A. J. Breivik

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.

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.

A possible Caledonide arm through the Barents Sea imaged by OBS data

Abstract The assembly of the crystalline basement of the western Barents Sea is related to the Caledonian orogeny during the Silurian. However, the development southeast of Svalbard is not well understood, as conventional seismic reflection data does not provide reliable mapping below the Permian sequence. A wide-angle seismic survey from 1998, conducted with ocean bottom seismometers in the northwestern Barents Sea, provides data that enables the identification and mapping of the depths to crystalline basement and Moho by ray tracing and inversion. The four profiles modeled show pre-Permian basins and highs with a configuration distinct from later Mesozoic structural elements. Several strong reflections from within the crystalline crust indicate an inhomogeneous basement terrain. Refractions from the top of the basement together with reflections from the Moho constrain the basement velocity to increase from 6.3 km s−1 at the top to 6.6 km s−1 at the base of the crust. On two profiles, the Moho deepens locally into root structures, which are associated with high top mantle velocities of 8.5 km s−1. Combined P- and S-wave data indicate a mixed sand/clay/carbonate lithology for the sedimentary section, and a predominantly felsic to intermediate crystalline crust. In general, the top basement and Moho surfaces exhibit poor correlation with the observed gravity field, and the gravity models required high-density bodies in the basement and upper mantle to account for the positive gravity anomalies in the area. Comparisons with the Ural suture zone suggest that the Barents Sea data may be interpreted in terms of a proto-Caledonian subduction zone dipping to the southeast, with a crustal root representing remnant of the continental collision, and high mantle velocities and densities representing eclogitized oceanic crust. High-density bodies within the crystalline crust may be accreted island arc or oceanic terrain. The mapped trend of the suture resembles a previously published model of the Caledonian orogeny. This model postulates a separate branch extending into central parts of the Barents Sea coupled with the northerly trending Svalbard Caledonides, and a microcontinent consisting of Svalbard and northern parts of the Barents Sea independent of Laurentia and Baltica at the time. Later, compressional faulting within the suture zone apparently formed the Sentralbanken High.

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

Abstract The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s −1 ) P-wave velocities, while the eastern has higher (6.3–6.9 km s −1 ) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjornoya. South of Bjornoya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s −1 is significantly lower than the high (7 km s −1 ) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.

Magmatic and tectonic evolution of the North Atlantic

The primary aim of the present paper is (1) to review the tectonomagmatic evolution of the North Atlantic, and (2) constrain evolutionary models with new lithosphere strength estimates and interpretation of potential field data north of Iceland. Our interpretations suggest that the breakup along the entire eastern Jan Mayen Ridge occurred at c. 55 Ma. Calculations of lithospheric yield strength indicate that the continental rifting in East Greenland, which led to oceanic crustal formation west of the Jan Mayen Ridge at c. 25 Ma, could have started at c. 42.5 Ma. Symmetrical V-shaped gravimetric ridges, which can be traced back to c. 48 Ma, document large-scale asthenospheric flow both north and south of Iceland. Such flow is predicted by geodynamic models of mantle plumes, but has yet to be predicted by other mechanisms. The results from the compartments north of Iceland, viewed in a regional context, strengthen the hypothesis attributing the anomalous magmatism in the North Atlantic area from c. 70 Ma to the present to the Icelandic plume.

SOUTHWESTERN BARENTS SEA MARGIN : LATE MESOZOIC SEDIMENTARY BASINS AND CRUSTAL EXTENSION

Abstract The deep sedimentary basins of the southwestern Barents Sea were formed in response to several late Mesozoic–early Cenozoic tectonic events within the North Atlantic rift zone, which culminated with continental breakup and formation of a mainly sheared margin in early Tertiary times. Due to deteriorating data quality, the development of the margin-bordering Sorvestsnaget Basin is not well known. To improve on this, we use both seismic interpretation and gravity modelling to estimate the depth to the Middle Jurassic (MJ) sequence boundary, which marks the onset of Mesozoic rifting. The horizon represents a first-order contrast in seismic velocity and density, and the gravity field correlates well with its depth of burial. Only one seismic line enables tracing of the MJ from surrounding areas into the Sorvestsnaget Basin, down to a depth of at least 17 km in the northeastern part of the basin. The deep basin is reflected in a general thinning of the crust from 30–36 km within the Barents Sea to 20–24 km in the margin area. The gravity modelling show that the Middle Jurassic–Lower Cretaceous sequences are of comparable thicknesses in the Tromso and Sorvestsnaget Basins. However, adjacent to the rifted margin segment the deep northern part of the Sorvestsnaget Basin is affected by additional large Late Cretaceous normal faulting and subsidence. These faults also caused additional structuring of the intrabasinal Veslemoy High. Post-Middle Jurassic crustal thinning in the southwestern Barents Sea shows maximum cumulative β-values exceeding 4 within the Tromso and the northern Sorvestsnaget Basins. The corresponding crustal extension is estimated to 70–85 km in a west to northwest direction. There is no uniform increase in crustal thinning toward the continent–ocean boundary, which we attribute to the transform origin of the margin. However, there is a gradual migration of tectonic activity toward the incipient margin as the time of continental breakup was approached.

Crustal structure of the Vøring Margin, NE Atlantic: a review of geological implications based on recent OBS data

Modelling of extensive seismic datasets recorded on Ocean Bottom Seismographs (OBS) on the outer Voring Margin, NE Atlantic, has provided significant new insights into deeper sedimentary structures, distribution of sill-intrusions in the sedimentary section, top of the crystalline crust, the lower crust and Moho. Primarily based on the modelling of S-waves, it is concluded that the high-velocity lower crust most likely consists of a mixture of plume-related Late Cretaceous/Early Tertiary mafic intrusions mixed with older continental blocks. Northeastwards in the Voring Basin, the landward limit of the lower crustal high-velocity layer steps gradually seawards, closely related to five crustal scale lineaments. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Iceland plume, and the passive component is illustrated by the fact that all resolved crustal lineaments to a certain degree acted as barriers to magma emplacement. A lithospheric delamination model is invoked to explain the observed variations in crustal velocities and thickness. The location of six Tertiary domal structures in the Voring Basin is between, or in the vicinity of, pre-breakup high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these high-velocity structures, subject to mild NW–SE compression, is the most important factor controlling the formation, spatial distribution and trend of the domes. Structures in the high-velocity lower crust may be the single most important element in controlling the formation of the domes; all modelled highs in the lower crustal Early Tertiary intrusive layer seem to be related to the formation of domes.

Magmatic development of the outer Vøring margin from seismic data

The Voring Plateau off mid-Norway is a volcanic passive margin, located north of the East Jan Mayen Fracture Zone (EJMFZ). Large volumes of magmatic rocks were emplaced during Early Eocene margin formation. In 2003, an ocean bottom seismometer survey was acquired over the margin. One profile crosses from the Voring Plateau to the Voring Spur, a bathymetric high north of the EJMFZ. The P wave data were ray traced into a 2-D crustal velocity model. The velocity structure of the Voring Spur indicates up to 15 km igneous crustal thickness. Magmatic processes can be estimated by comparing seismic velocity (VP) with igneous thickness (H). This and two other profiles show a positive H-VP correlation at the Voring Plateau, consistent with elevated mantle temperature at breakup. However, during the first 2 Ma magma production was augmented by a secondary process, possibly small-scale convection. From ∼51.5 Ma excess melting may be caused by elevated mantle temperature alone. Seismic stratigraphy around the Voring Spur shows that it was created by at least two uplift events, with the main episode close to the Miocene/Pliocene boundary. Low H-VP correlation of the spur is consistent with renewed igneous growth by constant, moderate-degree mantle melting, not related to the breakup magmatism. The admittance function between bathymetry and free-air gravity shows that the high is near local isostatic equilibrium, precluding that compressional flexure at the EJMFZ uplifted the high. We find a proposed Eocene triple junction model for the margin to be inconsistent with observations.