mainland source area and the offshore northern North Sea sink area since the Permian. The slope was faulted into N–S-trending rift fault-blocks through flexural down-bending during the large-scale extension and rapid rift basin subsidence in the Late Jurassic and Early Cretaceous. Mapping of 3D seismic data has revealed a profound network of E–W-oriented erosional submarine canyons. These canyons cut up to 500 m into the crystalline bedrock on the rift-related fault-block crests. We suggest that the canyons were first established prior to the faulting associated with late Jurassic rifting. The canyons may have been important feeders in the Oxfordian, Kimmeridgian and Tithonian like canyons in the Uer Terrace to the south, although we lack direct evidence for this. Further erosion and deepening of the canyons into the basement occurred during Cretaceous in a post-rift setting. The position of the main canyons sustained during recurring periods of erosion from the Late Jurassic until burial within the slope in the Late Cretaceous. By the aid of detailed bathymetric maps, the main canyons can be correlated with onshore faults and drainage systems (fjords and valleys). The evolution of the slope canyon system over time is controlled by both tectonic and isostatic movements and, as discussed in the text, can help understand when and where the pre-fjord drainage was established. Multiple incision events have been detected, and each of these express some correlation to regional tectonic events in (1) Late Jurassic–Earliest Cretaceous, (2) Late Aptian–Albian and (3) Turonian– Coniacian.publishedVersio

Structure and evolution of the Bellsund Graben between Forlandsundet and Bellsund (Spitsbergen) based on marine seismic data

Seismic interpretation of multi-channel seismic data acquired along the western shelf of Spitsbergen allowed identification of the main geological features of the area, including the Hornsund Fault Zone, and the Forlandsundet and Bellsund grabens. The Bellsund Graben is defined as a narrow, N-S trending graben structure which is approximately 20 km wide and 70 km in length. The graben represents a southern continuation of the Forlandsundet Graben in the north and in the south, it is limited by E-W trending dextral transverse faults external to Isfjorden and Van Mijenfjorden. Development of the graben structures was related to the formation of the West Spitsbergen Fold and Thrust Belt and opening of the Norwegian- Greenland Sea. Compressional structures observed within sedimentary strata infilling the graben may support the view that three stages can be discerned with respect to the evolution of these structures: 1) An initial stage of sediment accumulation in basins broader than those at present (probably latest Paleocene – Early Eocene); 2) graben formation, possibly as a pull-apart structures during a dextral strike-slip regime with local compression (latest Eocene); and 3) normal faulting and final graben development since the onset of sea-floor spreading between Svalbard and Greenland in early Oligocene. The lowermost reflector that underlies the Bellsund Graben has been interpreted as a detachment surface formed during the Late Eocene-Oligocene(?) extension as reactivation of a thrust plane developed during formation of the West Spitsbergen Fold and Thrust Belt.publishedVersio

How post-salt sediment flux and progradation rate influence salt tectonics on rifted margins: Insights from geodynamic modelling

Continental rifted margins can be associated with widespread and thick salt deposits, which are often formed during the final stages of rifting, prior to breakup. These salt-bearing margins are typically characterized by pronounced post-rift salt tectonics with variable and complex structural styles and evolution. We use a lithosphere-scale geodynamic numerical model to investigate the role of varying post-rift sediment fluxes and progradation rates on rifted margin salt tectonics. We focus on a single, intermediate, rifted margin type and salt basin geometry to explore scenarios with different: (i) constant and (ii) time-varying post-salt sediment fluxes. We demonstrate that these promote significant contrasts in the style and magnitude of salt tectonics in the proximal, transitional and distal margin domains. The differences are primarily controlled by the relationship between the rates of sediment progradation (Vprog) and salt flow (Vs). When Vprog > Vs, the salt is rapidly buried and both vertical and lateral salt flow are suppressed across the entire margin. When Vprog < Vs, the salt flows vertically and seaward faster than sediments prograde producing major diapirism in the proximal domain and major distal nappe advance, but only moderate overburden extension and distal diapirism. When Vprog ~ Vs, there is moderate proximal diapirism and distal nappe advance, but major updip extension and downdip shortening, which produces major distal diapirism. Modelling results are comparable to various natural systems and help improve our understanding of the controls and dynamics of salt tectonics along salt-bearing rifted margins.publishedVersio

Crustal domains in the Western Barents Sea

The crustal architecture of the Barents Sea is still enigmatic due to complex evolution during the Timanian and Caledonian orogeny events, further complicated by several rifting episodes. In this study we present the new results on the crustal structure of the Caledonian–Timanian transition zone in the western Barents. We extend the work of Aarseth et al. (2017), by utilizing the seismic tomography approach to model Vp, Vs and Vp/Vs ratio, combined with the reprocessed seismic reflection line, and further complemented with gravity modelling. Based on our models we document in 3-D the position of the Caledonian nappes in the western Barents Sea. We find that the Caledonian domain is characterized by high crustal reflectivity, caused by strong deformation and/or emplacement of mafic intrusions within the crystalline crust. The Timanian domain shows semi-transparent crust with little internal reflectivity, suggesting less deformation. We find, that the eastern branch of the earlier proposed Caledonian suture, cannot be associated with the Caledonian event, but can rather be a relict from the Timanian terrane assemblance, marking one of the crustal microblocks. This crustal block may have an E–W striking southern boundary, along which the Caledonian nappes were offset. A high-velocity/density crustal body, adjacent to the Caledonian–Timanian contact zone, is interpreted as a zone of metamorphosed rocks based on the comparison with global compilations. The orientation of this body correlates with regional gravity maxima zone. Two scenarios for the origin of the body are proposed: mafic emplacement during the Timanian assembly, or massive mafic intrusions associated with the Devonian extension.publishedVersio

Late-syn- to post-rift salt tectonics on wide rifted margins—Insights from geodynamic modeling

Rifted margins are often associated with widespread and thick evaporite (salt) deposits and pronounced salt tectonics. The largest salt basins formed during the latest stages of rifting, immediately prior to continental breakup. Salt tectonics along these rifted margins commonly exhibit structural domains characterized by gravity-driven updip extension, translation, downdip shortening, and salt nappe advance. The precise spatial and temporal links between these structural processes, their relative contributions and dynamics are still a topic of debate on many margins. We use 2D thermo-mechanically coupled finite-element modeling of lithospheric extension to investigate the evolution of salt tectonics along wide rifted margins and the interplay between rifting and post-rift deformation. The models integrate lithospheric extension with post-rift salt tectonics using a geodynamically self-consistent approach where the geometries of the lithosphere and salt basins are not prescribed. They confirm that wide salt-bearing rifted margins are characterized by gravity-driven updip extension and downdip shortening, but also that syn-depositional salt flow and salt stretching occurs in their distal portions prior to and during continental breakup. This produces widening of the basin and emplacement of a salt nappe over newly formed oceanic crust. Post-rift updip extension is mostly balanced by downdip diapir shortening, all related to Couette flow. The salt nappe initiated by late syn-rift stretching advances further by post-rift pressure-driven Poiseuille salt flow so that its final width is a product of both processes. The results can be directly compared to examples from various salt-bearing rifted margins and improve our understanding of their enigmatic genesis and evolution.publishedVersio

Coupling Crustal-Scale Rift Architecture With Passive Margin Salt Tectonics: A Geodynamic Modeling Approach

Continental rifted margins are often associated with widespread, thick evaporite (i.e., salt) deposits and pronounced salt tectonics. The majority of salt basins formed during the latest stages of rifting, prior to continental breakup. We use 2D thermo-mechanical finite element modeling of lithospheric extension to investigate the interplay between rifted margin architecture, late syn-rift salt deposition, and post-rift salt tectonics. We focus on four different types of continental margins: (a) narrow, (b) intermediate, (c) wide, and (d) ultra-wide margins. We evaluate the: (a) interplay between laterally variable syn-rift extension, salt deposition and salt tectonics, (b) influence of syn-rift basin architecture on post-rift salt flow, (c) spatial and temporal distribution of salt-related structural domains, and (d) contrasting styles of salt tectonics for different margin types. Narrow and intermediate margins form partially isolated salt basins associated with prominent base-salt relief, limited translation but significant diapirism, and minibasin development. Wide and ultra-wide margins form wide salt basins with subtle base-salt relief that results in significant seaward salt expulsion and overburden translation. These wide margins demonstrate significant updip extension with the development of post-rift normal faults and rollovers, mid-margin translation associated with complex diapirism and downdip diapir shortening. All margins contain a distal salt nappe that varies in width and complexity. We also test the effect of different salt viscosities, relative post-salt progradation rates, and pre-salt sediment thicknesses. The results are comparable to several examples of salt-bearing rifted margins and improve our understanding of their dynamics and on the controls on their salt tectonics variability.publishedVersio

From Caledonian collapse to North Sea Rift: The extended history of a metamorphic core complex

Extensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductile‐to‐brittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3‐D seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometer‐scale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a >100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift. Around 61°N, only minor N‐S faults (<100 m throw) cut through the core of the MCC. Major rift faults (≤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.publishedVersio

Paleobathymetric reconstructions of the SW Barents Seaway and their implications for Atlantic–Arctic ocean circulation

Unravelling past, large-scale ocean circulation patterns is crucial for deciphering the longterm global paleoclimate. Here we apply numerical modelling to reconstruct the detailed paleobathymetry-topography of the southwestern inlet of the Barents Seaway that presently connects the Atlantic and Arctic oceans. Subaerial topography was likely enough to block Atlantic Water from entering the Barents Seaway in the earliest Eocene (c. 55 Ma). The water may have entered in the middle Eocene (c. 47 Ma) as observed from major basin subsidence, but paleotopographic highs to the east may have hindered connections between the two oceans. From the Oligocene (c. 33 Ma) until the onset of the Quaternary (c. 2.7 Ma), basin shallowing and regional shelf uplift blocked Atlantic Water from entering the Barents Seaway. Our results imply that the Fram Strait remained the sole gateway for Atlantic Water into the Arctic Ocean since its opening in the Miocene until the Quaternary

Underexplored continental shelf gateways: timing, mechanisms and role of SW Barents Sea Gateway, Norwegian Arctic

Abstract of presentation given at the OCEANIC GATEWAYS: MODERN AND ANCIENT ANALOGUES AND THEIR CONCEPTUAL AND ECONOMIC IMPLICATIONS Conference, organised by The Geological Society of London, London, 23-25 November 2022.Ocean gateways connecting ocean basins are crucial for water and heat circulation, which influence global temperature, climate evolution and sediment distribution. While deep-water gateways have been a major research focus by the community, very little attention has been drawn to shallower gateways located on the continental shelves, where such circulation also takes place. In this study, we investigate the evolution of a shallow gateway in SW Barents Sea that presently connects NE Atlantic and Arctic oceans. This gateway contributes to about half of the Atlantic–Arctic water exchange, whereas the other half is occurring through the deeper Fram Strait Gateway. When and how this SW Barents Sea Gateway formed are debated and still poorly understood. Outcomes from this study will thus be relevant for regional and global models of ocean circulation. Moreover, this study will contribute to climate evolution models over longer timescale in a climate sensitive region where an Arctic amplification of warming is presently seen