Senja Fracture Zone Research Articles

Southeastern Flank of Knipovich Ridge (Northern Atlantic): Basement Structure and Neotectonics from Geophysical Data and Experimental Modeling

Acoustic basement of the Knipovich Ridge southeastern flank was interpreted on the time-domain CDP seismic sections and calibration of Bouguer gravity anomalies to depth was done with construction of basement structural map for the area with an oceanic crust type. On this map, to the east from Knipovich Ridge, there is the longitudinal uplift, which is the northern continuation of the Senja fracture zone and interpreted as a transverse ridge on the transform fault board. This uplift is framed by linear clusters of the off-axis seismicity epicenters, indicating the activation of this area structures. The CDP seismic data above the identified uplift show deformations of the Pliocene‒Quaternary sedimentary cover with reverse fault and shear kinematics. Physical modeling of structure formation in the area of the Knipovich Ridge clearly demonstrated the fe-atures of the main tectonic elements during oblique spreading. The result, especially close to reality, was obtained by conducting combined experiments with bending the weakened zone to large angles between the direction of stretching and perpendicular to the axis of the weakened zone. At the same time, the a-ppearance of typical accretion swells and nontransform axis displacements simulating the structures of the southeastern flank of the Knipovich Ridge is close to reality. The series of experiments conducted to study the possible formation of the spreading axis jump in an easterly direction to the continuation of the Senja fracture zone showed the fundamental possibility of this structure activation, which we consider as one of the reasons for the formation of features observed in geophysical data. The current position of the active zones of the region, seismicity, the structure of the basement and the structure of the sedimentary cover indicate a shift in the activity of the main tectonic elements in the east direction relative to the current position of the extension axis. The likely scenario for further development of the region will be the transformation of the Knipovich Ridge into one or the series of transform faults parallel to the western edge of the Barents Sea shelf and the series of short spreading segments between them.

Geomorphology and topography of relict surfaces: the influence of inherited crustal structure in the northern Scandinavian Mountains

Low-relief surfaces in northern Norway are mapped and analysed to explore (1) whether surfaces were once continuous and possibly correlative, recording an ancient, relict landscape, (2) whether the distribution of (now disrupted) surfaces reveals a tectonic history related to rifting of the North Atlantic and (3) how topography changes across the transition between the North Atlantic and Barents Sea margins. Elevation contours on surfaces, a smoothed fit to mean elevations and histograms of elevation show three distinct, coast-parallel zones onshore northern Norway, and a different pattern in coastal Lofoten–Vesterålen. A sharp transition from continuous surfaces to lower, discretely stepped, discontinuous surfaces is located where the crust thins rapidly to the NW from >39 to <25 km. In margin-parallel transects, elevations remain high for >100 km north of the Senja Fracture Zone and then decrease gradually further NE. The spatial pattern suggests that a continuous, low-relief, relatively low-elevation surface formed in late Mesozoic time and was preserved despite modification by Cenozoic faulting, uplift and slow erosion. Scandinavia's present-day topographic envelope reflects a crustal strength profile set up during earlier hyperextension and maintained by slow Neogene erosion rather than resulting from opening of the North Atlantic or caused by glacial erosion. Supplementary material: A map of polygons of all surface heights within the study area and data on the location of median centres of polygons, assigned attributes and selected zonal statistics are available at https://doi.org/10.6084/m9.figshare.c.3312960

From pull-apart basins to ultraslow spreading: Results from the western Barents Sea Margin

This paper describes results from a geophysical study in the area between the ultraslow Knipovich Ridge and Bear Island, western Barents Sea. The objective was to map the crustal structure along a profile crossing a pull-apart rifted continental margin and oceanic crust generated by ultraslow spreading. The results are based on modeling of wide-angle seismic and gravity data, together with interpretation of multichannel reflection data. Our results show a two layered oceanic crust in the western part of the profile. The thickness of the oceanic crust is variable in the western 130 km, ranging from 3.5 to 5.5 km. East of km 130 the crustal thickness is relatively constant, with values close to the global average for oceanic crust. The oceanic crust is buried by a thick package of Cenozoic sedimentary rocks. The continent–ocean transition (COT) is placed in the interval 207–255 km, between unequivocal oceanic crust and the foot of the westernmost fault in the Hornsund Fault Zone. It is not possible to conclude whether this interval is oceanic crust or thinned and intruded continental crust, but we favor the latter interpretation, at least for the eastern part of the COT. Stretched continental crust is observed between Hornsund Fault Zone and the Knølegga Fault. Here the sedimentary rocks have high velocities and are interpreted to be mainly of Mesozoic and Late Paleozoic age. In this interval Moho depths increase abruptly from 15 km in the west to 27 km in the east. Crystalline basement velocities are observed close to the seafloor east of the Knølegga Fault. We suggest that continental breakup north of Greenland–Senja Fracture Zone occurred around 33 Ma, after a period of pull-apart tectonics. The spreading rate of the earliest seafloor spreading may have been higher than the present day spreading, creating thicker oceanic crust close to the COT.

Late Mesozoic–Cenozoic structural and stratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins

Late Mesozoic–Cenozoic tectonostratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins are provided, based on the recently refined structural and stratigraphic framework off mid-Norway, new early opening plate reconstructions of Eurasia versus Greenland between the Jan Mayen and Senja fracture zones, and a sparse regional grid of seismic reflection profiles off NE Greenland. The Norwegian margin exhibits a distinct along-strike margin segmentation governed by across-margin transfer systems. In particular, the Bivrost Fracture Zone and its landward transfer zone prolongation are well-defined features. Corresponding features are here interpreted on the conjugate NE Greenland margin. Together, these conjugate transfer/fracture zones represent a first-order, across-margin tectonomagmatic boundary of prolonged structural inheritance. Regional transects across both margins reveal important vertical and lateral variations in crustal configuration and composition resulting from a complex history of rifting prior to and during the last rift episode in Late Cretaceous–Early Tertiary time, leading to break-up and volcanic passive margin formation. Although the composite Late Jurassic–earliest Cretaceous rifting was the dominant tectonic episode, one also observes structural and stratigraphic relations that indicate an Aptian–?Albian rift phase, probably co-eval with similar events elsewhere on the NE Atlantic margins. Late Cretaceous rifting, with onset in middle Campanian time, was characterized by low-angle detachment faulting, culminating in regional uplift, intrusive igneous activity and subsequent erosion towards the end of the Paleocene. Thick seaward-dipping reflector sequences indicate massive eruptions of lavas during break-up at the Paleocene–Eocene transition. The post-break-up passive margin development was characterized by the transport and deposition of large amounts of sediment in response to margin subsidence and continental uplift, particularly during two distinct phases of outbuilding in Oligocene?/Miocene and Plio-Pleistocene times. Of special interest are a number of mid-Cenozoic intra-basin inversion features recognized both off Norway and off Greenland, revealing a regional compressive regime.

An admittance study of the Reykjanes Ridge and elevated plateaux between the Charlie-Gibbs and Senja fracture zones

SUMMARY The North Atlantic between the Charlie–Gibbs and Senja Fracture Zones shows a number of oceanic plateaux, continental fragments and unusual mid-oceanic ridges, the Reykjanes Ridges and the Iceland Plateau being the most prominent ones. While rifted continental fragments and marginal volcanic plateaux have been formed during the initial formation process of the North Atlantic, others, such as the Reykjanes Ridge and plume-related plateaux such as Iceland and possibly also Jan Mayen, are still under development. By studying the ratio of geoid to topography in the long-wavelength range for these regions of elevated topography and comparing the results with theoretical models of crustal loading and subsurface thermal uplift we can show that the Rockall Bank and the Voring Plateau are compensated by a thick crust alone, while the Iceland Plateau, the outer Faeroe Plateau and the Jan Mayen Block have a strong component of deeply situated mass anomalies. The admittance, the spectral ratio of geoid to topography, for a number of profiles across the Reykjanes Ridge clearly indicates a decreas ei n compensation depth with distance from Iceland. This strengthens the argument for a mass flux from the Iceland Plume into the Reykjanes Ridge.

Early Eocene sea floor spreading and continent-ocean boundary between Jan Mayen and Senja fracture zones in the Norwegian-Greenland Sea

Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of ∼4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr−1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.

Cambro‐Ordovician stratigraphy of Bjørnøya and North Greenland: constraints on tectonic models for the Arctic Caledonides and the Tertiary opening of the Greenland Sea

The southern tip of Bjørnøya is composed of poorly known sedimentary rocks of Precambrian and Lower Palaeozoic age. Biostratigraphic and sedimentological investigations have demonstrated that the sediments of the Younger Dolomite are of Early, but not earliest, Ordovician age and that those of the conformably overlying Tetradium Limestone are of Black Riveran (Mid‐Ordovician) age. A significant hiatus lies between the base of the Younger Dolomite and the underlying Vendian glacial diamictites of the Sørhamna Formation. Comparison of this distinctive stratigraphic context with other sequences in Arctic Laurentia shows that a similar hiatus is present only in eastern North Greenland, where the Early Ordovician Wandel Valley Formation rests unconformably on a variety of Proterozoic units. Consideration of other geological characters, including the presence of west‐directed Caledonian thrusting and the location of the platform‐trough boundary in the Franklinian Basin, indicates that Bjørnøya was an integral part of the Franklinian Basin during Early Palaeozoic deposition and was attached to eastern North Greenland until early Tertiary rifting. A corollary of this interpretation of the Phanerozoic location of Bjørnøya indicates that any Caledonian strike‐slip, orogen‐parallel, displacements must have been located outboard of the combined Bjørnøya–Greenland craton. Furthermore, it provides strong constraints on interpretations of the geometry of Greenland Sea opening during early Tertiary time, and of the Vestbakken Volcanic Province and pull‐apart basin with its bounding faults, the Hornsund and Senja fracture zones.

Uplift at thermo-mechanically coupled ocean-continent transforms: Modeled at the Senja Fracture Zone, southwestern Barents Sea

Estimates of uplift at ocean–continent transform margins vary significantly between models incorporating both thermal conduction and viscous coupling across the transform and models taking only one of these processes into account. More heat is predicted to be conducted and advected into the continental lithosphere in combined models than in conductive models. Nevertheless, tectonic uplift predicted by the combined models is only half that predicted conductive models. This is because viscous coupling implies crustal thinning close to the transform. The form and amplitude of uplift in the combined models agree well with erosion estimates along a seismic transect across the Senja shear margin, in the southwestern Barents Sea.

The late Mesozoic and Cenozoic evolution of the Sørvestsnaget Basin: A tectonostratigraphic mirror for regional events along the Southwestern Barents Sea margin?

Seismic interpretation and mapping of the Sørvestsnaget Basin at the southwestern Barents Sea Margin has revealed a complex basin history, related to large scale plate tectonic movements in the Norwegian-Greenland Sea area. The pre-Cenozoic evolution of the Sørvestsnaget Basin is poorly understood, due to lack of well data and poor seismic data quality. Regional pre-Cenozoic geologic models, that includes the Sørvestsnaget Basin, should await more and better data. Prior to break-up and sea-floor spreading in the Norwegian-Greenland Sea, Late Cretaceous and Palaeocene shear movements along the Senja Fracture Zone led to structuring and possibly elevation of the southern part of the basin. As sea-floor spreading took place and propagated northwards along the Senja Fracture Zone, the western areas of the basin were uplifted and eroded. Two large and two smaller intrabasinal features in the southern Sørvestsnaget Basin are characterised by: (a) low values on the gravimetric anomaly maps; (b) surrounding synforms; and (c) little response on magnetic anomaly maps. The features have been interpreted as salt structures. The Tertiary evolution of intrabasinal salt-structures seems to have been governed by extensional faulting and subsidence of the margin during the Late Palaeocene and Eocene periods. The Oligocene and Miocene periods were characterised by local and regional uplift of the Sørvestsnaget Basin, respectively. During these periods, the area in particular the western part of the basin, was subjected to transpressional/compressional forces, which can be tentatively correlated to contemporaneous, large scale plate reorganisations and variations of sea-floor spreading rates in the Norwegian-Greenland Sea. The Oligocene and Miocene transpressional/compressional events influenced the salt structures, and is here suggested to be partially responsible for the complex expression of the largest salt structures. The Late Neogene and Pleistocene periods are represented by a thick, westward prograding wedge, indicating subsiden3e with coinciding high sediment input to, and minor adjustment of the salt structures in, the Sørvestsnaget Basin.

The Norwegian continental margin from the Northern North Sea to the Senja Fracture Zone

The Norwegian passive continental margin extends some 1100 km along-strike, from the UK border in the SW to the Senja Fracture Zone in the NE. The margin can be divided into three segments. The More segment in the south is a relatively simple, deep and broad Cretaceous basin close to the Norwegian mainland. The middle part is the Voring/Haltenbanken segment, a broad (400 km) and complex margin with a platform area (Trondelag Platform), terraces (Halten and Donna Terrace) and the deep, broad, Cretaceous/Early Tertiary Voring Basin on the seaward side. The northern part is the Lofoten/Vesteralen segment, where the peninsular Lofoten islands form a high, uplifted and narrow horst between the steep and structured Tertiary margin to the west, and the deep Vestfjord Basin of assumed Cretaceous age to the east. The development into passive continental margin segments occurred during the opening of the NE Atlantic in the earliest Eocene. The break-up was associated with massive volcanic activity and in particular the More and Voring segments form type examples of volcanic passive margins. The area had a very long history of extensional tectonics and rifting before the final successful opening. Extensional tectonics dominate, both as deep basement-involved faulting, and as more shallow listric faults locally detaching into Triassic evaporite layers; inversion and doming are also seen. In terms of extensional deformation the three segments developed differently, creating a variety of structural features. Across the broadest segment, the Voring Margin, an apparent lateral migration of rifting events exposes a development from at least Permian time.

Evolution of North Atlantic volcanic continental margins

Structurally, the North Atlantic margins are characterized by Late Cretaceous–Early Tertiary extension, developing predominantly rifted segments south of the Greenland–Senja Fracture Zone, and by oblique extension and wrenching leading to large sheared margin segments further north. The rifted segments document an intense, transient volcanic phase during break-up, emplacing huge extrusive constructions on the outer margin. Commercial exploration has mostly been landward of the shelf edge, but lately there is increased interest in the deep water basins on the outer margins and in the perennially sea-ice-covered shelf off Greenland. Evaluation of these areas requires an understanding of the processes and events prior, during and after the Paleocene–Eocene opening of the North Atlantic. Although the margin is a Cenozoic feature, its segmentation and evolution are governed by the pre-opening history, particularly the Late Jurassic–Early Cretaceous tectonic episode which resulted in large Cretaceous sedimentary basins. Moreover, the first few multi-channel seismic profiles from the NE Greenland inner shelf north of 79°N show a deep Permo-Carboniferous rift basin. The similarity of tectono-magmatic features off NE Greenland and Norway shows the value that conjugate margin studies provide in understanding volcanic margin formation. The events at break-up affected both the subsidence and thermal histories of the marginal basins and were associated with crustal uplift and erosion of the rift region, intrusive activity within the adjacent Mesozoic basins and lava flows spilling onto large areas of the pre-opening crust. The transient magmatism/volcanism had important environmental implications. After break-up, the margin experienced thermal subsidence and little structural deformation, but the extrusive complexes controlled the Paleogene sediment deposition.

Deep seismic transects across the sheared western Barents Sea-Svalbard continental margin

We have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional MCS data to study the 1000 km long sheared western Barents Sea-Svalbard margin with the objective of determining its crustal structure and tectonic history. The margin developed from a megashear zone which linked the Norwegian-Greenland Sea and the Eurasia Basin during the Eocene opening. The margin is composed of two large shear segments and a central rifted margin segment associated with volcanism. The continent-ocean boundary is confined within a narrow zone (10–20 km) at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin. The crustal structure and tectonic history differ significantly between the three margin segments: In the north, across the Hornsund Fault Zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the Svalbard Platform to 6–8 km thick oceanic crust in the Greenland Sea. This margin segment experienced oblique continent-continent and partly continent-ocean shear with both transtensional and transpressional component; during Eocene time while post-Eocene extension caused crustal stretching west of Svalbard. The volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments. This formed at a releasing bend in the margin in response to Eocene-Oligocene extension and two phases of volcanism. In the south, the crustal thinning occurs over a wider area landward of the Senja Fracture Zone. Here, the margin bounds a basinal province in which as much as 18–20 km of sediments cover a highly attenuated crystalline crust. The Senja Fracture Zone evolved due to Eocene transtensional continent-continent shear followed by continent-ocean shear and has been passive since earliest Oligocone time. We conclude that the first-order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters: 1. (1) the pre-breakup structure, 2. (2) the geometry of the plate boundary at opening and 3. (3) the direction of relative plate motion. The interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments.

Crustal structure of the sheared southwestern Barents Sea continental margin

Eighteen expanding spread profiles from the southwestern Barents Sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data. The velocity-depth profiles from the slope-intercept, Herglotz-Wiechert and τ-sum methods were compared and contrasted to the velocity-depth curve calculated from the iterative forward modelling of the amplitudes. The velocity-depth profiles derived from the τ-sum and forward modelling of the amplitudes were used to produce synthetics in both the X- T and τ-P domains to choose the technique that most accurately represented the data. The results showed the oceanic crust at the Senja Fracture Zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5.7–6.4 km s −1 and slightly smaller crystalline crustal thickness. The continent-ocean transition at crustal depths was shown to be narrow, in the range of 10–20 km. Our results were consistent with two different crustal provinces east of the Senja Fracture Zone, the Tertiary marginal basin and the Cretaceous basin region. The velocity-depth curves enabled us, in a gross sense, to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles. Depths down to 20 km were inferred for the base of the sedimentary section in the southwestern Barents Sea. The crust thinned gently towards the continent-ocean boundary at the Senja Fracture Zone, whereas the crustal thickness changed abruptly below the Hornsund Fault Zone at the margin farther north. We associated the change in crustal structure with Cenozoic in the thick crust beneath the Svalbard Platform in the north. The depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area.

Pre-23 magnetic anomalies between Jan Mayen and Greenland-Senja fracture zones in the Norwegian sea

Analysis of magnetic data between the Jan Mayen and Senja fracture zones indicates that the anomaly 24A-B sequence extends from the Lofoten Basin onto the outer Voring Plateau. Anomaly patterns, including those on the conjugate margin, suggest that the pre-23 sea floor spreading was characterized by an unstable plate boundary between fracture zones. The pre-23 spreading rate was at least 2.5 cm yr-1 which is remarkably high compared with the post-23 rates. An evolutionary model which assumes Cenozoic oceanic crust as far landward as the Voring Plateau and Greenland escarpments is suggested.

Cenozoic continental separation between Europe and Greenland

Prior to the Cenozoic a shallow sea extending into the North Sea and the Barents Sea existed between Greenland and Europe. Sea-floor spreading along an axis within this epicontinental basin commenced in the Late Paleocene. However, the geologic development of the region is characterized by two major plate tectonic phases. During the first phase oceanic crust was created by sea-floor spreading south of the Greenland—Senja Fracture Zone, whereas the relative motion between northeast Greenland and Svalbard took place along a regional transform fault zone without formation of a deep basin. The second phase started in the Middle Oligocene when the relative plate motion became west-northwest with subsequent formation of the Greenland Sea by sea-floor spreading. Plate tectonic analysis also shows that generation of the Icelandic type ocean crust has been maintained throughout the entire Cenozoic as reflected by the shallow Faeroe—Iceland Ridge. The plate tectonic evolution has caused important constraints on the paleo-oceanography of the Norwegian—Greenland Sea and adjacent areas. In particular, two land bridges may at times have allowed migration of continental faunas and floras, thus by passing the growing North Atlantic Ocean. The land bridges have also acted as important barriers to the surface and deep water exchange between the Norwegian—Greenland Sea, the Arctic Ocean and the main basins of the North Atlantic Ocean. In the south, the main platform of the aseismic Faeroe—Iceland Ridge subsided below sea level in the Miocene. In the north, deep water did not flow between the margins off northeast Greenland and Svalbard before the Middle Oligocene.

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.

The seismicity of the norwegian and Greenland Seas and adjacent continental shelf areas

The seismic activity of the Norwegian and Greenland Seas and adjacent areas has been examined in view of the tectonic evolution of the North Atlantic. The 529 earthquakes used covered the period 1955–1972, and for fifteen of these events fault-plane solutions were available. An analysis was made of the location precision which turned out to be better than 20 km in most cases. Expectedly, little new evidence was obtained at the midoceanic ridges and major fracture zones, with possible exceptions of the Knipovich Ridge showing a well-defined seismicity belt supporting the idea of an active spreading ridge, and the Spitsbergen Fracture Zone, which seems to be a system of en-echelon faults. Most interesting is a weak linear event pattern in the Lofoten Basin, possibly giving evidence of unknown structures parallel to the Greenland and Senja Fracture Zones, although sediment loading also may be important. Earthquakes along the shelf edge off Norway are located at or near isostatic gravity belts which may act as hinge lines for the marginal subsidence, thus implying stress release caused by differential subsidence of the continental crust. Part of the seismicity of eastern Greenland and western Norway appears to be related to zones of weakness of pre-Cenozoic age. The seismic activity along the edges of the Norwegian Channel is very limited.