Search

On Varanger Peninsula, the roughly NW–SE-trending Trollfjorden–Komagelva Fault Zone separates Neoproterozoic successions that accumulated in a shallow-marine platformal domain to the SW of the fault from deep-marine basinal to deltaic sediments to the NE. In Ediacaran time the fault scarp acted as a buttress in a period of basinal inversion during the top-to-the-SW, contractional Timanian orogeny. During the subsequent, top-to-the-SE to -ESE, polyphase Caledonian orogenesis the fault acted as a dextral strike-slip megafracture and lateral ramp. In the northeastern terrane, Timanian and Caledonian fold interference has produced several examples of double-folding and intersecting cleavages. In the southwestern terrane, the Lower Allochthon Gaissa Thrust Belt overlies the Parautochthon, below which a 45 km long deformation front has been mapped. Tectonic shortening within this frontal zone varies from top-to-the-SSE in the west to top-to-the-ESE in the east. Imbricate thrust sheets disrupt the succession in the northeastern terrane, below which a floor décollement is speculated to emerge as a frontal fault along a prominent footwall of the precursor fault that acted on the seabed in outermost Varangerfjorden. By use of potential field data, the Caledonian structures on Varanger can be followed to the north into the Barents Sea where the nappe pile in the pre-Carboniferous basement reaches a thickness of several kilometres. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

[The rheological properties, locations, orientations and interaction of the Timanian and Caledonian structures together with two subsequent extensional phases strongly influenced the presalt rift architecture and gave rise to seven subbasins in the Nordkapp Basin. The relative depth of each subbasin in contrast to the margin, cross‐cutting master faults, structural highs arrangement and depositional paleo‐environment controlled the thickness and facies of the synrift to early postrift layered evaporite sequence. , Abstract Potential field data, reprocessed regional 2D seismic reflection profiles and 3D seismic tied to wells were used to study the late Devonian‐Permian development of the Nordkapp Basin in Barents Sea. The composite basin can be subdivided into the northeastern, central and southwestern segments that developed above a basement that contains elements of the Timanian and Caledonian regimes. The transition between the Timanian and Caledonian structures is positioned below the central basin segment. The rheological properties, locations, orientations and interaction of inherited structures together with two subsequent extensional phases, defined the presalt rift architecture and gave rise to seven subbasins within Nordkapp Basin. During the late Devonian‐early Carboniferous NE‐SW oriented extension, the basin consisted of two regional half‐grabens (northern and southern) separated by an interbasin ridge. During the late Carboniferous, the shift of extension direction to NW‐SE orientation reshaped the two regional half‐grabens. In particular, an interbasin transfer zone divided the northern regional half‐graben by separating its hinged‐margin portion (incipient northeastern segment) from the deeper part (incipient central segment). At the same time, the elevated interbasin ridge separated the incipient central and southwestern segments. Internally within the seven subbasins, the evolving structural elements including cross‐cutting master faults and structural highs have influenced the deposition and character of the Pennsylvanian‐lower Permian layered evaporite sequence and the distribution of subsequent salt structures. We suggest that the synrift to early postrift processes, relative depth of each subbasin, arrangement of the structural highs and depositional palaeo‐environment all controlled the thickness and facies of the layered evaporite sequence. The latter in combination with the presalt architecture comprised a laterally varying in thickness and character substratum that during the earliest Triassic influenced the sediment routings and deposition of the progradational system arriving in Nordkapp Basin. The study outcomes are pertinent and applicable to other salt‐influenced rift basins worldwide.]

Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

The Caledonian Orogen preserves the record of a complete Wilson cycle from rifting to continent–continent collision and orogenic collapse. The Revsegg Allochthon, the uppermost tectonostratigraphic unit of the Hardangervidda–Ryfylke Nappe Complex of the southern Scandinavian Caledonides, is an understudied example illustrating the key temporal and tectonic stages in a Wilson cycle. It overlies 1600–1500 Ma gneisses of the Kvitenut Allochthon, which were deformed, metamorphosed and juxtaposed onto the Dyrskard Allochthon at 1000 Ma. The Revsegg Allochthon consists of leucosome-bearing mica schists with metasandstone, and amphibolite and granitoid lenses. The timing of sedimentation of the metasedimentary rocks is constrained to the period 780–495 Ma, but its association with a 495 Ma bimodal mafic and felsic intrusive suite suggests concurrent sedimentation in a Cambrian extensional setting. The Revsegg Allochthon underwent peak metamorphism at 480–470 Ma, followed by several metamorphic stages from 460 to 440 Ma, probably at an active margin outboard of Baltica, as postulated for the eclogite-bearing Jaeren Nappe to the SE. The Revsegg Allochthon was thrust onto the Kvitenut–Dyrskard duplex from 437 to 434 Ma during an early Scandian phase, which is also recognized in the Seve Nappe. Syn-deformational pegmatites, emplaced at 428 Ma, represent the final stage in the development of the nappe stack. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

The Listafjord–Drangedal Fault Complex is a central structure in the NE-SW-trending Agder–Telemark Lineament Zone that dominates the structural grain and topography of southernmost tip of Norway. T...

It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.We have studied the prolonged deformation history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by combining K–Ar and 40Ar–39Ar geochronology with structural analysis. The HØDZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of HØDZ, called the Ørje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. 40Ar–39Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the Ørje shear zone.This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10–25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. 40Ar–39Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K–Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite 40Ar–39Ar plateau age, consistent with field observations of the former reworking the latter.In sum, the HØDZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic Ørje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.

[ , Abstract An extensive, reprocessed two‐dimensional (2D) seismic data set was utilized together with available well data to study the Tiddlybanken Basin in the southeastern Norwegian Barents Sea, which is revealed to be an excellent example of base salt rift structures, evaporite accumulations and evolution of salt structures. Late Devonian–early Carboniferous NE‐SW regional extensional stress affected the study area and gave rise to three half‐grabens that are separated by a NW‐SE to NNW‐SSE trending horst and an affiliated interference transfer zone. The arcuate nature of the horst is believed to be the effect of pre‐existing Timanian basement grain, whereas the interference zone formed due to the combined effect of a Timanian (basement) lineament and the geometrical arrangement of the opposing master faults. The interference transfer zone acted as a physical barrier, controlling the facies distribution and sedimentary thickness of three‐layered evaporitic sequences (LES). During the late Triassic, the northwestern part of a salt wall was developed due to passive diapirism and its evolution was influenced by halite lithology between the three‐LES. The central and southeastern parts of the salt wall did not progress beyond the pedestal stage due to lack of halite in the deepest evaporitic sequence. During the Triassic–Jurassic transition, far‐field stresses from the Novaya Zemlya fold‐and‐thrust belt reactivated the pre‐salt Carboniferous rift structures. The reactivation led to the development of the Signalhorn Dome, rejuvenated the northwestern part of the salt wall and affected the sedimentation rates in the southeastern broad basin. The salt wall together with the Signalhorn Dome and the Carboniferous pre‐salt structures were again reactivated during post‐Early Cretaceous, in response to regional compressional stresses. During this main tectonic inversion phase, the northwestern and southeastern parts of the salt wall were rejuvenated; however, salt reactivation was minimized towards the interference transfer zone beneath the centre of the salt wall.]

[Abstract Reprocessed, regional, 2D seismic reflection profiles, 3D seismic volumes and well data (exploration and shallow boreholes), combined with 2D structural restorations and 1D backstripping were used to study the post‐salt evolution of the Nordkapp Basin in Barents Sea. The post‐salt evolution took place above a pre‐salt basement and basin configuration affected by multiple rift events that influenced the depositional facies and thickness of Pennsylvanian‐lower Permian‐layered evaporite sequence. Initially, regional mid‐late Permian extension reactivated pre‐salt Carboniferous faults, caused minor normal faulting in the Permian strata and triggered slight salt mobilization towards structural highs. The main phase of salt mobilization occurred during earliest Triassic when thick and rapidly prograding sediments entered from the east into the Nordkapp Basin. In the early‐mid‐Triassic, the change in the direction of progradation and sediment entry‐points shifted to the NW led to rotation of the earlier‐formed mini‐basins and shift of dominant salt evacuation direction to the south. The prograding sediment influx direction, sediment transport velocity and thickness influenced the dynamics of the early to late passive diapirism, salt expulsion and depletion along the strike of the basin. The basin topography resulting from salt highs and mini‐basin lows strongly affected the Triassic progradational fairways and determined the distinct sediment routing patterns. Minor rejuvenation of the salt structures and rotation of the mini‐basins took place at the Triassic–Jurassic transition, due to far‐field stresses caused by the evolving Novaya Zemlya fold‐and‐thrust belt to the east. This rejuvenation influenced the sediment dispersal routings and caused formation of dwarf secondary mini‐basins. The second and main rejuvenation phase took place during likely early‐mid‐Eocene when propagated far‐field stresses from the transpressional Eurekan/Spitsbergen orogeny to the NW inverted pre‐salt normal faults, reactivated the structural highs and rejuvenated the salt structures. The studied processes and study outcomes can be applicable to other evaporite‐dominated basins worldwide., Sequential structural restoration of an interpreted and depth‐converted section showing the supra‐salt evolution of the NENB segment in the Nordkapp Basin. The detailed temporal evolution shows the interplay between the base‐salt relief, laterally‐varying syn‐ to early post‐rift LES and prograding sediments. The models shows the salt flow direction (within and out of the plain or three‐dimensions), was influenced by the far‐field stresses during the Triassic‐Jurassic transition and early‐middle Eocene, and by the rotation of the mini‐basins. ]

Intrusive and extrusive mafic igneous rocks in the Embla oil field, central North Sea, testify to repeated post-Caledonian magmatism on the northern flank of the Mid North Sea High. The igneous rocks are highly clay- and carbonate-altered, but retain their High Field Strength element signatures on the sample scale. These signatures are used to group, classify and investigate the tectonic significance of the rocks. Three magmatic events are identified. Late Devonian transitional basalts are interpreted as part of a bimodal volcanic assemblage that includes ca. 375 Ma alkali rhyolites, suggested to record rifting in a proto-Central Graben. Early Permian volcanic and hypabyssal alkaline rocks in the Embla oil field display lamprophyric traits and represent low degree melts. They likely correlate either to ca. 300 Ma lamprophyre magmatism leading up to, or 298-292 Ma alkaline magmatism in the Midland Valley - Southern Uplands coeval with the northwest European magmatic flare-up at the Permo-Carboniferous boundary. Kimmeridgian-Tithonian submarine tholeiitic basalts in the Embla oil field post-date the alkaline continental mid Jurassic magmatism associated with doming and initial rifting in the North Sea, and are suggested to represent a hitherto unknown volcanic expression of the late Jurassic main rift phase in the Central Graben.

Mapping of the structural pattern by remote sensing methods (Landsat, SPOT, aerial photography, potential field data) and field study of selected structural elements shows that the cratonic basement of South Norway is strongly affected by a regular lineament pattern that encompasses fault swarms of different orientation, age, style, attitude and frequency. Albeit counting numerous fault and fracture populations, the faults are not evenly distributed and N-S to NNE-SSW/NNW-SSE and NE-SE/ENE-WSW-systems are spatially dominant. N-S to NNW-SSE structures can be traced underneath the Caledonian nappes to the Western Gneiss Region in western and central South Norway, emphasizing their ancient roots. Dyke swarms of different ages are found within most of these zones. Also, the Ostfold, Oslo-Trondheim and the Mandal-Molde lineament zones coincide with trends of Sveconorwegian post-collision granites. We conclude that the N-S-trend includes the most ancient structural elements, and that the trend can be traced back to the Proterozoic (Svecofennian and Sveconorwegian) orogenic events. Some of the faults may have been active in Neoproterozoic times as marginal faults of rift basins at the western margin of Baltica. Remnants of such fault activity have survived in the cores of many of the faults belonging to this system. The ancient systems of lineaments were passively overridden by the Caledonian fold-and-thrust system and remained mostly, but note entirely inactive throughout the Sub-Cambrian peneplanation and the Caledonian orogenic collapse in the Silurian-Devonian. The system was reactivated in extension from Carboniferous times, particularly in the Permian with the formation of the Oslo Rift and parts of it remain active to the Present, albeit by decreasing extension and fault activity.

Summary We utilize available seismic and well data, and focus on the distribution, orientation of deep-seated Carboniferous graben structures and their control on the domes in the southeastern Norwegian Barents Sea. We map prominent NW-SE trending that represents Timanian Orogeny trend, deep-seated Carboniferous graben structures separated by platforms and structural highs. Evaporites were accumulated in the deep basins, while carbonates occupied the highs. Several distinct domes at top Gipsdalen to Cretaceous levels are present within the study area, including the Haapet, Veslekari, and Signalhorn domes, and the central domal feature of the Fedynsky High. The spatial analysis of graben structures, evaporite accumulation, and domes reveal that the deep-seated Carboniferous structures strongly control the distribution and partially the evolution of the domes along with facies variations. Effect of salt mobilization on the dome evolution is dependent on the lithological variations (mobile and immobile evaporites and carbonates) and thickness. Lateral thickness variations of the uppermost Triassic to the lowermost Cretaceous sediments, the Cretaceous onlaps to the Jurassic successions and erosion of the residual Cretaceous strata, all suggest a multiphase evolution of the domes. We propose Paleogene timing for the main phase of reactivation of the domes, probably in response to regional compressional stresses.

Tectonic inversion of rift basins is most commonly reported in the literature to occur after rifting has ceased. In contrast, we present evidence for synrift, localized tectonic inversion from the Loppa High area, southwestern Barents Sea and present a model for the formation of inversion structures as a result of differential uplift. The structures are of early Barremian to mid-Albian age (c. 131 – 105 Ma) and are focused in or near pre-existing extensional boundary faults along the margins of the Loppa High. Inversion is interpreted to be the result of uplift of the high along its inclined boundary faults, leading to space accommodation problems as uplift was not properly compensated by extension in the region. The model constrains the initiation of uplift of the Loppa High to the early Barremian and shows that the asymmetric margin configuration of the high may have led to a bulk clockwise rotation of the high around a vertical axis during uplift. The cause of uplift is not fully understood, but is suggested to be linked to contemporaneous extreme lithospheric thinning in neighbouring basins to the west. Processes involved may include isostatic flexure, thermal heating, lithological phase changes and/or far-field stresses, although these aspects need to be further tested. The final version of this reseach has been published in the Journal of the Geological Society. © 2016 Geological Society

A series of analogue experiments utilizing sequences of sand with interlayered silicone polymer have been performed to investigate the effects of multistage extension on rock sequences of different strength, with particular reference to the Hoop Fault Complex of the Barents Sea. It was found that the width and style of the graben systems as seen in map view depend strongly on the extension velocity. Wide areas of graben formation are promoted by fast extension, whereas narrowly constrained deep-graben structures are typical of slow extension rates. Furthermore, the decoupling strata are likely to be characterized by flow rather than by distinct detachment faults. The scaled experiments produced units of contrasting fault frequencies and styles in individual sand layers positioned between layers of silicone polymer. It was also found that the fault segments developed from different levels were related to varying extents (hard-linked, soft-linked, firm-linked, unlinked). The fault configurations and the general fault pattern obtained in the experiments is similar to that observed in natural faults where salt or unconsolidated mudstone separate sequences of sand.

This study focuses on the tectonic evolution of the greater Egersund Basin in the Norwegian central North Sea, with special emphasis on Late Paleozoic extensional tectonics following Caledonian collapse and the Variscan Orogeny, and the impact of the basement structural grain on this evolution. The Caledonian collapse likely resulted in development/rejuvenation of the deepest E-W trending structures/depocenters by Late Devonian time. Thus, a late Devonian-?early Carboniferous age can be assigned to the initial extension, which was associated with the development of an E-W striking basin system, to be overprinted by N–S extensional structures of similar age. A phase of regional magmatism at the Carboniferous-Permian transition (≈300 Ma) may be associated with a large igneous province centred on the Skagerrak area. Faulting during late Carboniferous-early Permian was minor within the study area as reflected by uniform sedimentary thicknesses of the uppermost Carboniferous and lower Permian sequences. Major normal faults, mainly trending N–S, were active during a late Permian-Early Triassic rift phase affecting large parts of the central and northern North Sea area. A later phase of extension was initiated in late Middle Jurassic time and the Egersund Basin proper formed during the Late Jurassic-Early Cretaceous. The depocenters that developed during this phase was influenced by the deep Late Paleozoic (sub-salt) structural grain, including strike-slip movements along the Sorgenfrei-Tornquist Zone. Later events include mild inversion along the northern flank of the Egersund Basin, possibly as a Late Cretaceous response to far-field Alpine compression, and Cenozoic regional tilting.

High quality reprocessed seismic reflection profiles and available wells were used to study the little studied southeastern Norwegian Barents Sea and east Finnmark Platform. The study area comprises prominent structural elements such as the Haapet, Veslekari, and Signalhorn domes, the West Fedynsky High, and the Tiddlybanken and Nordkapp basins. Seven deep-seated Carboniferous grabens, not formally described earlier, were defined and informally named; and similarly, five evaporite bodies that are tapered stratigraphically above the grabens have been mapped in detail. In the late Devonian, the region comprised a central structural high (Fedynsky High), and two depressions to the north and south, and has subsequently experienced transtensional deformation during a late Devonian-early Carboniferous NE-SW regional extensional phase. As a result, a NW-SE trending graben system was created over the paleotopography, following the inherited Timanian orogeny lineaments and giving rise to the deep-seated Carboniferous grabens. Pennsylvanian to early Permian evaporites were deposited and were characterized by mobile and non-mobile lithologies. The Carboniferous structures controlled the volume, thickness and lithological alterations of the evaporites, and have later influenced the distribution and development of the salt wall and domes. The Haapet, Veslekari, composite West Fedynsky (two domes informally named Alpha and Beta) and Signalhorn domes were generated and the salt wall of the Tiddlybanken Basin was rejuvenated during the late Triassic due to compressional stresses propagating from the evolving Novaya Zemlya fold-and-thrust belt. The domes and salt wall were subsequently reactivated during the upper Jurassic and earliest Cretaceous. Furthermore, we infer that the main phase of reactivation of these structures took place during the early-middle Eocene due to far-field stresses from the transpressional Eurekan/Spitsbergen orogeny.

The Sub-Cambrian Peneplain in southern Norway formed in the Cryogenian–early Cambrian. It was transgressed in the Cambrian–early Ordovician, and subsequently broken up by Caledonian thrusting, post-Caledonian normal faulting and regional uplift. In southern Norway the Sub-Cambrian Peneplain is generally expressed as an angular unconformity or nonconformity surface with no apparent deep weathering. In some localities the basement is characterized by a weathered top. Furthermore, the basement may have a mineralized top in some cases. The peneplain is commonly overlain by in situ regolith, and/or Cryogenian glacial diamictite, Lower–Middle Cambrian siliciclastic sediments, Lower–Upper Cambrian black shale or Lower Ordovician limestone. It has commonly mildly tectonized contacts, but occasionally displays contacts that have been completely destroyed by Caledonian thrusting and reworked into fault rocks. Tracing the peneplain across southern Norway reveals a fault-affected and fault-delineated plateau with a sloping eastern margin. Onshore cumulative offsets record vertical movements of > 2000 m. A 1–5° dip below Caledonian nappes suggests a flexural component related to tectonic overburden. As the oldest identifiable and dateable surface covering much of Scandinavia the peneplain constitutes the reference surface for studies of the younger regional morphotectonic evolution. Its shape supports the interpretation that Cenozoic uplift combined with faulting, not smooth flexural doming, caused the Southern Scandes high in South Norway.

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 Loppa High area has been subject to several events of uplift and subsidence from the Late Paleozoic to Present. The driving mechanisms behind the vertical movements, however, are not fully understood. We propose that uplift and subsidence were influenced by the combination of density changes caused by metamorphic phase changes in a 90 × 140 km wide mafic lower crustal body below the Loppa High and local (rift-related) and far-field stress. Through a numerical modelling approach we analyse the tectonically induced variations in pressure and temperature in the lower crust, their influence on phase changes in the mafic body and the affiliated vertical movements. Results show that (1) densification of the mafic body caused by far-field compression associated with the late Triassic westward translation of Novaya Zemlya could cause surface subsidence, (2) heat and fluid influx provided by early Cretaceous rifting could trigger density reduction and surface uplift and (3) the present-day geometry of the Loppa High as observed in seismic data can be reproduced by combining the modelled effect of rift flank uplift and phase changes in the mafic body. Phase change-driven vertical movements may also have affected other structural highs in the western Barents Sea, including the Stappen High. The final version of this reseach has been published in the Journal of the Geological Society. © 2017 Geological Society

The Oligocene sedimentary succession in the eastern North Sea is revised and re-interpreted by applying new state-of-the-art reflection seismic data integrated with new bio- and Sr-stratigraphy data from three key wells in the study area. The Oligocene succession in the eastern North Sea is divided into four transgressive–regressive (T-R) sequences, characterized by non-accretional and/or aggradational transgressive systems tracts and prograding regressive systems tracts. Detailed studies of three wells, including biostratigraphy and Sr analysis, constrain the age relationships between the T-R sequences. Internal clinoform geometry indicates that the sediments were sourced from the present southern Norwegian mainland to the north of the depositional area. The direction of progradation shifted from being SE-directed in the earliest Rupelian (early Oligocene) to S- and SW-directed during Chattian time (late Oligocene). Rapid basin subsidence is indicated by the development of non-accretionary transgressive systems tracts, with subsequent progradation into water depths of hundreds of metres. The creation of accommodation space was out of phase relative to eustatic sea-level changes, and mainly controlled by regional-scale differential vertical movements where uplift and exposure of landmasses of the hinterland (southern Norway) occurred concurrently with basin subsidence. Halokinesis had an intra-basinal influence on the main sediment transport direction, but probably did not contribute much in creation of accommodation space.

The normalization of overthickened orogenic crust after a continent-continent collision typically involves several different processes, including erosion, plate divergence, gravity-driven collapse of the orogenic wedge, and viscous flow of the lower crust. As a contribution to this discussion, we here utilize reprocessed deep seismic data to image the Scandinavian part of the major Caledonian continent-continent collision zone where we identify a double set of extensional shear zones: one in the upper to middle crust (Hardangerfjord shear zone) and a major oppositely dipping Moho-offsetting shear zone in the lower crust and upper mantle. The latter shear zone may have a true displacement close to 50 km and appears to offset the Moho vertically by ~10 km, extending downward beyond the 50 km depth limit of the seismic data. While the upper structure offsets the basal Caledonian thrust, the Moho shear zone is more difficult to date. However, they are kinematically consistent and located at the transition zone between thick- and thin-skinned post-collisional tectonics, and we suggest that they may represent a crustal “pinch” or zipper-like structure that separated viscously flowing Caledonian lower crust from cooler rigid basement representative for the rest of the Baltic Shield. Later extension created the North Sea rift, but the general rift axis and the associated riftrelated thinning developed in a narrower zone that runs oblique to the Devonian trend, hence the two generations of extension and thinning can be distinguished at the crustal scale.

Fracture lineaments in crystalline and metamorphic rocks of southern Norway can be subdivided into joint swarms, fracture corridors and faults, depending on displacement, the fracture mode and patterns, and the presence of fault rocks. Their physical appearance as lineaments seen by remote sensing is not discernible, as they define km-long and narrow tabular zones of high fracture intensity. Intrinsically, fracture zonation becomes better expressed from joint swarms to fracture corridors and especially faults as a consequence of increasing accumulate strain. Joint swarms and fracture corridors commonly reveal a symmetric fracture zonation on both sides of its core, whereas inclined extensional faults tend to have asymmetric patterns with enhanced strain and a wider damage zone in the hanging wall. Fracture lineament can be mapped in subzones A–B (core), which are typically some cm up to some tens of meters wide. Common structural elements are fault rocks/shear zones, lenses, and a network of fractures often with very high fracture frequency. Secondary minerals are common. Outside this, subzones C–D (damage zone) are commonly 20–50-m\ wide with lower fracture intensity of lineament-parallel fracturing, defining the topographic boundary of the lineament. Mineralisation is rarer. The transitional subzone E of multi-orientation fractures defines the transition to the background fracture system. We propose a model for the classification and development of fracture lineaments, applying their architecture (intrinsic geometry, spatial fracture pattern and spatial distribution of fault rocks) as tools for the systematic description. This links fault growth processes and mechanisms that can be ascribed to strain hardening and softening scenarios in a model of fault architecture.

Two methods for digital mapping and analysis of topographic surfaces of moderate relief, such as erosional surfaces, are introduced and discussed in this paper. The surface fit method uses least squares to solve the problem of finding the best surface fit to the data, whereas the region grow method uses the surface slope and a set of properties (e.g. maximum slope between adjacent pixels) to generate surfaces from single seed GPS-positions. In both methods the analysed surface was identified from field controlled geological maps. The surface fit method is applicable to landscapes with great topographic varieties, whereas the region grow method is most applicable to relatively flat areas. Both methods were used to identify the sub-Cambrian peneplain (SCP). The result suggests that the SCP occurs across significant parts of the Hardangervidda area, with a greater lateral extent in the central part of the area compared to the northern part. This can be attributed to more extensive faulting of the SCP in the northern part.

The Triassic sedimentary succession in the Central North Sea has been investigated to establish a broader understanding of the Triassic Period, from the combined interpretation of seismic reflection data and well data. The Triassic succession has been subdivided into four seismic units, where unit boundaries are characterized by regional seismic amplitude anomalies, reflecting changes in gross sedimentary facies or rock properties. A successful correlation between sedimentary facies, interpreted within the well sections and distinct seismic reflection patterns, allowed a thorough mapping of the gross palaeoenvironment throughout the Triassic. The method presented of subdividing a continental sedimentary succession into seismic units should be applicable elsewhere in other basins. The main source area during the Triassic was Scandinavia to the north, and sediment transportation was mainly along north–south- and NE–SW-trending lineaments, which are at present located onshore southern Norway, and in the Asta Graben and the Varnes Graben offshore. An uplifted Skagerrak Graben area acted as source area in the Early and early Middle Triassic, with sediment dispersal to the south and SW. High relief existed for a longer period in western Scandinavia than in eastern Scandinavia, which supports an asymmetric shape of the Scandinavian mountains during the Triassic. Accommodation space in the Early Triassic was mainly controlled by the relief inherited from a Late Carboniferous–Permian rift phase. Although thermally induced regional subsidence continued in the Middle and Late Triassic, creation of local accommodation space was mainly limited to halokinesis, including redistribution and withdrawal of salt from the subsurface. The Upper Triassic succession is eroded across the western and central parts of the study area, although the Upper Triassic unit is preserved in synforms adjacent to salt structures. In the western part of the study area, dry, playa conditions prevailed during the Early Triassic, although fluvial systems supplied long-transported sandy detritus southeastwards in the late Early Triassic. More sandy detritus was transported into the sedimentary basin in the Middle and Late Triassic, concurrently with a gradually wetter climate. Supplementary material: Uninterpreted seismic sections are available at www.geolsoc.org.uk/SUP18726.

[1] Shear-enhanced compaction in shallow sandstone reservoirs has been investigated in laboratory experiments using high-pressure triaxial testing of poorly lithified sandstone and sand. We have studied the deformation mechanism involved during shear-enhanced compaction and controlling parameters for yield stress at varying confining pressures for sandstone/sand with different grain sizes, porosities, and packing. Experimental testing provides insights into the deformation mechanism during hydrostatic and axial compression of coarse- and fine-grained sands with different packing including (1) natural coarse-grained sandstone, (2) densely packed fine-grained sand, and (3) loosely packed fine-grained sand. Monitoring of deformation and ultrasonic velocity during deformation indicates porosity loss, compaction, and strain hardening for most of the samples. Visualization of deformation using pretest and posttest X-ray imaging and thin sections demonstrates localized deformation fabrics and grain damage. The results show grain rearrangement as the controlling deformation mechanism for material at low stress and high porosity, whereas for lower porosity and higher stress, grain fracturing controlled the deformation. The most pronounced localization of deformation was observed for the coarse-grained, low-porosity material. A Cam-Clay cap model was used to describe the porosity loss during compaction and shear-enhanced compaction, demonstrating large inelastic compaction with increasing confining pressure. Yield stress and end caps for poorly lithified sandstone are observed for various porosities and stress conditions and found to be lower than predicted using empirical relationships for sandstone.

A dense grid of 2D multichannel seismic data was used for the interpretation of sub-sea-floor structures in the area of Isfjorden in western Spitsbergen. West Spitsbergen underwent Eocene transpressional deformation that resulted in formation of the West Spitsbergen Fold-and-Thrust Belt. Three horizons were defined for the seismic interpretation as well-expressed and continuous reflections: (1) the top of the metamorphic basement; (2) the base of the upper Carboniferous Nordenskioldbreen Formation; (3) the base of the Lower Cretaceous Helvetiafjellet Formation. Time–structure maps and analysis of the sub-bottom structural trends were generated for each horizon. The top of the metamorphic basement displays north–south-trending graben structures, apparently representing continuation of the Devonian grabens from northern Spitsbergen. The tectonostratigraphic unit bounded by the base of the upper Carboniferous Nordenskioldbreen Formation and base of the Helvetiafjellet Formation encloses the fold-and-thrust belt and is affiliated with WSW–ENE shortening involving three decollement levels. Within this unit the strata between the middle (Triassic shales) and upper (Upper Jurassic shales) decollements have undergone the most intense strain, whereas sediments situated between the basal (lower Permian evaporites) and middle decollements underwent a relatively mild deformation. The strata above the base of the Helvetiafjellet Formation are characterized by minor Tertiary deformation only.

Magmatic and metamorphic events in two of the nappes of the Hardanger–Ryfylke Nappe Complex in the Caledonides in SW-Norway, and in the intervening thrust zone, have been investigated by means of ID-TIMS U–Pb zircon and titanite data. Orthogneiss protoliths in the upper Kvitenut Nappe are dated at 1615 ± 6 Ma, showing analogies to the Gothian terrane, including the Western Gneiss Region. By contrast, the Dyrskard Nappe is composed of metasedimentary rocks and metarhyolites with a 1508 ± 4 Ma extrusion age and shows an affinity to rocks of the Telemarkia terranes. We argue that the time of thrusting and juxtaposition of the two nappes along the shear zone is constrained by the age of 999 ± 5 Ma of a syndeformational granite body and co-genetic pegmatitic leucosomes, with late Sveconorwegian movements and fluid activity being recorded by titanite at 924 ± 6 Ma. Both nappes behaved as one block during the Silurian emplacement in the Caledonian nappe stack, sharing a 434 ± 1 Ma metamorphic peak and later overprints, as young as 414 ± 2 Ma, related to retrogression. The distinct origin and Sveconorwegian age of coupling of the Dyrskard and the Kvitenut nappes suggest that, in their pre-Caledonian location to the west-northwest, they represent the now hidden boundary zone between the Western Gneiss Region and Telemarkia.

Caprock quality is crucial when wanting to evaluate the storage potential of a reservoir. The objective of this study is to evaluate the quality of the primary and secondary seal in case of CO2 storage in sandstones form the Mid to Late Jurassic in the Norwegian North Sea. These seals are composed respectively of Lower Cretaceous and Upper Jurassic shales for the primary, and from the three Cenozoic groups (Nordland, Hordaland and Rogaland) for the secondary. The primary seal is in general of sufficient thickness according to the recommendations from the Norwegian Petroleum Directorate (over 50 meters), except in a thin North-South corridor to the West of the platform, in the inner part of the grabens. This seal is brittle in the entire study area due to a deep burial depth. The only area where it could be ductile is to the North of the Utsira High, which is due to a warmer geothermal gradient but also possibly the movement of a basement fault beneath. Since its rheology is brittle, the primary seal could be breached during fault movements. To ensure its efficiency, it is thus important to evaluate the properties of the faults from this area that have a direction less than 30° away from the regional maximum horizontal stress (ENE-WSW) The secondary seal is much shallower and therefore has a ductile rheology which makes it less affected by fault movements. This is due to the nature of ductile deformation which creates less conductive faults that have self- healing properties. The thickness is also at least 500 meters close to the shore, but most commonly between 1000 and 2000 meters. These properties make it a high quality secondary seal, as has been demonstrated by the successful Sleipner storage site in the Utsira sands that are sealed by Nordland shales.

Regional assessment studies for CO2 storage plays have been carried out in the Norwegian-Danish Basin of the Central North Sea and in the Skagerrak-Kattegat area. The development of the reservoir models is a part of an ongoing interdisciplinary project with the overall goal to establish a basis for large-scale handling of CO2 in this area, including regional CO2 source and capture possibilities, transportation and infrastructure, possible storage sites as well as legal aspects relating to the whole CCS chain. The study shows that all the necessary premises for a safe, long term CO2 storage, are present in the area. Two trap types for storage have been studied more closely: 1) large gently inclined, unfaulted reservoirs in the northern Skagerrak area and 2) dome structures with four-way closures above salt pillows in the Norwegian Danish Basin. We have closely focused on the Upper Triassic-lowermost Jurassic Gassum Formation and the Middle Jurassic Haldager Sand Formation. The current study presents reservoir characteristics of the sandstones of the Gassum and Haldager formations in the Fjerritslev Trough and on the Skagerrak-Kattegat Platform.

In the Embla oil field on the northern flank of the Mid North Sea High, the central North Sea, multiple quartz porphyric volcanic beds at ca. 4600 m depth form part of a volcano-sedimentary interval above the Caledonian basement as interpreted from seismic data. Zircon U–Pb laser ablation ICPMS date one bed to 374 ± 3 Ma, indicating that the volcanic rocks and interbedded sediments are early Famennian and correlate to the Buchan Formation. The volcanic rocks have been extensively clay and carbonate altered in a near-surface environment, but high field strength element data show that the protoliths were alkali rhyolites, yielding intra-plate signatures in tectonic discrimination diagrams. Famennian quartz porphyric volcanic rocks have also been reported from well A17-1 on the southern flank of the Mid North Sea High. The Famennian volcanism on the northern and southern flanks testify to an active magmatic environment in the central North Sea in the early Famennian, supporting the existence of a late Devonian proto-Central Graben rift extending northwards into the central North Sea. The rift is likely an early example of strain localisation to a zone of reduced crustal strength along the Caledonian suture between Avalonia and Baltica.

Magmatic zircon in the syenite (bostonite) part of a composite NE–SW-trending cogenetic bostonite–camptonite dyke in Orkney, Scotland, yields a laser ablation inductively coupled plasma mass spectrometry age of 313 ± 4 Ma and e Hf (313 Ma) values of +6 to +11. This suggests that the NE–SW-, east–west- and NW–SE-trending Scottish lamprophyre dyke swarms were emplaced during one late Carboniferous magmatic event, contrasting with published K–Ar dates ranging between c . 325 and 250 Ma. This magmatism is interpreted as a response to late Variscan regional extension or an early response to the Skagerrak mantle plume. Lamprophyre magmatism was initiated some 10 Ma earlier in Scotland than in the Oslo Rift. Supplementary material: Thin-section images of the investigated rocks, geochemical data and an REE plot, U–Pb and Hf isotopic data, and a Hf v. time diagram are available at www.geolsoc.org.uk/SUP18501.

The regional resultant stress field of the northeastern North Atlantic has shifted significantly throughout the Phanerozoic. In Fennoscandian parts of the Caledonian orogen, mountain building, which was characterized by NW-SE contraction (reference to present North), was followed by a collapse with transport both parallel and transverse to the mountain chain. The Late Palaeozoic – Mesozoic saw several stages of E-W to NW-SE extension, varying in time and position. Local episodes of inversion are traceable in some cases, particularly in connection with deep-seated and long-lived zones of weakness. The Cenozoic has to a larger degree been affected by compression, including folding and basin inversion. Again some of the more pronounced effects of local inversion are related to pre-existing fault systems. Neogene uplift of the western mountainous area in Scandinavia can be unravelled by potential field study, AFT data and reflection seismic sections. Assuming that the region is close to isostatic equilibrium, the uplifted areas must be supported at depth by substantial volumes of low-density material within the crust or the mantle, close to the crust/mantle interface or close to the lithosphere/asthenosphere interfaces. A series of NW-SE-oriented cross-sections, representing successive stages of development from the late Caledonian to Present were prepared and used to analyse the tectonomorphological development as related to the shifting stress configurations. Three stages are inferred: Stage 1 ; Devonian – late Permian denudation of the Caledonian mountain chain : During the latest stage of closing of the Iapetus Ocean in early to mid Devonian times, the Caledonian mountain chain reached its maximum elevation of perhaps as much as 8–9 km. Including the foreland basins to the east, the average topographic gradient of the eastern flank may have been in the order of 1,3°. The denudation of the Caledonian mountains continued into the Permian and by the end of the Permian, the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0,1°. Stage 2 ; Triassic – late Cretaceous; Tectono-thermal uplift of central south Norway . Uplift of central south Norway was rejuvenated in Triassic times and continued throughout the Jurassic and early Cretaceous. The gradient of the eastern flank increased to a maximum of approximately 0,15° and master structural elements like the Laerdal-Gjende-Olestol Fault Complex again played important roles in the development. Stage 3 ; Tertiary – Present; Renewed tectono-thermal activity and post-glacial rebound . A second phase of post-Caledonian tectono-thermal uplift of south central Norway started in Oligocene time, again increasing the gradient of the eastern flank of south Norway (around 0,2°).

Investigations were conducted in several regions of Norway with the purpose of detecting and measuring stress-relief features and to derive from them valuable information on the shallow-crustal stress state. Stress-relief features are induced by blasting and sudden rock unloading in road construction and quarrying operations, and are common in Norway and probably in other regions of Fennoscandia, as they are in NE America. Stress relief at the Earth9s surface is diagnostic of anomalously high stress levels at shallow depths in the crust and characterizes the formerly glaciated Baltic and Canadian Precambrian shields. The mean orientation of the maximum horizontal compressive stress axis, as deduced from the orientation of stress-relief features, was found to be NW–SE, consistent with North Atlantic ridge-push forces. We show that our determined stress orientations are in excellent agreement with other kinds of stress data. As a final step, we estimate ridge-push force magnitudes from gravity data in the North Atlantic region. Our computations suggest that strong ridge-push forces characterize the Baltic and Canadian shields. We conclude from this quantitative analysis and our fieldwork results that present-day stress relief, in Fennoscandia and NE America, is mostly triggered by plate-scale ridge-push forces and not by residual glacial loading stresses.

This work is a review of Boris Choubert’s paper (1935), which was published in French under the rather devalorizing title: “Research on the Genesis of Palaeozoic and Precambrian Belts.” Despite its innovative content, this article had no impact either at the time of its publication or even later. It begins with the construction of a remarkable fit of the circum-Atlantic continents. This was based on the −1.000 meters isobath instead of the shoreline. Thirty years before Bullard et al. (1965), it demonstrated in an indisputable way the reality of the continents motion on the surface of the Earth. Therefore, Choubert designated Wegener’s “continental drift” as the main cause of tectonics. Even going beyond Wegener’s theory, he argued that this mechanism was efficient well before the formation of the Triassic Pangæa, during the whole Palaeozoic to result in the building of the Caledonian and Hercynian mountains. Although he was still encumbered by the vocabulary of the time regarding geosynclines, Boris Choubert described tectonics based on the horizontal mobility of the Precambrian continental blocks. Oddly enough, he did not apply this model to the Precambrian structures, which he attributed to the effects of the Earth’s rotation on the continental crust during its solidification. At the time of its publication, this paper was a very important step towards understanding global tectonics. Unfortunately, Choubert’s contemporaries did not generally recognize its significance. Ce travail est une analyse de l’article de Boris Choubert (1935) paru en français sous le titre un peu réducteur: « Recherches sur la genèse des chaînes paléozoïques et antécambriennes. » Malgré son contenu novateur, cet article n’a eu aucun retentissement à l’époque de sa publication, ni même plus tard. Il débute par la construction d’un remarquable assemblage des continents péri-atlantiques fondé sur l’isobathe −1.000 mètres et non pas sur la ligne de rivage. Trente ans avant Bullard et al. (1965), il démontre de façon indiscutable la réalité du déplacement des masses continentales à la surface de la Terre. Dès lors, Choubert privilégie totalement la « dérive des continents » comme moteur de la tectonique. Allant au-delà même de la vision de Wegener, il fait jouer ce mécanisme bien avant la formation de la Pangée triasique, pendant toute la durée de l’ère primaire, au cours de l’érection des chaînes calédonienne et hercynienne. Bien que restant prisonnier du vocabulaire de l’époque sur les géosynclinaux, Boris Choubert décrit une tectonique fondée sur la mobilité des blocs continentaux précambriens. Curieusement, il n’applique pas ce modèle aux déformations précambriennes qu’il attribue aux effets de la rotation de la Terre sur une croûte continentale en voie de solidification. Lors de sa parution, cet article représentait un pas très important vers la compréhension de la tectonique globale et il est regrettable qu’il n’ait pas été mieux reçu par ses contemporains. ISSN:0037-9409 ISSN:1777-5817

The major Pisia and Doumena faults of the Corinth Rift, Greece, are both hosted by micritic carbonate rock. Their common fault elements are principal slip surfaces, layers of fault rocks, fault rock and host rock lenses, corrugations, and smaller shear and tension fractures. Along planar fault segments, a corrugated slip surface with an associated thin fault rock layer is formed. Well-developed down-dip corrugations indicate movement along the same surface over many fault increments. Bends/jogs in the faults are sites of complex structural relations, hosting frequent lenses, more porous fault rocks, and dense networks of fractures. Major jogs in the slip surfaces have both strike and dip-oriented undulating curvature that are associated with higher order fracture networks as well as frequent lenses. Fault core lens shapes of the Doumena fault show relationship of length vs. width of 3/2. Similarly, principal slip surfaces of the two faults show undulations with aspect ratios of amplitude vs. wavelength (1:19 and 1:14) in strike-direction, despite a prominent difference in down-dip geometry. The intrinsic deformation in fault cores of micritic carbonate supports a model in which fluids entered the core by damage zone fractures that are linked to the principal slip surface. Flow along the fault core was controlled by the fracture system of lenses, and thereby lens connectivity, but may have been hampered by flow-retarding fault rocks. The flow potential is especially well developed in fault bends and breached relays due to frequent lenses and well-developed fracture networks.

Geological storage of CO2 is a key technical solution to the climate-energy challenge, but it has a number of technological constraints (Baines and Worden 2004; Halland et al., 2011), broadly under the themes of assuring adequate storage capacity and long-term storage integrity. A suitable CO2-storage reservoir should consist of rock formations with sufficient porosity, permeability and connectivity in order to provide an adequate storage volume. The role of faults and their associated deformation structures (such as deformation bands and fractures) in controlling both storage capacity and long-term storage integrity is thus a key factor in achieving globally significant CO2 storage (Figure 1). Although some sedimentary basins on the Norwegian continental shelf already harbour operational CO2- injection and storage projects such as Sleipner (Zweigel et al., 2004) and Snohvit (Hansen et al., 2013), our understanding of reservoir fluid communication due to compartmentalization is far from complete and will be important for further use of the offshore basins for CO2 storage. In addition to the inherited structural features, elevated injection pressures may cause hydraulic fractures or stimulate fault reactivation which both point to the need to characterize the geomechanical response of the rock system to CO2 injection (Rutqvist, 2012; Iding and Ringrose, 2010). In the present work, we investigate the effects of faults and their related structures on the geomechanical and petrophysical properties of sandstone reservoirs. Important components of fault systems include fractures and deformation bands in the damage zone and fault core (Caine et al., 1996; Shipton and Cowie, 2003; Fossen et al., 2007). Fault systems may enhance or suppress fluid communication, which in turn may affect the storage capacity and conductivity of the candidate reservoirs (Figure 1). As a case study, a reservoir model of the Tubaen Formation at the Snohvit CO2 injection site in the Barents Sea (Grude et al., 2013; Hansen et al., 2013) was investigated using 4D seismic data and fault attribute analysis. The characteristics of deformation structures (e.g. sub-seismic faults, deformation bands and fractures) were investigated by field studies of outcrop analogues and by triaxial laboratory experiments to provide a basis for numerical modelling. Fault architecture within reactivated fault systems was studied by the use of analogue modelling. Key questions addressed in the work include: a) Where and when might strain localize in the reservoir? b) How does rock strain influence fluid communication? c) How might structural architecture affect CO2 storage effectiveness?

In the preceding chapters we have included only a few regional examples and case studies because of space limitations. The present chapter will, however, provide some examples. The North Sea and other parts of the Norwegian continental shelf contain several different petroleum provinces which can illustrate some of the general principles of petroleum geology and geophysics. The geological evolution of these sedimentary basins provides a necessary background to understand the distribution of source rocks and the timing of petroleum migration. The structural history of rifted basins, passive margins and also uplifted basins such as the Barents Sea is critical to the trapping of oil and gas. These basins are very well documented by seismic and well data.

Fieldwork was conducted in Finnmark, northern Norway, with the purpose of detecting and measuring stress-relief features, induced by quarrying and road works, and to derive from them valuable information on the shallow-crustal stress orientations and magnitudes. Two kinds of stress-relief features were considered in this study. The first consists of drillhole offsets that were found along blasted road-cuts and which were triggered by the sudden rock unloading following the actual blasting. Vertical axial fractures found in the concave remains of boreholes represent the second kind of stress-relief feature. The axial fractures are tension fractures produced by gas overpressure inside the drillhole when the blast occurs. As such, their strike reflects the orientation of the ambient maximum horizontal stress axis. The borehole offsets show mostly reverse-slip displacements to the E–SE and the axial fractures trend NW–SE on average, in agreement with NW–SE compression induced by North Atlantic ridge-push forces. Mechanical considerations of the slip planes offsetting some of the drillholes lead to the conclusion that the magnitude of the maximum horizontal stress at the surface is in the range ∼0.1–∼1 MPa. This range of magnitudes is 1–2 orders less than the horizontal stress magnitudes measured at the surface in other post-glacial environments (e.g. Canada). It is suggested that this difference is related to the marked decline in stress that followed the tremendous post-glacial burst of earthquake activity that affected Fennoscandia but apparently not the Canadian Shield.

Low temperature apatite fission track (AFT) data from two horizontal transects across the Møre-Trøndelag Fault Complex (MTFC) and along the coast of the Fosen peninsula have defined a complicated series of structural blocks whose exhumation to temperatures cooler than the uppermost apatite Partial Annealing Zone (PAZ) occurred during a broad Mesozoic to Cenozoic time span. The ∼100 million year cooling difference between the innermost and outermost blocks indicates they were subjected to very different exhumation pathways. Track length histograms and inverse model results also suggest that significant differences in thermal histories are present over very short horizontal distances. In conjunction with the offshore geological evidence, the AFT data and models strongly suggest that the innermost zones of the MTFC underwent between 2 and 4 km of structurally controlled net vertical rock column uplift/subsidence following latest Cretaceous time, and potentially much of the Tertiary as well. The up-to-the-east down-to-the-west relative offset at the Norwegian margin is interpreted in terms of a flexed, nearly broken lithospheric plate architectural model for late Mesozoic and Tertiary Scandes mountain building, whose driving force was likely dominanted by erosionally induced loading/unloading at the margin. © 2004 Elsevier B.V. All rights reserved.

The Njord oil field of the mid-Norwegian continental margin comprises a hydrocarbon trap structure, which is delineated by segments of master faults with complex geometry. The Vingleia Fault Complex includes the major fault segments that delineate the Njord structure to the southeast and east. Three master fault segments with a change in strike orientation are recognized. Reflection seismic sections reveal that these segments show strong variations in fault plane geometry. The central part is characterized by a pronounced ramp–flat–ramp geometry, which diminishes both southwestward and northeastward, so that the northeastern and southwestern segments have a more listric geometry. Hence, an along-strike profile shows an antiform-like culmination in the master fault surface. To the characteristic deformation of ramp–flat–ramp faults, additional deformation in the Njord Field is obvious and marked by extensional horses, extensive roll-over folding, varying strata rotation, characteristic fault bending and variable influence of synthetic and antithetic faults. The occurrence of the style of deformation and geometrical variations in fault pattern are closely related to the structural position above the detachment fault. The following steps in the development of the Vingleia Fault Complex and the associated deformation of the hanging-wall fault block are proposed. (1) Initiation of the master faults in early Triassic. This stage was characterized by slow, extensional displacement. The positions of the master fault segments were possibly influenced by basement in homogeneities marked by a magnetic/gravity anomaly. (2) It is likely that the ramp–flat–ramp had already become established at this stage and that the master fault geometry and a proposed weak salt layer promoted this. (3) During accelerating extension in mid to late Jurassic, the master fault segments became connected by one-sided, lateral linking of fault tip and fault plane or by mutual curved linking of the fault tips. (4) Simultaneously, a roll-over anticline developed within the hanging wall of the master fault. Its relatively large amplitude is produced by a differential movement on the fault surface in combination with a dip-parallel shift of the vertical displacement maximum, which is positioned above a change in the footwall dip.

A coupled pressure and stress simulator has been applied to the reservoir sandstone unit, the Middle Jurassic Garn Formation in the Halten Terrace, offshore Mid-Norway, in order to simulate hydraulic fracturing and leakage due to overpressure. The over- and underlying rocks are used as vertical seals to the compartments. The overpressure generation within the pressure compartments was modelled quantifying the mechanical and chemical compaction. An empirical model for the minimum horizontal stress was applied to the Griffith–Coulomb failure criterion to estimate the pressure levels at which hydraulic fracturing occurs. Sensitivity tests of Poisson9s ratio and Young9s modulus, which control Biot9s constant, show an effect on the accumulations of overpressure during time. Defining Biot9s constant equal to 1 results in too early pressure build up and hydraulic leakage in the western parts of the basin. Lower values of Biot9s constant (>0.85) give a present-day pressure distribution closer to the measured pressures in wells. The Kristin compartment is simulated to experience hydraulic failure around 1.5 Ma. High values of the coefficient of internal friction ( μ ) and the coefficient of sliding friction ( μ ′`), which are used in the failure criteria, results in a time-delay in failure and, thereby, less leakage from overpressured compartments.