Møre Margin: Crustal structure from analysis of Expanded Spread Profiles

Analysis in both the x—t and —p domains of high-quality Expanded Spread Profiles across the Møre Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the Møre Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s^–1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s^–1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted.     

Crustal structure of the Lofoten–Vesterålen continental margin, off Norway

By compiling wide-angle seismic velocity profiles along the 400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, 25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of 26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels.     

Regional setting of Håkon Mosby Mud Volcano, SW Barents Sea margin

The Håkon Mosby Mud Volcano (HMMV) is a seafloor mud volcano, having a 1-km-diameter circular shape and a relief of 8–10?m. HMMV is located within a slide scar on the Bjørnøya glacial submarine fan on the SW Barents Sea slope, and is underlain by a >6-km-thick Cenozoic sequence. Multichannel seismic data reveal a 1- to 2-km-wide disturbed zone, which extends to a depth of >3?km below the HMMV. We relate the zone to the presence of free gas. The seismic data are compatible with an intrasedimentary sourced mud volcano related to the glacial sedimentation history and mass movements.     

Continental margin off Lofoten-Vesterålen northern Norway

The continental margin off the Lofoten-Vesterålen islands between 67° and 70°N becomes progressively narrower northwards. The continental shelf west of the islands and in the Vestfjord is underlain by a relatively thin sedimentary sequence which has been subjected to block faulting, forming local basins and highs. The structural deformation had ceased in the mid-Creataceous. The Tertiary sediments are generally missing, but reappear in the Træn Basin south of about 67.5°N. The continental margin seaward of the shelf edge changes structural style from south to north. In the north, the marginal subsidence is characterized by major faults, whereas minor faults and flexuring dominate south of 69°N. A smooth acoustic basement reflector, which in places is underlain by dipping sub-basement interfaces, is typical for the area between anomaly 23 and the Vøring Plateau Escarpment. In the northern area, the acoustic basement extends almost to the shelf edge. These observations relate to the early Tertiary history of rifting and passive margin formation within a preexisting epicontinental sea between Norway and Greenland. The abrupt change from continental to oceanic basement is defined by the extension of the Vøring Plateau Escarpment south of 69.1°N and by the change in magnetic character off Vesterålen.     

Malvinas (Falkland) Plateau structure versus Mjølnir crater: Geophysical workflow template for proposed marine impact craters

A diagnostic geophysical‐based template, supported by modelling, is suggested to be used prior to, or in combination with geological/drilling data, when proposing a marine impact crater. The latter refers to impacts occurring in a marine setting and resulting in structures that are currently partially or totally underwater. The methodology is based on the well‐documented Mjølnir crater in the Barents Sea. The template has been developed in conjunction with the recently proposed and debated impact crater on the Malvinas (Falkland) Plateau in the South Atlantic. Despite their different sizes, their comparison adds to the ambiguous nature of the Malvinas structure and shows that the integrated analysis of seismic and potential field data and modelling is crucial for any interpretation of a marine impact crater without relevant geological information. The proposed workflow template utilizes all available geophysical data and is composed of a series of iterative steps, including a range of alternative nonimpact interpretations that must be discussed and accounted for. Subsequently, further iterative geophysical modelling is required to support and decipher the impact related processes. A more complex impact crater model and additional impact crater features can be resolved by physical property modelling. In all cases, a close spatial correspondence of the defined impact structure with potential field anomalies is a necessity to establish a causal relationship. We suggest that the diagnostic workflow template provides a methodology to be applied to future studies of the Malvinas structure, as well as to proposed marine (and, with minor adaptions, to nonmarine) impact craters in general.     

Thermal implications for the evolution of the spitsbergen transform fault

Heat flow taken between Svalbard and Greenland reveal three thermal provinces:

1. (1) the Molloy Ridge within the Spitsbergen Transform,

2. (2) the Yermak Plateau

3. (3) the northeastern margin of Svalbard (Nordaustlandet).

The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position.It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard.Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.     

The western and northern margin off svalbard

Recent geophysical measurements, including multi-channel seismic reflection, on the Svalbard passive margin have revealed that it has undergone a complex geological history which largely reflects the plate tectonic evolution of the Greenland Sea and the Arctic Ocean. The western margin (75–80°N) is of a sheared-rifted type, along which the rifted margin developed subsequent to a change in the pole of plate rotation about 36 m.y. B.P. The north-trending Hornsund Fault on the central shelf and the eastern escarpment of the Knipovich Ridge naturally divide the margin into three structural units. These main marginal structures strike north, paralleling the regional onshore fault trends. This trend also parallels the direction of Early Tertiary plate motion between Svalbard and Greenland. Thus, the western Svalbard margin was initially a zone of shear, and the shear movements have affected the adjacent continental crust. Although, the nature and location of the continent—ocean crustal transition is somewhat uncertain, it is unlikely to lie east of the Hornsund Fault. The northern margin, including the Yermak marginal plateau, is terminated to the west by the Spitsbergen Fracture Zone system. This margin is of a rifted type and the preliminary analysis indicates that the main part of the investigated area is underlain by continental crust.     

SW Barents Sea continental margin heat flow and Håkon Mosby Mud Volcano

The high thermal gradient and heat flow >1000?mW?m^-2 on Håkon Mosby Mud Volcano are ascribed to rapid transport of pore water, mud, and gas in a narrow, deep conduit within a 3.1-km-thick glacial sediment unit. The instability is caused by rapid loading of dense glacial sediments on less dense oozes. Changes in pressure–temperature conditions by sudden, large-scale downslope mass movement may induce structural deformation, opening transient pathways from the base of the glacial sediments to the sea floor. This model may also explain slope maxima elsewhere on the margin.     

Cenozoic uplift of Fennoscandia inferred from a study of the mid-Norwegian margin

Studies of the mid-Norwegian margin reveal that the Fennoscandian continental uplift represents a flexural intraplate deformation event separated in time and space from the regional syn-rift uplift associated with crustal breakup at the Plaeocene-Eocene transition. In the area 64–68°N, the uplift occurred from late Oligocene through Pliocene. During Late Pliocene and Pleistocene times the tectonic uplift was amplified by isostatic rebound in response to the Northern Hemisphere glaciation. The tectonic uplift component reaches 1 km in the northern part of the study area decreasing to the south. The shelf stratigraphy and sediment composition record the combined effects of tectonic uplift, eustatic sea level changes and Neogene climatic deterioration. The coeval uplift and climatic change may suggest causal relations. The resulting depositional model has three stages: (1) late Miocene ( 10.5-5.5 m.y.) increased continental erosion and deposition of prograding wedges most of which were later removed; (2) early-middle Pliocene (5.5-2.6 m.y.) development of extensive local ice-sheets reaching the coastline and deposition of the prominent, oldest Pliocene wedges; (3) Northern Hemisphere glaciation (2.6-0.01 m.y.) resulting in the younger wedges farther west covered by Quaternary deposits. The model is consistent with the development of landforms on the adjacent mainland. Both the tectonic and isostatic components of the Fennoscandian uplift appear to vary in magnitude along the uplift axis, however separation of the syn-rift plate boundary related uplift and the intraplate event support the Neogene age of the main Fennoscandian uplift. We document a correspondence between structural and physiographic margin segmentation and uplift magnitude and suggest that the intraplate deformation has a thermal origin. A hot-cold asthenosphere boundary beneath the Caledonide-Baltic Shield transition combined with pre-Tertiary relief at the base of the lithosphere might induce small-scale convection and preferential volume expansion beneath the observed elongate uplift.     

Geological events during the early formation of a passive margin

The concept of plate tectonics implies that the normal sea floor spreading stage is preceded by a sequence of events associated with the break-up of continental crust. Thus, evidence of the early development of “non-failed” rifts is to be found at passive continental margins. Of special interest is the question of the extent of the continental crust and the structural and compositional changes associated with the change in crustal type. In addressing these topics, we have focused attention on the Norwegian margin between the Jan Mayen and Senja fracture zones (66°–70°N) in an attempt to understand its history of rifting and early sea floor spreading. p ]The southern part of this rifted margin is characterized by a wide shelf and the marginal Vøring Plateau interrupts a gentle slope at a level of about 1500 m. However, the margin becomes progressively narrower towards the north and a typical narrow shelf and steep slope emerge off the Lofo—tenVesterålen Islands (Fig. 1). In a reconstructed pre-opening configuration (Talwani and Eldholm, 1977) the narrowest part of the juxtaposed EastGreenland margin is found in the south and a wide shelf and slope corresponds to the Lofoten-Vesterålen margin.The most prominent structural element is a buried basement high underneath the Vøring Plateau. The high is bounded landward by the Vøring Plateau Escarpment, a major structural boundary which defines typical changes in the geophysical parameters. These are: (1) a sudden increase of depth to acoustic basement; (2) changes in the velocity-depth function; (3) a gravity gradient; and (4) a magnetic edge anomaly separating sea-floor spreading type anomalies from a quiet zone on the landward side (Talwani and Eldholm, 1972). These observations were interpreted in terms of a sharp ocea—ncontinent crustal transition along the escarpment with sea-floor spreading commencing between anomaly 24 and 25 time (56–58 m.y. B.P.). Alternatively, the concept of ancient oceanic crust landward of this escarpment and the possible existence of continental crust under the outer basement high have been argued and we refer to Eldholm et al. (1979) for a detailed discussion.