Rolf Mjelde

Crustal structure of the northern part of the Vøring Basin, mid-Norway margin, from wide-angle seismic and gravity data

Abstract Three regional Ocean Bottom Seismograph (OBS) profiles with a total of fifteen recovered OBSs were acquired in the northern part of the Voring Basin, mid-Norway margin, in 1996. The data have been modelled by inversion and forward modelling of the OBS vertical components (P-waves). The velocity is found to increase downwards within the sedimentary layers, due to increasing depth of burial (confining pressure). Within the two deepest sedimentary layers there seems to be an increase of 0.5–1 km/s in velocity northwestwards. This increase is most likely caused by high-velocity sill-intrusions in the sedimentary rocks, emplaced during the rift episode leading to Early Eocene opening of the NE Atlantic. The presence of significant amounts of sills seems to terminate close to the Nyk High along profile 5 (dip-profile), and close to a dome northwest of the shelf edge along profile 6 (dip-profile). The velocity in the top of this dome is anomalously high (3.4 km/s at only 1 km depth beneath the seafloor), suggesting that the high consists of volcanic rocks, or heavily intruded sedimentary rocks. The relatively low velocities derived from within the upper crystalline crust (5.9–6.2 km/s) confirms that the crust is of continental origin. Within the lower crust there seems to be a clear increase in velocity northwestwards (from about 7.0 km/s to about 7.4 km/s), suggesting that the amount of high-velocity intrusions (underplating) in the lower crust decreases landwards. The modelling of the strike-profile (profile 7) suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile. The model for profile 7 ties closely southwestwards to the models derived from OBS data acquired in 1992, and northeastwards to Lofoten (OBS data acquired in 1988). The models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey, and that the transition zone is related to the Surt Lineament. The distribution of magmatic rocks thus seems to correlate strongly with pre-breakup structures. In addition to the P-wave modelling, gravity modelling has been performed along all profiles. The gravity modelling confirmed all main aspects of the OBS models, and provided constraints on the crustal structures towards the ends of the profiles.

Crustal structure of the central part of the Vøring Basin, mid-Norway margin, from ocean bottom seismographs

Abstract Regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Voring Basin, mid-Norway margin, have been modelled by use of two-dimensional (2-D) ray-tracing. The regional dataset comprises thirty OBSs deployed along seven 100–170 km long profiles. The deeper part of the Voring Basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels (2–5 km below sea-floor), but the modelling of the OBS-data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea-floor to the upper mantle. The shallow and intermediate sediments (to 5 km below sea-floor) are characterized by a vertical increase in velocity due to increased confining pressure. There is also considerable lateral variation in the velocities within sedimentary layers at all levels. The OBS-data confirm that intrusions of sills are important at intermediate and deep sedimentary levels (2–10 km below sea-floor) in most parts of the area. The sills seem to vary in lateral extent from about 20 km to more than 100 km, and their thickness is generally inferred to be about 200 m. The low velocity in the upper crystalline crust (6.2 km/s) confirms that the crust in the Voring Basin is of continental origin. In most parts of the area the velocity of the lower crust is as high as 7.3–7.6 km/s. This high-velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness. The base of the 7.3 km/s layer is interpreted as the Moho, and the upper mantle velocity is estimated to 8.2 km/s.

Crustal structure of the outer Vøring Plateau, offshore Norway, from ocean bottom seismic and gravity data

Four ocean bottom seismograph (OBS) profiles acquired across the continent-ocean transition zone of the outer Voring Plateau have been modeled by inversion and forward modeling of the OBS vertical component traces (P waves). The thickness of the sedimentary layers deposited since the early Eocene continental breakup varies from up to 2.2 km above the basaltic inner flows (IF) to a minimum of 0.2 km about 20 km seaward of the Voring Escarpment (VE), with a velocity of 1.6–2.55 km/s. The thickness of the uppermost layer of basalt varies from 0.5 to 4.5 km, and its velocity ranges from 3.4 to 5 km/s. The modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend ∼40 km seaward of the VE. Farther seaward, another layer of extrusives/intrusives is found beneath the uppermost layer of basalt. The lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent/ocean transition extends over a 30–50 km wide zone in between (strike) profile 3 (oceanic; 6.6–7.35 km/s) and (strike) profile 4 (continental; 6.1–7.0 km/s). An up to 7 km thick lower crustal high-velocity body (7.1–7.4 km/s) is interpreted in terms of intrusions in the lower crust beneath and landward of the continent/ocean transition. The total thickness of the normal oceanic crust is estimated to be 7–9 km, the anomalously thick oceanic crust reaches a maximum thickness of about 25 km, and the crustal thickness is estimated to be 17–22 km beneath the IF. Gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low.

Crustal structure of the Kolbeinsey Ridge, North Atlantic, obtained by use of ocean bottom seismographs

We present the results of a seismic survey performed around the Kolbeinsey Ridge, North Atlantic. The seismic data were acquired along three profiles: on the ridge axis (Ll), 12 km off-ridge (L2), and perpendicular to the ridge axis (L3), The models indicate lower velocity beneath the ridge axis than in the off-ridge area. Along L3, we obtain velocities of 2.6–3.6 km/s (upper crust), 4.8–6.0 km/s (middle crust), and 6.6–6.9 km/s (lower crust) beneath the ridge axis, while at more than 12 km off-ridge, the model shows velocities of 3.6–4.6 km/s (upper crust), 5.6–6.4 km/s (middle crust), and 6.9–7.2 km/s (lower crust). The model of L1 also shows significant lateral variation along the axis in the lower crust. The velocity in the lower crust decreases to 6.3 km/s in the southwestern part of Ll, where the Moho depth decreases to 7.2 km. The low-velocity structure beneath the ridge axis is interpreted to be due to high porosity in the upper crust and to higher temperatures, as well as a few percent porosity, in the middle and lower crustal material. According to this interpretation, the crustal model of L3 implies that the cracks have been sealed and the temperatures have decreased to normal in 1.2-m.y-old crust. The anomalous low velocity structure along Ll can be interpreted as related to the most recent injection processes or a thermal influence of the Spar Fracture Zone situated 50 km south of the profile.

A crustal study off Lofoten, N. Norway, by use of 3-component Ocean Bottom Seismographs

Abstract Twenty-four 3-component Ocean Bottom Seismographs (Obs) were used in August 1988 in a combined seismic refraction and reflection study off Lofoten, N. Norway. The purpose of the experiment was to map the crustal structure from the continental shelf to the oceanic crust off Lofoten. The very high data quality is demonstrated by the strong P-wave and shear-wave reflections, as well as converted waves from the Moho observed on the continental shelf. These arrivals are observed continuously from near vertical to wide angle incidence. Very high seismic sea-floor velocities in this area (3.1 km/s to 5.1 km/s) indicates absence or very thin sequences of Mesozoic sediments. The 5.1 km/s refractor coincides with the base Cretaceous reflection interpreted from the multichannel reflection data. The crystalline continental crust is here divided in layers with velocities of 6.0 km/s, 6.4 km/s and 6.8 km/s, respectively. On the seaward side of the escarpment Tertiary sediments varying in thickness from 1.0–1.5 km are situated on top of a 1.5-2.0-km-thick layer of flood-basalt containing seaward-dipping reflectors. A layer with velocity of 6.7 km/s is observed above the lower crust, which in this area is found to have a velocity as high as 7.3 km/s. These high velocities indicate that the crust in this area is of oceanic origin or, alternatively, the high-velocity layer in the lower crust might represent a magmatic body underplating highly thinned and intruded continental crust. Seven obss were deployed in the area that was covered by landward-flowing basalt deposited during the early Eocene breakup between Norway and Greenland. The survey was partly performed to investigate whether this method can be used to map structures below the basalt, which is impenetrable with conventional seismic reflection techniques. The obs data contain considerable information about structures below the flood-basalt; pre-opening sediments up to 4.0 km thick are indicated below the 1.0–2.5-km-thick landward-flowing basalt. The velocities of the crystalline portion of the crust are found to be similar to those observed under the continental shelf (6.0–6.8 km/s), which implies that the crust east of the escarpment is of continental origin. The crystalline crust is strongly thinned in this area, showing a minimum thickness of about 6 km. The depth to the Moho increases from about 15 km in the western part of the area to about 26 km on the continental shelf. The success of the obs survey indicates that such measurements can become an important tool in investigations on passive volcanic margins, and, potentially, in other areas where highly reflective surfaces make the reflection technique inefficient.

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.

High-velocity breakup-related sills in the Vøring Basin, off Norway

Multichannel seismic reflection profiles in the Hel Graben, V0ring Basin, reveal a sill complex at approximately 5 km depth. It is associated with exceptionally high, 7.4 km s−1, seismic wide-angle velocities. The existence of observable wide- angle arrivals shows that the sills act as efficient waveguides. Seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m. Sonic logs from sills of similar thickness on the nearby Utgard High show an average velocity of 7.0 km s−1. Such high velocities require an olivine-gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes. A possible reason for such an emplacement environment is the HeI Grabens role as an intrusion center during breakup volcanism. This would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times. Sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex, affecting depth conversion and gravity modeling. Furthermore, the results question the concept of lower crustal bodies as large-scale, homogeneous accumulations of mafic melt.

Crustal lineaments, distribution of lower crustal intrusives and structural evolution of the Vøring Margin, NE Atlantic; new insight from wide-angle seismic models

Abstract Five lineaments on the volcanic Voring Margin, NE Atlantic, have been identified in crustal scale models derived from Ocean Bottom Seismograph (OBS) data. It is suggested that the Voring Basin can be divided in four compartments bounded by the Jan Mayen Fracture Zone/Lineament, a new lineament defined from this study, the Gleipne Lineament, the Surt Lineament and the Bivrost Lineament. The NW–SE trending Jan Mayen-, Gleipne- and Bivrost lineaments probably represent old zones of weakness controlling the onset of the early Eocene seafloor spreading, whereas the Surt- and New lineaments, rotated ca. 30° symmetrically from the azimuth of the Gleipne Lineament, may represent adjustment features related to the early Cretaceous/early Tertiary rifting. The longest landward extent of a lower crustal high-velocity body, assumed to represent intrusions related to the last phase of rifting, is found between the New Lineament and the Gleipne Lineament, where the body extends across the Helland Hansen Arch. Northeastwards in the Voring Basin, the landward limit of the body steps gradually seawards, closely related to the interpreted lineaments. Northeast of the Gleipne Lineament, the body terminates close to the Fles Fault Complex, north of the Surt Lineament, it extends across the Nyk High, and northeast of the Bivrost Lineament the intrusions terminate around the Voring Escarpment. 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 Icelandic plume, and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement. The increased thickness of the continental crust on the seaward side of the Voring Escarpment, the upwarping of Moho and thinning of the lower crustal high-velocity layer in the western part of the Voring Basin, as well as a strong shallowing of the Moho observed in parts of the area between the Jan Mayen Fracture Zone/Lineament and the New Lineament, can be explained by lithospheric delamination models.

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.