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We integrated high-resolution aeromagnetic data and 2D/3D seismic data from the Norwegian Southwestern Barents Sea. The main objective is to address the long-standing question on the role of pre-existing basement structures in controlling strain accommodation and extension in the Finnmark Platform and adjacent rift basins. The thorough qualitative analysis of the high-resolution magnetic data reveals fault geometries, regional kinematics, magmatism and inheritance of older Precambrian/Caledonian structures. Through the application of second order derivative filters and depth-to-magnetic-source modelling, the trends of the Caledonian metamorphic fabrics are identified and correlated with the structure of buried basement faults and shear zones also imaged at the same level of resolution on 2D/3D seismic data. The magnetic data reveal an unprecedented detail of the basement fabrics dominated by high-frequency NW-SE trending magnetic lineaments associated with the semi-regional Sørøya-Ingøya Shear Zone. The high-frequency magnetic lineaments are superimposed by lower frequency NNW-SSE trending magnetic lineaments that reflect the inheritance of older Precambrian structures. At the edge of the Tromsø Basin, the new magnetic data highlight sill intrusions also visible on seismic data. Fault geometries, regional kinematics, and spatial distribution of the magnetic sources suggest that old detachments and younger Mesozoic faults reactivated the basement fabrics found along the graben borders. Focusing of strain accommodation at the edge of the Hammerfest Basin is helped as well as modulated by the presence of back-thrusted Caledonian nappes interpreted on the Finnmark Platform. Offshore, surface ruptures associated with graben formation align with the dominant NNW-SSE trending magnetic lineaments defining steeper normal faults that are characterised by right-stepping segments along the southern flank of the Hammerfest Basin. Based on potential field models, we finally quantify the crustal architecture of the rift and platform system. At upper crustal level, we test the presence and significance of potential Palaeozoic basin preserved at the edge of the basement hinge-zones. Potential field modelling also highlights and quantifies several rift domains defined by moderate to extreme thinning of the crust (low-β stretched domain, necking, and high-β hyperextended regions). The development of the necking zone is clearly influenced by the existence of former first-order and multi-scale inherited basement features preserved in the Finnmark Platform.

The Nordland area in NW Norway is one of the tectonically most active areas in Fennoscandia. It exhibits patterns of extension, which are in contradiction to the first-order regional stress pattern that reflects compression from ridge-push. The regional stress field stems from the interaction of ridge push and GIA (glacial isostatic adjustment); the local stress field mainly results from gravitational stresses, as well as the flexural effects of sediment erosion and re-deposition.We develop 3D finite element numerical models of crustal scale, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses, which are often omitted in stress models. We show that these can strongly influence the near-surface stress regime, in particular for the continental-margin setting we are considering. Similarly, existing weakness zones (such as faults) control the local stress field.We apply the far-field stress fields (GIA, ridge-push, sediment redistribution) as effective force boundary conditions to the sides or base of the model. This way, we can account for all stress sources at once, but can also vary them separately in order to examine their relative contributions to the observed stress and strain rate fields.We compare our models to the stress and strain observations derived from different recent seismological and geodetic data sets. These point to a correlation of seismicity with major changes in the crustal geometry.

SUMMARY The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. However, the exact cause of seismicity in this region is still debated. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. Here, we address this problem with the aid of a new high-resolution 3-D VP and VP/VS ratio images of the crust in Nordland using seismic traveltime tomography. These images show the existence of a localized, 10–15 km Moho step that runs parallel to the coast. The north–south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho step. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP/VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture.

The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s−1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed.

The distribution of Cenozoic compressional structures along the NW European margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling, and with the distribution of high-velocity lower crust and/or partially serpentinized upper mantle detected by seismic experiments. Only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle. Notably, partially serpentinized upper mantle is well documented beneath the central part of the southern Rockall Basin, but compressional features are sparse in that area. Where compressional structures have formed but the upper mantle is not serpentinized, simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling. The presence of pre-existing weak zones (associated with the properties of the gouge and overpressure in fault zones) and local stress magnitude and orientation are important contributing factors.

Seismic refraction data and results from receiver functions were used to compile the depth to the basement and Moho in the NE Atlantic Ocean. For interpolation between the unevenly spaced data points, the kriging technique was used. Free-air gravity data were used as constraints in the kriging process for the basement. That way, structures with little or no seismic coverage are still presented on the basement map, in particular the basins off East Greenland. The rift basins off NW Europe are mapped as a continuous zone with basement depths of between 5 and 15 km. Maximum basement depths off NE Greenland are 8 km, but these are probably underestimated. Plate reconstructions for Chron C24 (c. 54 Ma) suggest that the poorly known Ammassalik Basin off SE Greenland may correlate with the northern termination of the Hatton Basin at the conjugate margin. The most prominent feature on the Moho map is the Greenland–Iceland–Faroe Ridge, with Moho depths >28 km. Crustal thickness is compiled from the Moho and basement depths. The oceanic crust displays an increased thickness close to the volcanic margins affected by the Iceland plume.

Salt canopies are present in many of the worldwide large salt basins and are key players in the basins’ structural evolution as well as in the development of associated hydrocarbon systems. This study employs 2D finite-element models which incorporate the dynamical interaction of viscous salt and frictional-plastic sediments in a gravity-spreading system. We investigate the general emplacement of salt canopies that form in the centre of a large, autochthonous salt basin. This is motivated by the potential application to a mid-basin canopy in the NW Gulf of Mexico (GoM) that developed in the late Eocene. Three different salt expulsion and canopy formation concepts that have been proposed in the salt-tectonic literature for the GoM are tested. Two of these mechanisms require pre-existing diapirs as precursory structures. We include their evolution in the models to assure a continuous, smooth evolution of the salt-sediment system. The most efficient canopy formation takes place under the squeezed diapir mechanism. Here, shortening of a region containing pre-existing diapirs is absorbed by the salt (the weakest part of the system), which is then expelled onto the seafloor. The expulsion rollover mechanism, which evacuates salt from beneath evolving rollover structures and expels it both laterally and to the surface, was not successfully captured by the numerical models. No rollover structures developed and only minor amounts of allochthonous salt emerged to the seafloor. The breached anticline mechanism requires substantial shortening of salt-cored, pre-weakened folds such that the salt breaches the anticlines and is expelled to the seafloor. The amount of shortening may be too large to occur in the central part of a salt basin, but may explain canopy evolution closer to the distal end of the allochthonous salt. When applying the different concepts to the northwestern GoM, none of the models adequately describes the entire system, yet the squeezed diapir mechanism captures most structural features of the Eocene paleocanopy. It is nevertheless possible that different mechanisms have acted in combination or sequentially in the northwestern GoM.

The Messinian evaporites in the Eastern Mediterranean represent a world class site to study thin-skinned salt tectonic processes like gravity gliding or gravity spreading. In contrast to the Mesozoic evaporites in the Atlantic the related structures are just slightly overprinted by additional tectonic events. New high-resolution reflection seismic data image for the first time the entire lateral succession of gravity gliding-related features as they were suggested by analog models and demonstrate therewith the validity of those models for the Messinian evaporites. All three regimes of extension, translation and compression are present within the range of the seismic profiles. However, additional observations lead to an adapted and improved model. The generation of a critical slope angle caused by subsidence of the Levantine Basin is considered to be the main driving force for the thin-skinned salt tectonics and the lateral basinward creep of the basinal Messinian evaporites. In the extensional section beneath or near to the continental slope, where the basinal evaporites have been withdrawn, listric and antithetic listric growth faults, rollovers and keystone grabens form asymmetric turtleback structures above salt welds and salt rollers. Further basinwards an area with little or no deformation of the evaporites defines the translational segment. In the compressional region the lower and thickest part of the evaporites show ductile deformation (buckle folds), and the upper evaporites reveal brittle deformation (reverse faults and pop-up structures). Here the basinal evaporites have a maximum thickness of about 1500 m. The extensional zone shows continuous synsedimentary tectonic activity until recent times, whereas the compressional structures have developed later in a rather sudden event. There is clear evidence for neotectonics off the Israeli coast which is not related to salt dynamics. Faults cut through the entire pile of evaporites and the Post-Messinian overburden. Where the faults pierce the sea floor, mud escape structures have been observed. Deep rooted faults have been interpreted at the base of the Messinian paleo-slope, suggesting a shear zone off Israel and parallel to the Dead Sea Transform.

[1] In paper 1 we described a methodology to model coupled fluid flow and deformation in composite salt and siliciclastic tectonic systems and investigated their compaction and overpressuring behavior prior to and during continental margin-scale gravitational spreading. Compaction-driven Darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional-plastic yield and mechanical deformation. Viscous flow of the underlying salt is independent of fluid pressure. Paper 1 presented prototype models that are limited to single uniform sediment lithologies, either sandstone-type or shale-type, that undergo mechanical and volumetric viscous compaction. In this paper we present models with layered sandstone-type and shale-type lithologies designed to better approximate the more complex stratigraphy of the Gulf of Mexico, our natural example. A first set of models demonstrates that layered lithologies can produce fluid pressure regimes similar to those observed in sedimentary basins. We then introduce an improved formulation of viscous compaction that includes a stronger dependence on porosity and depth (used as proxy for temperature), thereby more effectively self-limiting viscous compaction. A second set of models with the improved viscous compaction formulation demonstrates that the onset of gravity spreading is mainly controlled by overpressuring in the landward end of the salt basin and that resulting shortening in the distal part is partly accommodated by horizontal compaction. Models with moderately high fluid pressure best reproduce conditions considered to have been necessary for large-scale gravitational spreading in the northwestern Gulf of Mexico, which led to the formation of the Perdido Fold Belt.

[1] A methodology is presented to model coupled fluid flow and deformation in rifted continental margin composite salt and siliciclastic tectonic systems; and we investigate their compaction and overpressuring behavior associated with continental margin-scale gravitational spreading. Compaction-driven Darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional-plastic yielding and mechanical deformation. Viscous flow of underlying salt is independent of fluid pressure. Numerical models are adapted to the Oligo-Miocene phase of large-scale gravitational failure in the northwestern Gulf of Mexico, and represent the first study of this system that includes dynamically evolving fluid pressure. Here we present the methodology and prototype models with single uniform sediment lithologies and simple parameterizations of their properties. The models serve to illustrate the interactions among compaction, generation of fluid overpressure, and gravitational failure and spreading. Mechanical and viscous compaction behavior of sandstone-type and shale-type sediments are investigated. Results demonstrate that mechanical compaction can generate moderate overpressure in thick shale-type material, whereas high overpressure requires viscous compaction. In sandstone-type material, only viscous compaction can generate significant overpressure, though this requires tens of millions of years. Changes in the stress regime during gravitational-driven deformation enhance compaction and overpressure. Although illustrative of the methodology and basic processes, none of the prototype single-lithology models satisfactorily reproduces Oligo-Miocene fluid pressure and deformational regimes of the Gulf of Mexico. Numerical models of layered sediments together with an improved formulation of viscous compaction, presented in part 2 of this set of companion papers, are more successful.

[1] The Perdido Fold Belt (PFB) is a prominent saltcored deep water structure in the northwestern Gulf of Mexico. It is characterized by symmetric, kink-banded folds of a � 4.5 km thick prekinematic layer and its vicinity to the extensive Sigsbee Salt Canopy. We use 2-D finite element numerical models to study the evolution of the PFB as a gravity-driven fold belt both in a local context and in the context of the larger-scale passive margin, influenced by adjacent allochthonous salt structures. We show that parameters such as overburden strength, salt geometry, or salt viscosity determine timing, extent, and location of the modeled fold belt. Simplified models of the Gulf of Mexico show that toe-of-slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden. In this scenario, the PFB likely represents the terminal folding of a much larger, diachronously formed fold belt system. Citation: Gradmann, S., C. Beaumont, and M. Albertz (2009), Factors controlling the evolution of the Perdido Fold Belt, northwestern Gulf of Mexico, determined from numerical models, Tectonics, 28, TC2002, doi:10.1029/2008TC002326.