T. Raum

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.

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.

The transition from the continent to the ocean: a deeper view on the Norwegian margin

We present a regional, crustal-scale, 3D structural model of the Norwegian continental margin integrating sedimentary and crustal layers from the continental and the oceanic domain. The model includes six sedimentary units, underlain on the continental side by a thinned crystalline crust and a lower-crustal high-velocity body. In the oceanic domain, three crustal layers (2AB, 3A and 3B), thickened at the continent–ocean transition (COT), are modelled below the post-breakup deposits. Two major rift phases with different rift axes (Late Jurassic–Early Cretaceous and Late Cretaceous–Early Tertiary) have caused post-Jurassic subsidence and post-depositional deformation of the pre-Cretaceous units. The modelled COT suggests that the pre-breakup rifting event was related to differential stretching focused at the outer margin and that breakup took place in a ‘base-up’ magmatic process as a continuation of underplating. For the earlier rift event, stretching was distributed over the entire margin and led to accumulation of up to 12 km of Cretaceous deposits. The large sediment thickness of the Cretaceous units requires deep-water conditions and abundant sediment supply and thus coeval offshore subsidence and onshore uplift. All layers indicate a sinistral offset along the Jan Mayen Fracture Zone and its continentward continuation.

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Voring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The Vp/Vs ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The Vp/Vs ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Voring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Voring Basin, as well as on the southern Lofoten Margin further north. The Vp/Vs ratio in the extrusive rocks on the Voring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the Vp/Vs ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The Vp/Vs ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Voring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the Vp/Vs ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).

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.

Sub-basalt structures east of the Faroe Islands revealed from wide-angle seismic and gravity data

Two semi-regional wide-angle Ocean Bottom Seismograph (OBS) profiles, acquired east of the Faroe Islands, have been analysed by use of forward and inverse modelling to map the crustal structures. In the present wide-angle data, the Tertiary basalt shows a maximum thickness of 3 km under the Faroe Islands, decreasing towards the Faroe–Shetland Channel where it terminates. Sedimentary rocks are present below the basalts and vary in thickness from 2 km to a maximum of 8 km towards the Faroe–Shetland Channel. These sedimentary rocks appear mainly as a low-velocity zone, and the presence of high-velocity intrusions in these layers generate several step-back features in the wide-angle refraction data. Pre-Cretaceous sedimentary rocks are only inferred north of the Clair fracture zone, while Cretaceous rocks dominate southeast of the Westray fracture zone. The crystalline basement is divided into an upper granitic and a lower granodioritic part. P-wave velocities around 7.0 km s−1 are modelled in the lowermost part of the crust, indicating that magmatic underplating is not present below the Faroe Islands. The depth to the Moho is modelled with a maximum depth of 29–30 km below the northern part of the Faroe Islands, decreasing both southeastwards and southwestwards to 25 km and 17 km, respectively.

Spatial relationship between recent compressional structures and older high-velocity crustal structures; examples from the Vøring Margin, NE Atlantic, and Northern Honshu, Japan

Abstract The geological evolution of the volcanic passive margin off mid-Norway, NE Atlantic, and the active volcanic arc in northern Honshu, related to the subduction of the Pacific plate beneath Japan, are similar in the sense that long periods of extensional deformation has shifted to recent compression with formation of domal structures. Seismic wide-angle data have been acquired across six contractional domes in the Voring Basin, mid-Norwegian Margin, and over the Ou Backbone Range ‘pop-up’ structure, northern Honshu. The models derived from the seismic data reveal that the location of all investigated structures in the Voring Basin is between, or in the vicinity of pre-Eocene high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these 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 controling the domal formation; all modelled highs in the lower crustal early Tertiary intrusive layer seem to be related to the formation of domes in the NW–SE direction. The Ou Backbone Range pop-up structure is localised at a low rigidity zone mapped as low upper crustal seismic velocities. The structure coincides with the present day volcanic front, which may be interpreted as the main reason for the existence of low velocities and the main factor controling the localization of compressional deformation. The Ou Backbone Range is squeezed between a high velocity upper crustal block to the east and a high velocity lower crustal block to the west, and it is speculated that the corresponding lateral and vertical variations in crustal rigidity may have contributed to the actual localization of the present day volcanic front.