Jan Inge Faleide

Late Mesozoic-Cenozoic evolution of the south-western Barents Sea in a regional rift-shear tectonic setting

Abstract On the basis of sedimentary fill, tectonic style and crustal structure, the study area may be divided into three main geological provinces, separated by major fault zones: (1) the oceanic Lofoten Basin and the Vestbakken Volcanic Province in the west; (2) the south-western Barents Sea basin province; and (3) the eastern region which has acted as a largely stable platform since Late Palaeozoic tirnes. The seismic stratigraphy, calibrated with lithostratigraphic units in exploration wells, has provided the timing of the main tectonic events. The structural evolution of the south-western Barents Sea since Middle Jurassic times comprises two main stages: Late Mesozoic rifting and basin formation, and Early Tertiary rifting and opening of the Norwegian-Greenland Sea. The basin formation was controlled by pre-existing structural elements which were probably established in Late Palaeozoic times. The Late Mesozoic-Cenozoic evolution reflects the main plate tectonic episodes in the North Atlantic-Arctic breakup of Pangea. The Middle-Late Jurassic and Early Cretaceous structuring were characterized by regional extension accompanied by strike-slip adjustments along old structural lineaments developing the Bjornoya, Tromso and Harstad Basins as prominent rift basins. The Late Cretaceous development was more complex with extension still dominating west of the Senja Ridge and the Veslemoy High, while halokinesis and continued thermal subsidence prevailed in the Tromso Basin. The Tertiary structuring was related to the two-stage opening of the Norwegian-Greenland Sea and the formation of the predominantly sheared western Barents Sea continental margin. The tectonic activity was shifted towards the west in successive phases. The south-western Barents Sea basin province developed within the De Geer Zone in a region of rift-shear interaction, having affinities to both the North Atlantic and Arctic regions; initially, as an area of oblique extension linking the Arctic and North Atlantic rift systems (Middle Jurassic-Early Cretaceous), then in a continental megashear setting (Late Cretaceous-Palaeocene), and finally a combined sheared-rifted margin setting during opening of the Norwegian-Greenland Sea (Eocene-Present).

Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin

Abstract Seven regionally correlatable reflectors, named R7 (oldest) to R1, have been identified in the Upper Cenozoic sedimentary succession along the western continental margin of Svalbard and the Barents Sea. Regional seismic profiles have been used to correlate between submarine fans that comprise major depocentres in this region. Glacial sediment thicknesses reach up to 3 seconds two-way time, corresponding to 3.5–4 km. Despite limited chronostratigraphic control, ages have been assigned to the major sequence boundaries based on ties both to exploration wells and to shallow boreholes, and by paleoenvironmental interpretations and correlations with other regions. Lateral and vertical variations in seismic facies, between stratified and chaotic with slump structures, have major implications for the interpretation of the depositional regime along the margin. The main phases of erosion and deposition at different segments of the margin are discussed in the paper, which also provides a regional seismic stratigraphic framework for two complementary papers in the present volume. Reflector R7 marks the onset of extensive continental shelf glaciations, but whereas the outer Svalbard shelf has been heavily and frequently glaciated since R7 time, this did not occur, or occurred to a much less extent, until R5 time in the southern Barents Sea. The present study provides the background for a quantification of the late Cenozoic glacial erosion of Svalbard and the Barents Sea. The rates of erosion and deposition exhibit large temporal and spatial variations reflecting the importance of glacial processes in the Late Cenozoic development of this nearly 1000 km long margin.

Evolution of the western Barents Sea

Abstract Information from multichannel seismic reflection data complemented by seismic refraction, gravity and magnetics forms the basis for a regional structural and evolutionary model of the western Barents Sea during post-Caledonian times. The western Barents Sea contains a thick succession, locally > 10 km, of Upper Paleozoic to Cenozoic sedimentary rocks covering a basement of probably Caledonian origin. The area is divided into three regional geological provinces: (1) an east-west trending basinal province between 74°N and the coast of Norway; (2) an elevated platform area to the north towards Svalbard; and (3) the western continental margin. Several structural elements of different origin and age have been mapped within each of these provinces. The main stratigraphic sequence boundaries have been tentatively dated from available well information, correlation with the geology of adjacent areas, and correlation with the interregional unconformities caused by relative changes of sea level. The main structural elements were developed during three major post-Caledonian tectonic phases: the Svalbardian phase in Late Devonian to Early Carboniferous times, the Mid and Late Kimmerian phase in Mid Jurassic to Early Cretaceous times and Cenozoic tectonism related to the progressive northward opening of the Norwegian-Greenland Sea. The sediments are predicted to be of mainly clastic origin except for a thick sequence of Middle Carboniferous — Lower Permian carbonates and evaporites. Salt diapirs have developed in several sub-basins, especially in the Nordkapp Basin where they form continuous salt walls that have pierced through > 7 km of sediments.

Late Palaeozoic structural development of the South-western Barents Sea

Abstract A regional grid of multichannel seismic reflection profiles records the Late Palaeozoic structure and tectonic development of the south-western Barents Sea. A 300 km wide rift zone, extending at least 600 km in a north-easterly direction, was formed mainly during Middle Carboniferous times. The rift zone was a direct continuation of the north-east Atlantic rift between Greenland and Norway, but a subordinate tectonic link to the Arctic rift was also established. The overall structure of the rift zone is a fan-shaped array of rift basins and intrabasinal highs with orientations ranging from north-easterly in the main rift zone to northerly at the present western continental margin. The structural style is one of interconnected and segmented basins characterized by halfgraben geometries. A less prominent north-westerly fault trend abuts against the main rift zone from the south-east. From the beginning of Late Carboniferous times, the tectonic development was dominated by regional subsidence, and the entire Barents Sea region gradually became part of a huge Permian-Triassic interior sag basin. This development was interrupted by renewed Permian-Early Triassic rifting and formation of north trending structures in the western part of the rift zone. The tectonic link between the northeast Atlantic and Arctic rifts, initiated in the Middle Carboniferous, then became the primary locus of deformation. The tectonic relationship of north-east Atlantic-Arctic rifting to the development of Late Palaeozoic basins, which dominate the structure of the eastern Barents Sea, remains poorly understood. The rapid Late Permian-Early Triassic subsidence of these earlier fault-controlled basins also affected the western Barents Sea. This suggests possible influence on rifting in the Barents Sea by active-margin processes operating at the eastern Barents Sea margin during subduction of the Uralian Ocean floor. Strong control on the Late Palaeozoic structural development by zones of weakness in the basement is interpreted to be inherited from three major compressional orogens-Baikalian, Caledonian and Innuitian-converging and partly intersecting at a major tectonic junction in the south-western Barents Sea. Local observations indicate that the Barents Sea Caledonides were affected by a Devonian phase of late-orogenic extensional collapse.

Continent-ocean transition at the western Barents Sea/Svalbard continental margin

The change in crustal type at the western Barents Sea/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism in Svalbard and the western Barents Sea, and might explain the prominent marginal gravity and velocity anomalies.

NE Atlantic continental rifting and volcanic margin formation

Abstract Deep seismic data from the Hatton-Rockall region, the mid-Norway margin and the SW Barents Sea provide images of the crustal structure that make it possible to estimate the relative amounts of crustal thinning for the Late Jurassic-Cretaceous and Maastrichtian-Paleocene NE Atlantic rift episodes. In addition, plate reconstructions illustrate the relative movements between Eurasia and Greenland back to Mid-Jurassic time. The NE Atlantic rift system developed as a result of a series of rift episodes from the Caledonian orogeny to early Tertiary time. The Late Palaeozoic rifting is poorly constrained, particularly with respect to timing. However, rifted basin geometries, inferred to be of this age, are observed at depth in seismic data on the flanks of the younger rift structures. Intra-continental rifting in Late Jurassic-Cretaceous times caused c. 50–70 km of crustal extension and subsequent Cretaceous basin subsidence from the Rockall Trough-North Sea areas in the south, to the SW Barents Sea in the north. In late Early to early Late Cretaceous times, new rifting occurred in the Rockall Trough and Labrador Sea associated with the northward propagation of North Atlantic sea-floor spreading. When sea-floor spreading was approached in the Labrador Sea the Rockall rift apparently became extinct. The final NE Atlantic rift episode was initiated near the Campanian-Maastrichtian boundary, lasted until continental separation near the Paleocene-Eocene transition, and caused c. 140 km extension. The late syn-rift and the earliest sea-floor spreading periods were affected by widespread igneous activity across a c. 300 km wide zone along the rifted plate boundary. The deep seismic data provide lower-crustal structural geometries that represent boundary conditions for a better mapping and understanding of the extensional thinning of the crust. The crustal geometries question extension estimates previously made from basin subsidence analysis, and aid in the definition of bodies of magmatic underplating beneath the outer volcanic margins.

Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas

The Cenozoic succession in the central and northern North Sea has been investigated to establish a regional sequence stratigraphic framework. Changes in sediment distribution indicate a complex pattern of regional vertical movements along older Palaeozoic and Mesozoic structures in Cenozoic times. These vertical movements, mainly related to tectonic processes along the continental margin to the north and north-west, were responsible for the generation and removal of provenance areas for sediments delivered to the North Sea basin through Cenozoic time. During the early through middle Palaeogene, sediments were mainly sourced from areas in the west and from the Atlantic margin in the north. Uplift in the northern North Sea in the late Eocene was, in the earliest, Oligocene followed by marked basin subsidence and tectonic uplift of southern Norway. A similar pattern of tectonic movements resulted in subaerial exposure of the northern North Sea in the early Miocene, followed by significant tectonic subsidence in the basin and along the Atlantic margin in late Miocene-Pliocene times. At the same time, southern Norway was uplifted and became a major sediment source. After the Miocene-Pliocene subsidence-uplift events, glacial processes in southern Norway and fluvial processes in a large drainage area east and south-east of the North Sea were responsible for the main sediment influx to the North Sea. The Cenozoic depositional sequences in the North Sea developed in close interaction with regional tectonic movements and changes in provenance areas. Tectonic movements in north-west Europe overprinted global sea-level changes, so that the generation of depositional sequences and sequence boundaries apparently occurred independently of the rate of eustatic sea-level change.

Cenozoic sedimentation along the southwestern Barents Sea margin in relation to uplift and erosion of the shelf

Abstract The distribution of Cenozoic sediments along the southwestern Barents Sea margin reflects the progressive opening of the Norwegian-Greenland Sea towards the north. The Lofoten Basin fill consists of Early to Middle Tertiary mostly undeformed sequences of great lateral extent, overlain by a huge sedimentary wedge which extends over both continental and oceanic crust. This wedge, the Bjornoya Fan, comprises 3–4 km of Late Cenozoic deposits. Regional seismic lines enable tracing of sequence boundaries westward until they terminate onto oceanic basement of known age, thus providing maximum ages of the sediments above. Additional age constraints come from exploration wells and shallow boreholes in the southwestern Barents Sea as well as paleoenvironmental indicators from deep sea drilling. The pre-glacial units (sequences Te1–Te4) are controlled by the plate tectonic setting and basin geometry at the time of deposition. The seismic stratigraphy of the Bjornoya Fan (reflectors R1–R7) indicates three main phases of glacial erosion and sedimentation: (1) 2.3 Ma (R7) dates the onset of extensive continental shelf glaciations, (2) 1.0 Ma (R5) marks an increase in the intensity of glaciations, giving rise to increased accumulation rates and the onset of large-scale mass movements and (3) 0.44 Ma (R1) marks the end of ne erosion in the outer parts of the Bjornoya Trough. The wedge represents about 70% of the sediments in the Lofoten Basin and its young age indicates that a large thickness of sedimentary strata has been eroded and deposited over a short geological time period. Using volumetric calculations, Cenozoic erosion and sedimentation rates have been quantified. Furthermore, the sediment yield and denudation rates for the main glacial sequences in the Bjornoya Fan are calculated.

Mjølnir structure: An impact crater in the Barents Sea

A systematic search for impact indicators was conducted on a core of Late Jurassic-Early Cretaceous sedimentary strata from the vicinity of the proposed Mjolnir impact structure, Barents Sea. A 0.8 m-thick section of the core was found to contain unequivocal indicators of meteoritic impact: shocked quartz grains and a strong enrichment in iridium. The ejecta-bearing strata were discovered only 30 km north-northeast of the structure, within a stratigraphic interval corresponding to the seismically defined deformation event at Mjolnir. Further study of this unusually well presented impact-crater-ejecta-layer pair may help constrain poorly understood aspects of large-magnitude meteorite impacts into the oceans. 14 refs., 4 figs.

Effect of thermal contrasts on gravity modeling at passive margins: Results from the western Barents Sea

The western Barents Sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling. This margin developed by shear motion between the Eurasian and Greenland plates during the early Tertiary, and it is a significant border zone between young, hot oceanic lithosphere and cooler continental lithosphere. We construct two-dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling. The approach has a substantial impact over traditional shallow gravity models, here demonstrated on a previously published model. On the basis of a 140 mGal free-air anomaly, the old model proposes an anomalous, high-density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development. However, the lithospheric models predict a homogeneous oceanic crust, while preserving regional isostasy at base lithosphere from continent to ocean. Two further tests agree with this conclusion: A map of Bouguer corrected ERS-1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin. Admittance analysis shows a strong oceanic lithosphere, and the high coherence between bathymetry and free-air gravity discounts a significant subsurface load. The high gravity anomalies at the margin are thus an edge effect, enhanced by sedimentation onto the strong oceanic lithosphere, and shaped by the effect of the lithospheric thermal field. Other results of this work include a new continent-ocean boundary map and two crustal transects across the margin.