Abstract
Our knowledge of crystalline basement rocks flooring continental rift basins, rifted margin deposits, and foreland basins is poor, yet important for understanding basin evolution, crustal heat production, petroleum maturation, and for offshore mineral exploration. Using the northern North Sea rift as an example, we demonstrate how modern broadband seismic data can be utilized to unravel basement lithology and structure at a previously unprecedented level of detail. In our case study, we have discovered a pre-rift Caledonian basement with a folded structural pattern very similar to Devonian collapse-related structures onshore Western Norway. The data indicate that the collapse-generated Caledonian infrastructure extends >100 km into the northern North Sea, defining a wide zone of Caledonian crust that turned hot and mobilized vertically by upright folding and detachment faulting during the post-collisional stage. This extensive post-orogenic thermally induced collapse makes the south Scandinavian Caledonides unique among Phanerozoic orogens.
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Introduction
Crystalline crustal rocks of the continents continue under continental rifts and passive margins, where they are buried under kilometer-thick packages of sedimentary rocks. These sedimentary packages dampen and obscure magnetic, gravimetric, and seismic signals, and most of the relatively few wells that are drilled into the basement only sample the upper few meters or tens of meters of crystalline rocks. Hence, in contrast to the sedimentary strata, which in places host large petroleum accumulations, the underlying crystalline basement is poorly known.
There is a growing academic interest in these buried portions of the continents. Offshore and buried basement conceal an important evolutionary record of stretched continental crust, as knowledge of these basement regions is essential for a complete understanding of tectonic processes, notably orogeny and rifting. For example, knowledge of pre-rift basement can help to understand the location, structure and geometry of rift systems. Furthermore, the amount of pre-rift extension taken up by the crust in rifts and passive margins is important for reconstructions of former (super)continents.
In this work, we show how modern reflection seismic data can reveal new aspects of buried basement, particularly in the shallower parts of passive margins and rifts. We utilize RGB blending of the spectral components of the seismic signal. While spectral decomposition techniques are widely used to extract information about sedimentary structures and architecture such as river channels in clastic deposits1, our application to the basement itself is new. Broadband 3D seismic data, where a wide range of frequencies are recorded, are particularly well suited for imaging buried basement. Naturally, shallowly buried crust is better imaged than deep crust, but features can be observed throughout the crust. In our case study, we use 3D broadband seismic data from the northern North Sea rift, focusing on the basement on the Norwegian (eastern) side, notably the rift shoulder offshore Western Norway (the Horda Platform) (Fig. 1). The data reveal large-scale folds and associated shear zones in the offshore basement, suggesting a more extensive collapse of the central Caledonian orogen than previously realized.
Results
Seismic imaging of basement
To explore buried crystalline crust, we make integrated use of seismic data in two different ways. The first approach is to explore the top basement unconformity, i.e., the interface between the rift basin deposits and the older and much more complexly deformed crystalline basement. The second approach is to map intra-basement reflectors below the top basement unconformity. Combining these two approaches is elevating our understanding of the buried North Sea rift basement to a new level of confidence and detail (Fig. 1).
Regarding the top basement unconformity, our detailed seismic mapping and use of spectral decomposition methods have revealed patterns of frequency variations that in part reflect the overburden and geomorphic expressions such as channels eroded into the basement, and in part the basement with its variations in lithology, layering and structure. In places where a uniform sequence of sedimentary layers covers the top basement surface, the patterns appearing on blend images are largely related to the basement itself. Internally in crystalline crust underneath rift basins and continental margin deposits, reflectivity and frequency are typically low and seismic imaging poor. Nevertheless, with modern 3D-seismic broadband data, intra-basement packages of reflectors can be observed and mapped in three dimensions. Even though individual reflectors may not be continuous over large areas, reflectors at different depths and from different parts of the data cube can be combined to constrain the overall structural pattern of the crust. In our North Sea rift example (Fig. 1), we find abundant evidence of an extensively folded basement that obtained its final structure during the collapse of the Caledonian orogen.
Nature of onshore Caledonian crust
The onshore basement is dominated by the collapsed and exhumed Silurian–Early Devonian Caledonian orogen. The Caledonian collisional event was relatively short (~20 million years) and initially cold, with deep (locally >100 km deep) subduction of a rigid Baltica margin. During the collision, a growing orogenic wedge of allochthonous crustal units developed2 (Fig. 2a), consisting of continental nappes overlain by oceanic terranes often referred to as outboard terranes or the Upper Allochthon3.
After the collision of Baltica with Laurentia around 425–405 Ma (Fig. 2a), the subducted hinterland portion of the Baltica continental crust, now exposed along the coast of Western Norway (Fig. 1), was heated and then decompressed almost isothermally towards normal crustal depths (Figs. 2a, b)4,5. Structural and kinematic data show that decompression and the related crustal thinning occurred in response to divergent motions between Laurentia and Baltica in the northern North Sea region6. This divergence allowed for tectonic stretching of the orogenic crust and tectonic transport of the orogenic wedge toward the hinterland7 (Fig. 2b). Ar40–Ar39 cooling ages from Baltica basement show that exhumation to middle to upper crustal depths continued through the Middle Devonian8,9 (Fig. 3). The youngest Ar40–Ar39 mica cooling ages (time of cooling through 450–400 °C) are reported from the most high-pressure coastal region north of the Middle Devonian Hornelen basin (Fig. 1) at around 380 Ma8. Farther south, Ar40–Ar39 mica ages cluster around 400 Ma, and brittle structures transecting the late upright folds in the Øygarden Core Complex (ØCC in Fig. 1) have been dated to the Lower to Middle Devonian (Fig. 3)10. Hence, different parts of the Caledonian crust entered brittle crustal levels at different times, earlier in the south and later in the north. By the time that rifting is initiated in the Permian, the crust is assumed to have reached normal thickness. The Permian basement surface is preserved in deeply downfaulted parts of the northern North Sea rift, while the basement in elevated parts of rotated fault blocks and along the rift flanks experienced some further Triassic to Early Jurassic erosion due to footwall uplift11. The temperature–time paths shown in Fig. 3 show paths from the onshore coastal part of Fig. 1, while deep hanging walls to major offshore faults have preserved basement that reached the surface in the Permian.
Dating of migmatitic material in the Baltica crust shows that, during the initial rapid decompression (prior to the middle and upper crustal level cooling outlined in Fig. 3), exhumation was associated with partial melting12,13,14. This must have turned the middle crust into a rheologically soft and gravitationally less dense layer, prone to ascend buoyantly to form upright folds or dome-like structures (Fig. 2c). Associated differential vertical movements occurred in concert with activity on extensional shear zones. Extensional shear zones developed already at the initial and intermediate stages of orogenic extensional collapse. While those early shear zones were hinterland-dipping (Fig. 2b), a change into a system of bivergent extensional shear zones occurred as collapse-related exhumation continued toward the brittle-ductile transition (Fig. 3)15. The result was a series of metamorphic core complexes that involved large-scale folding and associated extensional shear zones with up to a hundred kilometers offset14 (Fig. 2c, d), all located in a vast region of sinistral Devonian transtension7,16.
Onshore Norway, this orogenic collapse event created deformation that increased in intensity from the inland to the coastal rift flank, where a set of plunging upright folds formed during this post-orogenic extensional evolution, folding the main ductile extensional shear fabric. These folds are associated with several kilometer-thick ductile shear zones16,17, locally with a brittle overprint18. The first-order folds have wavelengths of up to tens of kilometers and amplitudes of several kilometers, with Baltica crust occupying antiformal cores and with tectonically overlaying Caledonian allochthons and Devonian supradetachment basins in synformal positions. Regionally, the system of detachments and folds define an up to 100 km wide zone onshore, vanishing eastwards into the mainland. In the following we argue that this tectonic style extends far into the northern North Sea rift proper.
The offshore Gulen fold example
We here use an area in the northeastern part of the North Sea rift to demonstrate how modern seismic data and methodology can be utilized to reveal the nature of the buried basement. In this area, the seismic broadband survey extends close to the Norwegian coast and its coastal bedrock platform, with only a small gap between relevant onshore and offshore data11,19. Our selected example is a seismically imaged antiform offshore Sognefjorden, here referred to as the offshore Gulen antiform, which can be linked to the adjacent onshore Gulen antiform20. The basis for this correlation is its proximity to the onshore Gulen antiform, the way that its location matches the offshore projection of the onshore Gulen antiform, and the fact that both the onshore and offshore folds involve folding of a strong lithologic banding that offshore is identified by multiple subparallel seismic reflectors and onshore is a shear fabric related to the orogenic collapse. The offshore antiform is buried under 300–400 m of seawater and 1–2 km of Jurassic to Quaternary sediments, but still very well imaged at the top basement level in the spectral decomposition blends (Fig. 4).
Spectral decomposition blends, where RGB colors represent different frequency (or cycles/km for depth-converted data) bins, exhibit basement layers of different seismic properties and their large-scale fold pattern. The data also reveal second-order (parasitic) folds at the 0.1–1 km scale (Fig. 4d, also see Supplementary Fig. S1). The layering is strikingly similar to the gneissic layering observed onshore (Fig. 4b, c), but with a level of continuity that by far surpasses relevant onshore examples that are cluttered by vegetation, sedimentary cover, lakes, fjords, and human infrastructure. Different blend colors represent different parts of the seismic frequency (or cycles/km) spectrum and relate to variations in acoustic impedance between different basement layers and the unconformably overlying sandy Jurassic sedimentary strata. In the Gulen case, the cover is interpreted to be sandstone, so that the variations are mainly those within the basement, i.e., metamorphic rock layers or groups of layers with different seismic properties.
The visually impressive top basement blend images of the offshore Gulen antiform shown in Figs. 4a and 5 represent the subcrop pattern of the basement fold at the lower Jurassic paleotopographic denudation surface. The seismically defined layers that outline the fold on the blend images correspond to reflective layers dipping into the crystalline basement (Fig. 4b, d). While these reflectors represent basement heterogeneities, it is not straightforward to directly relate layers of specific frequency bins (colors) to specific lithologies, because the colors also depend on layer thickness, interference, tuning effects, noise, and other geophysical effects. However, a very similar looking onshore lithologic banding (Fig. 4c) suggests that the offshore blend features may relate to alternating granitic, gneissic (with biotite), and amphibolitic lithologies. Furthermore, information about bedrock properties can be deduced from variations in paleotopography. The brighter bands, which show up as around 200-350 m thick bands in the data (Fig. 4b), are locally seen to define paleotopographic ridges (see star in Fig. 5a, b). This indicates that they are more resistant to erosion than adjacent lithologies, as is the case also for onshore granitic layers.
Topography also correlates with the larger-scale fold structure, where a rougher (Fig. 5c, d) and slightly lower (Fig. 5b) topography is seen in the north (upper) part of Fig. 5a. The marked changes in terrane roughness likely relate to a change in lithology. Also, the more widely eroded core of the fold seems to consist of more easily erodible lithology. In this part a wider channel network is seen, focusing on narrower channels through the flat and, in terms of erosion, more resistant middle part of the fold.
The three-dimensional geometry of the fold was investigated by mapping out reflectors within the basement, as shown in Fig. 6. These reflectors are generated by rocks of different densities and/or sound velocities, which for typical onshore basement could represent mica-rich schist or gneiss, amphibolite and granitic layers. Seismic anisotropy caused by deformation fabrics characteristic of shear zones also create seismic reflections21. The seismic reflection pattern reveals a 20 km wide gentle to open upright west-plunging antiform, splitting westward into two slightly diverging hinges (Fig. 6a, arrows).
The offshore Gulen fold structure is defined by a series of parallel bands believed to represent different rock types. Onshore, this pattern is characteristic for strongly sheared or flattened basement, where originally non-parallel features (intrusive structures and tectonic fabrics) have been transposed into subparallel layers. Many Caledonian continental nappes show very strong fabrics and banding, as do the parts of the Baltica crust that have been involved in Caledonian collapse-related ductile flattening and shearing. The onshore Gulen fold is defined by a 3–5 km thick series of banded Baltica gneisses that were sheared during the Caledonian collapse shear zone (detachment zone)22. The offshore Gulen fold is also defined by sheared basement rocks, but it is possible that these basement rocks occupy a higher tectonostratigraphic level (continental allochthons). Hence the folded layers may represent an allochthonous continental unit, an interpretation favored in Fig. 7. The differently eroded basement rocks north of the offshore Gulen fold (Fig. 5) must represent a different unit, most likely higher (outboard) nappe units.
A regionally folded basement province
The offshore Gulen fold is only one of many large-scale folds that can be identified in the offshore seismic data cube. Although individual basement reflectors cannot be traced across the study area, the data as a whole reveal a uniform set of basement folds in the northern North Sea rift area east of the Viking–Sogn graben system with consistent orientation, geometry, wavelength, and amplitude. The fold pattern continues southwards, northwards, and westwards from the offshore Gulen antiform. This offshore fold pattern is outlined in Figs. 7 and 8 (also see Supplementary Figs. S2 and S3) and is very similar to the onshore fold pattern in both map view and cross-section. The onshore series of large upright E–W trending folds vanishes 50–100 km inlands and gets more pronounced westward into the North Sea rift. They affect the major Devonian extensional shear zones and to a lesser extent also the associated supradetachment basins of Middle Devonian age, which occupy synformal positions in the fold system. As may be expected, the synform-antiform pattern seen onshore does not project cylindrically into the offshore area. Instead, a non-cylindrical pattern with dome-like geometries is revealed (Fig. 8), consistent with the general style of large-scale late folds both onshore and offshore. For example, the synclinal structure containing the Hornelen basin is replaced by a large antiformal structure (named Hornelen antiform in Fig. 8) and several shorter-wavelength folds to the south.
In more detail, an adjacent east-plunging antiform (the Vette antiform) is imaged in the downfaulted block south of the west-plunging offshore Gulen antiform. The two antiforms are similar in size, orientation and style, except for their opposite plunge directions (Fig. 6). The Vette antiform is about 25 km wide and its dipping lithologic layering has clearly influenced the top basement paleotopography (Fig. 8b), which in this area is of late Permian to early Triassic age23. To the north and south of these two antiforms are wide synformal structures (e.g., the Solund synform in Fig. 8), containing a down-folded unit of lower seismic reflectivity. From onshore-offshore correlation, this tectonostratigraphically higher unit most likely represents the Caledonian Upper allochthon, overlaying more reflective Baltica continental crust. Although attempts to relate lithology to seismic facies have been made before24,25,26, it is more convincingly expressed in the new broadband 3D seismic data. We found that coherency or semblance attributes (also called discontinuity calculations) are sometimes useful to separate different parts of the crust, as shown in Fig. 9. This profile clearly shows that the upper transparent part of the crust gradually thins northward. A similar eastward thickening across the Utsira High can be seen in E–W sections (Supplementary Fig. S5). However, we did not find evidence for a previously26 postulated dome-shaped core complex geometry within the Utsira High basement itself.
The style of folding described above can be traced 50–100 km west of the present coastline (Fig. 7). Farther into the deeper central parts of the North Sea rift, the basement reflectivity becomes too low for internal structures to be mapped out. The fold pattern can also be traced southward into the Utsira High (Fig. 7). Vertically, the folded structural pattern can be traced into the middle crust, in a few places close to the base of the seismic data (20 km depth). Altogether, this information suggests that the onshore pattern of the folded basement extends more than 50–100 km westward into the rift interior and vertically into the middle crust.
The Ordovician–Silurian Utsira High basement
The Utsira High (Fig. 1) is a major N–S oriented intra-rift horst complex that was peneplanized after late Paleozoic to early Triassic rifting. The peneplanation exposed the crystalline basement to erosion, but some half-grabens with rotated Permian deposits are locally preserved. The Utsira High is special in that a number of wells have been drilled into the crystalline basement, thus providing valuable information about crustal lithology and age.
Most of the cores from the Utsira High wells show that the basement contains magmatic rocks of granitic to gabbroic composition, but also gneiss, dacite, schist, phyllite, and metasandstone are represented27 (Supplementary Fig. S4). U-Pb dating of some of the magmatic rocks has yielded ages mostly ranging from 482–430 Ma28,29. This covers the time interval of Ordovician–Silurian convergence and subduction of the Iapetus Ocean up to the time of Caledonian continental collision30, and the dated igneous rocks can be correlated with the onshore Caledonian arc complex known as the Sunhordland Batholith31 (Fig. 1). The two wells located between the Utsira High and the mainland to the east also appear to sample the Upper Allochthon (e.g., well 17/3-1 in Fig. 7), suggesting that Upper Allochthon units dominate the area between the Utsira High and the mainland. Like the offshore Gulen fold, the top basement surface is an angular unconformity against which different tilted lithologic units subcrop. Hence, frequency decomposition blends can be used to further explore variations in basement structure below the unconformity.
Frequency blends from the top basement exhibit a folded banding or foliation, although locally suppressed by a strong signal from the overlying Permian Zechstein limestone. Where visible, the fold pattern shows folded bands or units of different seismic properties, from 10 km thick down to seismic resolution (Fig. 10). The folds are of similar scale (wavelength) as those to the north, but their geometry and orientations seem more complex and variable. The example shown in Fig. 10 has the characteristics of a classical fold interference pattern32. Although particular attention has been paid to the granitic rocks sampled by Utsira High basement wells, our observations emphasize that the basement in the Utsira Horst is heterogeneous and deformed, dominated by rocks that were foliated and folded during and after the Caledonian collision. This observation agrees with onshore exposure of Upper Allochthonous units, where more or less homogeneous plutonic rocks occur together with well-foliated metamorphic rocks of plutonic, volcanic, and sedimentary origin. An onshore example of the heterogeneous Upper Allochthonous units from the NW side of the Hardangerfjord Shear Zone (Fig. 7) is presented as an inset in Fig. 10.
Discussion
The reflective nature of the sheared Baltica basement offshore defines a consistent pattern of upright folds and domes, while more complex fold patterns are locally revealed in the overlying volumes of Caledonian allochthons, for instance in the Utsira High basement. It seems clear that this characteristically folded and sheared onshore Caledonian crust extends at least 50–100 km westwards.
The offshore Gulen antiform can be traced downwards to a depth of around 12 km. Not all fold structures are traceable to this depth, but the general structural style revealed by the seismic data is that of upright folds down to 10–15 km depth, i.e., from the current top of the crystalline basement through the middle crust. Ductile folding of these basement rocks would, depending on the temperature gradient, require a burial depth of roughly 10 km. Hence, some 10 km of crust must have been removed between the time of collapse-related Middle Devonian folding and the Mesozoic. These folds must therefore have formed at 10–25 km depth in a collapsing Devonian crust.
Whereas the onshore folding and shear fabric patterns exposed by these units were considered to be the result of Caledonian collision and thrusting prior to our enhanced understanding of kinematic indicators in the early 1980s, later work14,33,34 has demonstrated that post-collisional extensional collapse sheared, folded and fundamentally redefined the structural pattern after the Silurian to early Devonian Caledonian history of collision and thrusting. The offshore data presented here show that the Devonian collapse of the Caledonian hinterland was much more wide-reaching than previously thought. The Devonian age of the folding is constrained onshore by a marked difference in fold strain between the basins and their substrate. Folds are more frequent and tighter in the underlying extensional shear zone, and together with detailed structural observations, this is taken as evidence that the large-scale upright folding of extensional detachment mylonites and Devonian basins developed progressively during the Devonian extensional collapse history, starting before and continuing during deposition of the Middle Devonian basins, coeval with continued movement on the extensional shear zone system16,17.
One of the more convincing offshore pre-rift basement shear zones is the Utsira Shear Zone35 (Figs. 1 and 7c), which was exploited during rifting by the Utsira East Fault24. An interesting aspect of this shear zone is its mostly southeasterly dip, opposite to most major onshore collapse shear zones (west to northwest dipping). An onshore parallel is the curved Bergen Detachment (Fig. 1), which initiated in the Middle Devonian at an advanced stage of the extensional collapse (Fig. 2d). The east-dipping Bergen Detachment reactivates the top-W extensional fabric in the basement with an opposite (down-E) sense of shear, and is associated with the exhumation of the upwarping of the Øygarden Complex as a pre-rift core complex (Fig. 2d)15. These foreland-dipping shear zones mark the transition from hinterland-directed extensional displacement of Caledonian allochthonous units (Fig. 2b) to a stage of gravity-influenced vertical basement movements, formation of core complexes and associated development of foreland-dipping shear zones (Fig. 2c).
Offshore core complexes have been described farther north along the Norwegian rifted Atlantic margin, where they also have a Devonian origin, but locally continued to develop during the first stage of rifting in the late Paleozoic to Triassic36. We do not see much evidence for this development in the northern North Sea. The large N–S trending Permo-Triassic faults mostly transect the Devonian basement structures, forming as relatively steep normal faults that rotated during rift-related crustal stretching37 and cut the crust until they flatten and merge with the reflective pattern of the lower crust38. Some rift-related brittle reactivation within low-angle Devonian extensional shear zones probably occurred, and low-angle rift-related faulting has been documented onshore within the wide and mostly ductile Nordfjord–Sogn detachment zone18,39,40. However, the fold and shear zone pattern developed during Devonian extension17 was not significantly modified by this reactivation. This can be demonstrated for the Nordfjord–Sogn detachment zone, where Permian brittle reactivation created a new local detachment fault that cuts off the folded Devonian strata18,39. Offshore, large Devonian crustal structures contributed to the formation of transfer zones or domain boundaries that define 100 km scale segmentation of the rift35. Besides of the Hardangerfjord Shear Zone, which represents a major rift transfer zone immediately south of the study area (Fig. 1), the Gulen antiform is located at another such zone (the Sogn transfer zone; Fig. 1). This antiformal structure clearly caused deflection or termination of Permo-Triassic faults, e.g., the Øygarden Fault System (Figs. 1 and 8), as they attempted to follow the trend of the basement banding. The Utsira shear zone (Fig. 1) is another example, where the rift fault bounding the Stord basin seems to be controlled by the shear zone, particularly in the northern part where the Stord basin is at its deepest. These faults contributed to the rift structuring in the form of horst and graben structures, but there is no core complex development associated with this rift faulting. This difference between the North Sea rift and the rifted Atlantic margin farther north likely relates to their different orientations. The NE–SW trending Møre-Vøring margin runs subparallel to the Caledonian/Devonian structures, i.e., optimal for reactivation. The N–S trending North Sea, however, is highly oblique to many of the Devonian structures. In addition, the Atlantic rifted margin experienced higher amounts of extension than the failed North Sea rift.
The onset of buoyant Devonian doming can be explained in terms of decompression of the subducted Baltica crust, which during exhumation experienced a roughly isothermal decompression path to normal crustal depths2. This decompression promoted partial melting and rheologic softening as the subducted crust re-entered normal crustal depths. In addition, fluids released from metamorphic reactions contribute to the increased buoyancy and rheologic weakening of the crust12.
The Devonian extension of the Caledonian crust illustrated in Fig. 2 is interpreted to have occurred in an overall transtensional tectonic regime16,17 (Fig. 1, inset). Transtension can explain NE–SW to E–W trending upright folds16,41, but the dome-shaped core complexes and associated bivergent shear zones call for thermally controlled basement buoyancy within this transtensional setting. We postulate that Baltica crust extends at least 75 km west of the current coastline underneath Caledonian allochthons, demonstrating the presence of a hot collapse-related crustal structure that is much wider than previously assumed. This interpretation implies a new type of orogen that supplement the traditional cold and hot orogens42,43, namely a hybrid orogen that is cold throughout the collisional stage and then turning hot during its subsequent extensional collapse and exhumation stage. The results also suggest that strong orogenic collapse produces extensive low-angle structures that may not reactivate easily during later rifting. It is possible that this has contributed to the Atlantic Ocean not opening along the North Sea and the Caledonian suture, but west of the British Isles. The data and results presented here serve as an example of how basement buried below thick basin deposits now can be explored in terms of structure, morphology and lithology.
Methods
We have applied the spectral decomposition method to PSDM (pre-stack depth migration) broadband 3D seismic data owned by CGG (NVG NS legacy data cube), in addition to various attribute analyses of the data set. For the Utsira High, we used time-migrated 3D data owned by Aker BP. The spectral decomposition method transforms the reflection data into the frequency domain, or the similar cycles/km domain for the depth-converted data (we used both types of data), which generates a range of frequency cubes. Three band-restricted components of the seismic data were selected and blended into a single volume using an RGB color scheme, as specified in the caption to each blend image above. Color variations may reflect changes in layer thickness, lithology, velocity, or seismic anisotropy. Hence these variations may be difficult to explain in detail, but they reveal patterns that in the case of basement often can be assigned to the basement structure. Seismic interpretation was performed by means of Petrel software. Frequency domain blends were produced using GeoTeric software. Depth (elevation), dip and roughness maps shown in Fig. 5 were made in QGIS from the basement interpretation from CGG data in Petrel.
Data availability
Offshore seismic data are property of CGG (CGG.com) and AKER BP. Public seismic and well data available at https://public-enterprise-search.dsif.dawlmknpdprd01.ienergycloud.solutions/search. Well data used in this work are available at https://factpages.sodir.no/en/wellbore/PageView/Exploration/All/4541. Interpretations derived from CGG data are not public, except for the figures themselves as presented in this article.
References
Othman, A. A. A., Fathy, M. & Maher, A. Use of spectral decomposition technique for delineation of channels at Solar gas discovery, offshore West Nile Delta, Egypt. Egypt. J. Pet. 25, 45–51 (2016).
Hacker, B. R. et al. High-temperature deformation during continental-margin subduction & exhumation: the ultrahigh-pressure Western Gneiss Region of Norway. Tectonophysics 480, 149–171 (2010).
Gee, D. G., Fossen, H., Henriksen, N. & Higgins, A. K. From the early Paleozoic Platforms of Baltica and Laurentia to the Caledonide Orogen of Scandinavia and Greenland. Episodes 31, 44–51 (2008).
Chauvet, A., Kienast, J. R., Pinardon, J. L. & Brunel, M. Petrological constraints and PT path of Devonian collapse tectonics within the Scandian mountain belt (Western Gneiss Region, Norway). Geol. Soc. London 149, 383–400 (1992).
Kohn, M. J., Corrie, S. L. & Markley, C. The fall and rise of metamorphic zircon. Am. Mineral. 100, 897–908 (2015).
Fossen, H. Extensional tectonics in the Caledonides: synorogenic or postorogenic? Tectonics 19, 213–224 (2000).
Fossen, H. in Continental Tectonics and Mountain Building: The Legacy of Peach and Horne Vol. 335 (eds Law, R. D., Butler, R. W. H., Krabbendam, M. & Strachan R. A.) 767–793 (Geol. Soc. Spec. Publ., 2010).
Walsh, E. O., Hacker, B. R., Gans, P. B., Wong, M. S. & Andersen, T. B. Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology and fault-slip analysis. Tectonophysics 608, 1159–1179 (2013).
Dunlap, W. J. & Fossen, H. Early Paleozoic orogenic collapse, tectonic stability, and late Paleozoic continental rifting revealed through thermochronology of K-feldspars, southern Norway. Tectonics 17, 604–620 (1998).
Larsen, Ø., Fossen, H., Langeland, K. & Pedersen, R. B. Kinematics and timing of polyphase post-Caledonian deformation in the Bergen area, SW Norway. Nor. J. Geol. 83, 149–165 (2003).
Fossen, H. A Pleistocene origin of the strandflat coastal platform in southwestern Scandinavia. Commun. Earth Environ. https://doi.org/10.1038/s43247-023-00724-6 (2023).
Labrousse, L., Prouteau, G. & Ganzhorn, A. C. Continental exhumation triggered by partial melting at ultrahigh pressure. Geology 39, 1171–1174 (2011).
Gordon, S. M., Whitney, D. L., Teyssier, C. & Fossen, H. U–Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: significance for history of partial melting in continental subduction. Lithos 170-171, 35–53 (2013).
Wiest, J. D., Jacobs, J., Fossen, H., Ganerød, M. & Osmundsen, P. T. Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse. J. Geol. Soc. 178 https://doi.org/10.1144/jgs2020-199 (2021).
Wiest, J. D., Fossen, H. & Jacobs, J. Shear zone evolution during core complex exhumation – implications for continental detachments. J. Struct. Geol. 140 https://doi.org/10.1016/j.jsg.2020.104139 (2020).
Krabbendam, M. & Dewey, J. F. in Continental Transpressional and Transtensional Tectonics Vol. 135 (eds Holdsworth, R. E., Strachan, R. A. & Dewey, J. F.) 159–181 (Geol. Soc. Lond. Spec. Publ., 1998).
Chauvet, A. & Séranne, M. Extension-parallel folding in the Scandinavian Caledonides: implications for late-orogenic processes. Tectonophysics 238, 31–54 (1994).
Eide, E., Torsvik, T. H. & Andersen, T. B. Absolute dating of brittle fault movements: late Permian and late Jurassic extensional fault breccias in western Norway. Terra Nova 9, 135–139 (1997).
Bauck, M. S., Faleide, J. I. & Fossen, H. Late Jurassic to Late Cretaceous canyons on the Maloy Slope: source to sink fingerprints on the northernmost North Sea rift margin, Norway. Nor. J. Geol. https://doi.org/10.17850/njg101-3-1 (2021).
Wiest, J. D. et al. From Caledonian Collapse to North Sea Rift: The extended history of a metamorphic core complex. Tectonics https://doi.org/10.1029/2020tc006178 (2020).
Almqvist, B. S. G. & Mainprice, D. Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure. Rev. Geophys. 55, 367–433 (2017).
Wiest, J. D., Osmundsen, P. T., Jacobs, J. & Fossen, H. Deep crustal flow within postorogenic metamorphic core complexes: Insights from the southern Western Gneiss Region of Norway. Tectonics 38, 4267–4289 (2019).
Wrona, T. et al. Rift kinematics preserved in deep‐time erosional landscape below the northern North Sea. Basin Res. https://doi.org/10.1111/bre.12732 (2022).
Fazlikhani, H., Fossen, H., Gawthorpe, R. L., Faleide, J. I. & Bell, R. E. Basement structure and its influence on the structural configuration of the northern North Sea rift. Tectonics 36, 1151–1177 (2017).
Lenhart, A. et al. Structural architecture and composition of crystalline basement offshore west Norway. Lithosphere https://doi.org/10.1130/l668.1 (2019).
Serck, C. S. et al. From metamorphic core complex to crustal scale rollover: Post-Caledonian tectonic development of the Utsira High, North Sea. Tectonophysics https://doi.org/10.1016/j.tecto.2022.229416 (2022).
Riber, L., Dypvik, H. & Sørlie, R. Altered basement rocks on the Utsira High and its surroundings, Norwegian North Sea. Nor. J. Geol. https://doi.org/10.17850/njg95-1-04 (2015).
Slagstad, T., Davidsen, B. & Daly, J. S. Age and composition of crystalline basement rocks on the Norwegian continental margin: offshore extension and continuity of the Caledonian-Appalachian orogenic belt. J, Geol. Soc. 168, 1167–1185 (2011).
Lundmark, A. M., Sæther, T. & Sørlie, R. Ordovician to Silurian magmatism on the Utsira High, North Sea: implications for correlations between the onshore and offshore Caledonides. Geol. Soc. Lond, Spec. Publ. 390, 513–523 (2014).
Dunning, G. R. & Pedersen, R. B. U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus. Contrib. Mineral. Petrol. 98, 13–23 (1988).
Andersen, T. B. & Jansen, Ø. J. The Sunnhordland Batholith, W. Norway: regional setting and internal structure, with emphasis on the granitoid plutons. Norsk Geol. Tidsskr. 67, 159–183 (1987).
Nabavi, S. T. & Fossen, H. Fold geometry and folding – a review. Earth-Sci. Rev. https://doi.org/10.1016/j.earscirev.2021.103812 (2021).
Andersen, T. B., Jamtveit, B., Dewey, J. F. & Swensson, E. Subduction and eduction of continental crust: major mechanisms during continent-continent collision and orogenic extensional collapse, a model based on the Norwegian Caledonides. Terra Nova 3, 303–310 (1991).
Fossen, H. The role of extensional tectonics in the Caledonides of South Norway. J. Struct. Geol. 14, 1033–1046 (1992).
Fossen, H., Khani, H. F., Faleide, J. I., Ksienzyk, A. K. & Dunlap, W. J. Post-Caledonian extension in the West Norway-northern North Sea region: the role of structural inheritance. Geol. Soc. Lond. Spec. Publ. 439, 465–486 (2016).
Osmundsen, P. T., Péron‐Pinvidic, G. & Bunkholt, H. Rifting of Collapsed Orogens: successive incision of Continental Crust in the Proximal Margin Offshore Norway. Tectonics https://doi.org/10.1029/2020tc006283 (2021).
Yielding, G., Badley, M. E. & Freeman, B. in The Geometry of Normal Faults Vol. 56 (eds Roberts, A. M., Yielding, G. & Freeman, B.) 79–89 (Geol. Soc. Lon. Spec. Publ., 1991).
Odinsen, T., Christiansson, P., Gabrielsen, R. H., Faleide, J. I. & Berge, A. in Dynamics of the Norwegian Margin 167 (ed. Nøttvedt, A. et al.) 41–57 (Geol. Soc, Spec. Publ., 2000).
Torsvik, T., Sturt, B. A., Swensson, E., Andersen, T. B. & Dewey, J. F. Palaeomagnetic dating of fault rocks: evidence for Permian and Mesozoic movements and brittle deformation along the extensional Dalsfjord Fault, western Norway. Geophys. J. Int. 198, 565–580 (1992).
Fossen, H., Ksienzyk, A. K., Rotevatn, A., Bauck, M. S. & Wemmer, K. From widespread faulting to localised rifting: evidence from K‐Ar fault gouge dates from the Norwegian North Sea rift shoulder. Basin Res. 33, 1934–1953 (2021).
Fossen, H., Teyssier, C. & Whitney, D. L. Transtensional folding. J. Struct. Geol. 56, 89–102 (2013).
Vanderhaeghe, O. The thermal–mechanical evolution of crustal orogenic belts at convergent plate boundaries: a reappraisal of the orogenic cycle. J. Geodyn. 56-57, 124–145 (2012).
Jamieson, R. A. & Beaumont, C. On the origin of orogens. Geol. Soc. Am. Bull. 125, 1671–1702 (2013).
Ksienzyk, A. K., Dunkl, I., Jacobs, J., Fossen, H. & Kohlmann, F. From orogen to passive margin: constraints from fission track and (U-Th)/He analyses on Mesozoic uplift and fault reactivation in SW Norway. Geol. Soc. Lond. Spec. Publ. 390, 679–720 (2014).
Stipp, M., Stünitz, H., Heilbronner, R. & Schmid, D. W. The eastern Tonale fault zone: a natural laboratory for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C. J. Struct. Geol. 24, 1861–1884 (2002).
Acknowledgements
Seismic company CGG is thanked for allowing access to their broadband 3D seismic data and publication of basement data and interpretations based on these data. We are thankful for careful and constructive reviews by Tiago M. Alves and an anonymous reviewer. Open-access funding provided by the University of Bergen.
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I.P. ignited the idea for this basement paper by presenting North Sea frequency blend images based on his own interpretations to the first author. H.F. expanded on the subject through seismic interpretation and blend image investigations, in collaboration with I.P. and M.S.B. C.C. contributed to the investigation and understanding of the post-Caledonian collapse history. H.F. wrote the paper and produced the figures, with some input from the other authors.
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Communications Earth & Environment thanks Tiago Alves and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Joao Duarte and Carolina Ortiz Guerrero. A peer review file is available.
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Fossen, H., Polonio, I., Bauck, M.S. et al. The North Sea rift basement records extensional collapse of the Caledonian orogen. Commun Earth Environ 5, 206 (2024). https://doi.org/10.1038/s43247-024-01374-y
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DOI: https://doi.org/10.1038/s43247-024-01374-y
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