North Sea Rift Research Articles

Multi‐mode gravity tectonics during northern North Sea rifting: the Snorre fault block case

AbstractContinental rifts are characterized by up to 30 km wide rotated fault blocks with stratigraphic dip away from the central rift axis. Although gravity‐induced mass movements are well known features of collapsed fault block crests, I here demonstrate the occurrence of polymodal gravity‐driven mass transport down the back slope of a first‐order rift fault block. I identify (1) early sliding related to syntectonic crestal collapse of second‐order rift faults, (2) large‐scale bed‐parallel sliding of the L‐M Jurassic sedimentary package, and (3) the accumulation of two 7 km long, 1–2 km wide and up to 750 m thick volumes of complexly slumped material in the hanging walls of two ramp‐forming faults. Early sliding is documented by 100 m of repeated Brent Group stratigraphy in a cored well in the study area (well 34/4‐15A). These smaller slides have intact internal stratigraphy but show elevated deformation band densities. The seismic data also show evidence for ca. 2 km of massive translational sliding of the ca. 400 m thick and ca. 300 km2 large Jurassic section above a lowermost Jurassic bedding‐parallel detachment. This translational slide did not deform much internally, except for ductile folding where it slid over underlying active rift faults. Chaotic seismic facies in fault hanging walls are interpreted as contorted Jurassic beds, formed by multiple slumping and sliding events that stacked mobilized sediments into a 750 m thick column. These complex slump volumes occur where fault displacement is highest along two relayed faults. A model is favoured where the large translational slide ruptured with an opening of space against the fault that was progressively filled with slumped material from the footwall. While the large‐scale translational sliding only caused moderate internal subseismic deformation, early sliding and, particularly, the complex slumping caused significant internal deformation. This study shows the importance of carefully searching for and distinguishing between different types of mass movement in rift systems.

Facies models for rocky shorelines and their application to transgressed basement highs in the North Sea

Rocky shorelines form where basement highs are eroded and flooded during marine transgressive events. Although the Mesozoic North Sea rift generated numerous platform margins and rotated fault blocks that acted as basement highs, rocky shoreline deposits have not previously been reported in this region. Rocky shoreline deposits are usually represented in the rock record by thin conglomerates overlying major unconformities. They are typically characterized by their ichnological aspects rather than by their depositional facies. This study used the sedimentological aspects of modern and Miocene rocky shorelines from Spain and Austria to create facies models, which were then applied to the recognition of rocky shorelines in the Mesozoic of the Central North Sea. Our results demonstrate that structureless, clast-supported, poorly to moderately sorted conglomerate–breccia deposits are associated with competent basement lithologies, which produce hard, resistant coastal cliffs around previously overlooked volcanic centres in the subsurface of the North Sea. The basement lithologies in most of the Central North Sea favoured the formation of softer coastal cliffs, with less resistant lithologies that did not generate or preserve gravel-sized particles. These softer coastal cliffs were mostly characterized by low-angle, unconformity-bounded sandstones and fine-grained deposits, precluding the preservation and recognition of Mesozoic rocky shores in much of the North Sea's stratigraphic record.

The North Sea rift basement records extensional collapse of the Caledonian orogen

Our knowledge of crystalline basement rocks flooring continental rift basins, rifted margin deposits, and foreland basins is poor, yet important for understanding basin evolution, crustal heat production, petroleum maturation, and for offshore mineral exploration. Using the northern North Sea rift as an example, we demonstrate how modern broadband seismic data can be utilized to unravel basement lithology and structure at a previously unprecedented level of detail. In our case study, we have discovered a pre-rift Caledonian basement with a folded structural pattern very similar to Devonian collapse-related structures onshore Western Norway. The data indicate that the collapse-generated Caledonian infrastructure extends >100 km into the northern North Sea, defining a wide zone of Caledonian crust that turned hot and mobilized vertically by upright folding and detachment faulting during the post-collisional stage. This extensive post-orogenic thermally induced collapse makes the south Scandinavian Caledonides unique among Phanerozoic orogens.

Topological Characterization of a Fault Network Along the Northern North Sea Rift Margin

AbstractThe factors that control the spatial variation of the topological characteristics of normal fault networks at the rift‐scale are poorly understood. Here, we use 3D seismic reflection data from the northern North Sea to investigate the spatial variation of the geometry, topology, and strain heterogeneity of the Late Jurassic normal fault network along the rift margin. Our results show that fault orientation varies spatially along the rift margin. Normal faults within fault blocks that are adjacent to the North Viking Graben exhibits dominant N‐S and NE‐SW strikes that are sub‐parallel to the graben axis and associated step‐over, whereas in fault blocks farther from the graben, there is a dominant NW‐SE strike. Furthermore, we identify two broad topological domains within the fault network: (a) dominated by isolated nodes, partially connected branches, and low fault connectivity, and (b) dominated by abutting nodes, fully connected branches, and moderate to high fault connectivity. These topological domains correlate with previous sub‐division of the rift margin in the northern North Sea into platform and sub‐platform structural domains, respectively. There is also a positive correlation between the spatial variability of the fault orientations and intensity, with the fault network connectivity, highlighting the relationship between normal fault geometry and topology. We conclude that the across and along‐strike variation in strain, presence of pre‐existing structures, and accommodation zone‐related deformation are key factors influencing the spatial variation of fault network properties at the rift scale.

Quantitative characterisation of fracture connectivity from high-resolution borehole image logs

Fractures are fundamental structural elements in brittle rocks. They have a primary role in transporting geofluids, particularly in basement and carbonate rocks, where they have been the focus of reservoir characterisation and modelling for decades. Fracture networks are often described by statistical analysis of their geometrical attributes such as orientation, length and aperture. However, the spatial arrangement (topology) of fractures/fracture sets and its temporal evolution is equally important to better understand physical attributes of rocks. How much is a fracture network connected? Can this be quantified? Does connectivity change in different locations, if yes what is the reason for it? What is the brittle strain distribution in a fracture network? How much are fracture density and connectivity influenced by mechanical stratigraphy?This paper attempts to address these relevant questions and test quantitative fracture connectivity analysis on high resolution electrical and ultra-sonic borehole image logs in an appraisal well (Total depth, TD: 2195.36 mMD), drilled into the Utsira High basement, in the extensional Norwegian North Sea rift basin. Fracture connectivity and fracture size attributes were investigated in Permian Zechstein carbonates and Ordovician-Silurian granitic basement and separately for defined fracture zones at the interval of 2050–2195 mMD. The average degree of fracture/fault connection is good (D = 2.6) for the sediments and excellent (D = 3.5) for the basement. Accordingly, the block intensity (R22) is 0.4 for sediments and 0.2 for basement, which imply a moderately and strongly disintegrated host rock, respectively. There is a large fracture connectivity variation in different fracture zones, i.e. completely isolated fractures (Zone 1) vs highly interconnected and strongly disintegrated basement (Zones 9–10). The connectivity variation is governed by lithology, mechanical properties (stiffness), strain distribution and presence of weakness zones (faults). The study also highlights some possible applications of fracture connectivity results to reservoir modelling and future follow-up trends in quantitative spatial characterisation of fracture networks.

Structural Observations of the Northern North Sea: Insights into Rift Failure Dynamics

Lithospheric extension leads to rift formation and may continue to the point of breakup, with oceanic ridge initiation and the formation of two conjugate rifted margins. In some settings, extension can cease, and the rift may be abandoned. These so-called failed rifts archive snapshots of early phases of deformation, with geometries that may help better constrain the parameters that can prevent a rift from reaching breakup, such as lithospheric rheology, thermal state, rift opening direction and rate, inheritance. This contribution summarizes a study of the Norwegian Continental Shelf which includes the North Sea Rift and the Møre and Vøring rifted margins. We proceeded to the interpretation of a new dataset of deep penetrating seismic reflection profiles and worked at the regional scale, deliberately ignoring local particularities, to focus on the large-scale structural picture. The aim is to list architectural similarities and differences between the failed rift and the successful rifted margins. Our mapping shows that the North Sea structural geometries and basement seismic facies are very similar to the observations listed for the adjacent Møre and Vøring rifted margins. Various types of tectonic structures are observed, from thick anastomosing shear zones possibly evolving into core-complex geometries, to composite large-scale detachment faults and standard high-angle normal faults. These are categorized into five classes and interpreted as exemplifying the rift tectonic evolution through distinct generations of deformation structures that can activate, de-activate and re-activate. Based on these observations, rift failure dynamics are discussed, and it is proposed that the North Sea rift abandonment may not be related to pre-rift local conditions but rather to the ability to initiate specific tectonic structures such distal breakaway complexes.

Geological controls on petroleum plays and future opportunities in the North Sea Rift Super Basin

Geological controls on petroleum plays and future opportunities in the North Sea Rift Super Basin

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

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

Thick- and thin-skinned basin inversion in the Danish Central Graben, North Sea – the role of deep evaporites and basement kinematics

Abstract. Using borehole-constrained 3D reflection seismic data, we analyse the importance of sub-salt, salt, and supra-salt deformation in controlling the geometries and the kinematics of inverted structures in the Danish Central Graben. The Danish Central Graben is part of the failed Late Jurassic North Sea rift. Later tectonic shortening caused mild basin inversion during the Late Cretaceous and Paleogene. Where mobile Zechstein evaporites are present, they have played a significant role in the structural evolution of the Danish Central Graben since the Triassic. Within the study area, Jurassic rifting generated two major W- to SW-dipping basement faults (the Coffee Soil Fault and the Gorm–Tyra Fault) with several kilometres of normal offset and associated block rotation. The Coffee Soil Fault system delineates the eastern boundary of the rift basins, and within its hanging wall a broad zone is characterized by late Mesozoic to early Paleogene shortening and relative uplift. Buttressed growth folds in the immediate hanging wall of the Coffee Soil Fault indicate thick-skinned inversion, i.e. coupled deformation between the basement and cover units. The western boundary of the inverted zone follows the westward pinch-out of the Zechstein salt. Here, thin-skinned folds and faults sole out into Zechstein units dipping into the half-graben. The most pronounced inversion structures occur directly above and in prolongation of salt anticlines and rollers that localized shortening in the cover above. With no physical links to underlying basement faults (if present), we balance thin-skinned shortening to the sub-salt basement via a triangle zone concept. This implies that thin Zechstein units on the dipping half-graben floor formed thrust detachments during inversion while basement shortening was mainly accommodated by reactivation of the major rift faults further east. Disseminated deformation (i.e. “ductile” at seismic scales) accounts for thin-skinned shortening of the cover units where such a detachment did not develop. The observed structural styles are discussed in relation to those found in other inverted basins in the North Sea Basin and to those produced from physical model experiments. Our results indicate that Zechstein units imposed a strong control on structural styles and kinematics not only during rift-related extension but also during basin inversion in large parts of the Danish Central Graben. Reactivated thin-skinned faults soling out into thin Triassic evaporite units within the carapace above Zechstein salt structures illustrate that even thin evaporite units may contribute to defining structures during tectonic extension and shortening. We thus provide an updated and dedicated case study of post-rift basin inversion, which takes into account the mechanical heterogeneity of sub-salt basement, salt, and supra-salt cover, including multiple evaporite units of which the Zechstein is the most important.

Normal fault reactivation during multiphase extension: Analogue models and application to the Turkana depression, East Africa

We present crustal scale physical analogue models of multiphase rifting to provide new constraints on the role exerted by faults formed during early extension on structures developed during later episodes of rifting. Our laboratory experiments (sand mixture is used to simulate the upper crust and PDMS-Corundum mixture is used to simulate the lower crust) are inspired by the Turkana depression in the East African Rift System, a natural case of rift zone developed through multiple extensional phases. The models reproduce a first rifting phase with NE-SW extension direction followed by a phase of W-E trending extension. In the experiments, we varied the amount of extension during the initial phase, which caused variations in the development and prominence of sets NW-SE normal faults, representing pre-existing faults to be potentially reactivated during the later rifting phase. Our model results indicate that the amount of extension in the early phase is critical in controlling the reactivation: the greater the amount of extension in the 1st-phase, the more important early faults are, and the easier these 1st-phase normal faults are reactivated during the 2nd-phase. At a local scale, fault reactivation may result in faults with a typical zigzag pattern, whose importance increases with the increase in the amount of extension during the early phase. Extrapolation of model results to the Turkana depression suggests that large-scale fault reactivation is unlikely to have occurred in the region, possibly implying that the Cretaceous-Early Paleogene extension was limited in the central part of Turkana depression. Results have been also extrapolated to other regions interested by multiphase rifting, such as basins in the North Sea rift.

Syn‐rift sediment gravity flow deposition on a Late Jurassic fault‐terraced slope, northern North Sea

AbstractStructurally controlled bathymetry in rifts has a significant influence on sediment routing pathways and depositional architecture of sediment gravity flow deposits. In contrast to rift segments characterized by crustal‐scale half‐grabens, the tectono‐stratigraphic evolution of deep‐water rift domains characterised by distributed faulting on narrow fault terraces has received little attention. We use 3D broadband seismic data, calibrated by boreholes, from the Lomre and Uer terraces in the northern North Sea rift to investigate Late Jurassic syn‐rift sediment gravity flow systems on fault‐terraced slopes. The sediment gravity flow fairways were sourced from hinterland drainages via basin margin deltaic systems on the Horda Platform to the southeast. The deep‐water sedimentary systems evolve from initial, widespread submarine channelized lobe complexes, through submarine channels, to incised submarine canyons. This progressive confinement of the sediment gravity flow system was concomitant with progressive localization of strain onto the main terrace‐bounding faults. Although the normal fault network on the terraces has local impact on deep‐water sediment transport and the architecture of gravity flow deposits, it is the regional basin margin to rift axis gradient that dominantly controls deep‐water sediment routing. Furthermore, the gravity flow deposits on the Lomre and Uer terraces were predominantly sourced by rift margin deltaic systems, not from erosion of local uplifted footwall crests, emphasising the significance of hinterland catchments in the development of volumetrically significant deep‐water syn‐rift depositional systems.

From widespread faulting to localised rifting: Evidence from K‐Ar fault gouge dates from the Norwegian North Sea rift shoulder

AbstractAlthough seismic and stratigraphic well information put tight constraints on rift basin evolution, eroded rift shoulders commonly expose polydeformed prerift basement whose deformation history may be difficult to constrain. In this work, we apply K‐Ar dating of fault gouge samples from 18 faults to explore the brittle deformation of the well‐exposed eastern rift margin to the northern North Sea rift. We find evidence of clay gouge formation since the Late Devonian, with distinct Permian and Jurassic fault activity peaks that closely match early stages of the two well‐established North Sea rift phases. A marked decay in fault density away from the rift margin confirms a close relationship between rifting and onshore faulting. The results show that initial rift‐related extension affected a much wider area than the resulting offshore rift. Hence our data support a rift model where strain is initially distributed over a several 100 km wide region, as a prelude to the development of the ~150–200 km wide Permo‐Triassic northern North Sea rift as defined by large marginal faults. Towards the end of the second rift phase, strain localises even more strongly to the 25–50 km wide Viking Graben. Interestingly, a period of early widespread extension is seen for both phases of North Sea rifting and may be a general characteristic of continental rifting. The documented prerift faulting and fracturing of the basement since the Devonian weakened the basement and probably facilitated the widespread initial extension that subsequently localised to form the northern North Sea rift, with further localisation to its relatively narrow central part (Viking Graben).

Strain migration during multiphase extension, Stord Basin, northern North Sea rift

AbstractIn regions experiencing multiple phases of extension, rift‐related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre‐existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time‐structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β‐factor). Our results show that the locations of basin‐bounding fault systems were controlled by pre‐existing crustal‐scale shear zones. Within the basin, Permo‐Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E‐W extension direction. Rift faults control the locus of syn‐RP1 deposition, whilst during the inter‐rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β‐factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic‐Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.

From Caledonian Collapse to North Sea Rift: The Extended History of a Metamorphic Core Complex

AbstractExtensional 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.

Exploration play statistics in the central–northern North Sea region of UK–Norway–Denmark

Abstract The central–northern North Sea region represents one of the richest and most diverse petroleum basins in the world. This paper presents an analysis of the historical results of exploration in the Central Graben, Outer Moray Firth and Viking Graben, plus surrounding regions, including West of Shetlands. The richness and maturity of the main late Jurassic source rock accounts for almost 100 billion barrels of oil equivalent (boe) recoverable resource in the three North Sea rift arms in reservoirs as old as Devonian and as young as Quaternary. Within a contiguous area of 100 000 km 2 , 29 different proven part plays (area-confined, deposition-specific stratigraphic intervals or play segments) have been analysed in detail. These range in age from Triassic to Eocene, representing 717 discoveries with a total of 78.1 billion boe recoverable resource present in primary reservoirs. The most prolific plays in terms of total resource found are Mid Jurassic paralic sandstones (in the Viking Graben), Upper Cretaceous chalk (in the Central Graben) and Paleocene turbidite sandstones (in all three rift arms). The average discovery rate (historical chance of geological success) across all the plays is 44% with a relatively low variance. No significant improvement is seen in success rate over the last 50 years, despite increased well density, knowledge and huge advances in geophysical data. Of the discoveries, 404 are commercial fields representing 57% of all discoveries but the rate has decreased to 33% over the last 10 years, as average discovery sizes have decreased significantly. Discovery size is highly variable (10 to more than 600 MMboe) but the historical average is 114 MMboe, a world-class value. Dividing discovery recoverable resource size by footprint area produces a useful measure of reservoir yield, MMboe/km 2 , and the median value for the different part plays varies from 1.3 MMboe/km 2 for an Eocene turbidite to 5.3 MMboe/km 2 for a Jurassic turbidite. From a global perspective, the North Sea is not only unusual in the volume of petroleum, diversity of reservoirs and size of discoveries but also for its number of overlapping part plays and for the fact that the largest volume of total resource is found in reservoirs older than the source rock, syn-rift sandstones having been charged from an end-of-rift marine shale.

Palaeogeographical evolution of the Rattray Volcanic Province, Central North Sea

The Rattray Volcanics Member, at the triple junction of the North Sea rift, is here subdivided into two informal sub-members based on analysis of core, wireline and seismic data. The Lower and Upper Rattray Volcanics were emplaced during two distinct phases of volcanism separated by a sustained volcanic hiatus. The presence of hyaloclastite and abundant freshwater algae at the base of the Lower Rattray indicates that large lakes were present in the area prior to the volcanism, possibly indicating that collapse of the regional Jurassic Central North Sea dome began prior to volcanism. Pulsed subsidence probably occurred through the duration of the volcanism with lacustrine conditions becoming re-established during the mid-volcanic hiatus. Sediments were deposited across the Rattray Volcanic Province in fluvial systems and floodplain coal swamps after the final cessation of volcanism, with later marine transgression leading to drowning of the area in the Callovian to Oxfordian. In terms of hydrocarbon prospectivity, no evidence is currently found to confirm the presence of an intra-basaltic play analogous to the Rosebank Field in the Faroe–Shetland Basin, although post-volcanic Pentland Formation sedimentary sequences in the Fisher Bank Basin area indicate the possibility of supra-basaltic prospectivity in the triple junction. Supplementary material: A table containing the complete palynological record is available at https://doi.org/10.6084/m9.figshare.c.4857195

Structural and stratigraphic evolution of the Mid North Sea High region of the UK Continental Shelf

Interpretation of newly acquired seismic and legacy well data has led to a greater understanding of the Upper Paleozoic–Recent geological evolution of the Mid North Sea High (MNSH), an under-explored region of the North Sea. The position of granite-cored blocks controlled the distribution of Devono-Carboniferous highs and basins before Variscan uplift led to peneplanation and the creation of the Base Permian Unconformity. The MNSH became the dominant feature during the Permian when it formed a west–east-striking ridge between the Southern and Northern Permian basins. Following a period of non-deposition, sedimentation was renewed in the Late Permian–Triassic before Middle Jurassic doming caused uplift to the NE. Subsequent Late Jurassic North Sea rifting transected the MNSH to create the Western Platform between the Central Graben and Moray Firth rift arms. Following Cretaceous post-rift deposition, the area experienced a significant easterly tilt in the Cenozoic that led to the demise of the MNSH as a prominent topographical feature. The tectonic and stratigraphic evolution exerts a strong control over reservoir facies distribution, source-rock deposition and maturation. However, the area is not barren of petroleum potential. Despite the lack of Upper Carboniferous source rocks over large areas, hydrocarbon potential is evident through shows in legacy wells, indicating the Lower Carboniferous as a potential source rock. Cenozoic uplift to the west imparted a regional tilt, the effects of which remains key to unlocking the area's prospectivity since it reconfigured structures and formed remigration pathways from Lower Carboniferous and Jurassic source rocks. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

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

AbstractThe northern North Sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement. The geometry and evolution of syn‐rift depocenters during this multiphase evolution and the mechanisms and extent to which they were influenced by preexisting structural heterogeneities remain elusive, particularly at the regional scale. Using an extensive database of borehole‐constrained 2D seismic reflection data, we examine how the physiography of the northern North Sea rift evolved throughout late Permian‐Early Triassic (RP1) and Late Jurassic‐Early Cretaceous (RP2) rift phases, and assess the influence of basement structures related to the Caledonian orogeny and subsequent Devonian extension. During RP1, the location of major depocenters, the Stord and East Shetland basins, was controlled by favorably oriented Devonian shear zones. RP2 shows a diminished influence from structural heterogeneities, activity localizes along the Viking‐Sogn graben system and the East Shetland Basin, with negligible activity in the Stord Basin and Horda Platform. The Utsira High and the Devonian Lomre Shear Zone form the eastern barrier to rift activity during RP2. Toward the end of RP2, rift activity migrated northward as extension related to opening of the proto‐North Atlantic becomes the dominant regional stress as rift activity in the northern North Sea decreases. Through documenting the evolving syn‐rift depocenters of the northern North Sea rift, we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics, controlling the segmentation of depocenters, as well as the locations, styles, and magnitude of fault activity and reactivation during subsequent events.

Structural architecture and composition of crystalline basement offshore west Norway

Numerous studies have investigated the geodynamic history and lithological composition of the Proterozoic basement, Caledonian nappes, and Devonian extensional basins and shear zones onshore west Norway. However, the offshore continuation of these structures, into the northern North Sea, where they are suspected to have influenced the structural evolution of the North Sea rift, is largely unknown. Existing interpretations of the offshore continuation of Caledonian and Devonian structures are based on simple map-view correlations between changes in offshore fault patterns and pronounced onshore structures, without providing evidence for the presence, nature, and geometry of offshore, basement-hosted structures. By integrating three-dimensional (3-D) seismic, borehole, and onshore geological and petrophysical data, as well as two-dimensional (2-D) forward modeling of gravity and magnetic data, we reveal the structural architecture and composition of the crystalline basement on the Maloy Slope, offshore west Norway. Based on 3-D mapping of intrabasement reflection patterns, we identified three basement units that can be correlated with the Caledonian thrust belt, and the major Devonian Nordfjord-Sogn detachment zone, located only 60 km to the east, onshore mainland Norway. Similar to that observed onshore, offshore crystalline basement of the Proterozoic basement (Western Gneiss Region) and allochthons is folded into large-scale antiforms and synforms. These units are separated by the strongly corrugated Nordfjord-Sogn detachment zone. Our analyses show that different types of crystalline basement can be distinguished by their seismic reflection character, and density and magnetic properties. We speculate that the main causes of the observed intrabasement reflectivity are lithological heterogeneities and strain-induced structures such as shear and fracture zones. Our interpretation of the architecture of crystalline basement offshore west Norway has important implications for the location of the suture zone between Baltica and Laurentia.

Influence of fault reactivation during multiphase rifting: The Oseberg area, northern North Sea rift

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.