Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation-including examples from the Norwegian Sea

The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle

The Iceland Microcontinent and a continental Greenland-Iceland-Faroe Ridge

The breakup of Laurasia to form the Northeast Atlantic Realm was the culmination of a long period of tectonic unrest extending back to the Late Palaeozoic. Breakup was prolonged and complex and disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed, which are blanketed by lavas and underlain variously by magma-inflated, extended continental crust and mafic high-velocity lower crust of ambiguous and probably partly continental provenance. New rifts formed by diachronous propagation along old zones of weakness. North of the Greenland-Iceland-Faroe Ridge the newly forming rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis displaced ~ 150 km to the west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a transverse barrier. Thereafter, the ~ 400-km-wide latitudinal zone between the stalled rift tips extended in a distributed, unstable manner along multiple axes of extension that frequently migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day. It is the surface expression of underlying magma-assisted stretching of ductile mid- and lower continental crust which comprises the Icelandic-type lower crust that underlies the Greenland-Iceland-Faroe Ridge. This, and probably also one or more full-crustal-thickness microcontinents incorporated in the Ridge, are capped by surface lavas. The Greenland-Iceland-Faroe Ridge thus has a similar structure to some zones of seaward-dipping reflectors. The contemporaneous melt layer corresponds to the 3–10 km thick Icelandic-type upper crust plus magma emplaced in the ~ 10–30-km-thick Icelandic-type lower crust. This model can account for seismic and gravity data that are inconsistent with a gabbroic composition for Icelandic-type lower crust, and petrological data that show no reasonable temperature or source composition could generate the full ~ 40-km thickness of Icelandic-type crust observed. Numerical modeling confirms that extension of the continental crust can continue for many tens of Myr by lower-crustal flow from beneath the adjacent continents. Petrological estimates of the maximum potential temperature of the source of Icelandic lavas are up to 1450 °C, no more than ~ 100 °C hotter than MORB source. The geochemistry is compatible with a source comprising hydrous peridotite/pyroxenite with a component of continental mid- and lower crust. The fusible petrology, high source volatile contents, and frequent formation of new rifts can account for the true ~ 15–20 km melt thickness at the moderate temperatures observed. A continuous swathe of magma-inflated continental material beneath the 1200-km-wide Greenland-Iceland-Faroe Ridge implies that full continental breakup has not yet occurred at this latitude. Ongoing tectonic instability on the Ridge is manifest in long-term tectonic disequilibrium on the adjacent rifted margins and on the Reykjanes Ridge, where southerly migrating propagators that initiate at Iceland are associated with diachronous swathes of unusually thick oceanic crust. Magmatic volumes in the NE Atlantic Realm have likely been overestimated and the concept of a monogenetic North Atlantic Igneous Province needs to be reappraised. A model of complex, piecemeal breakup controlled by pre-existing structures that produces anomalous volcanism at barriers to rift propagation and distributes continental material in the growing oceans fits other oceanic regions including the Davis Strait and the South Atlantic and West Indian oceans

Crustal and basin evolution of the southwestern Barents Sea: from Caledonian orogeny to continental breakup

A new generation of aeromagnetic data documents the post-Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent-ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well-constrained onshore, swing from a NE-SW trend onshore Norway to NW-SE/NNW-SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post-Caledonian development of the late Paleozoic basins. Farther west, the pre-breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep-seated high-density body, interpreted as exhumed high-grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganization of the crustal extension toward a second necking zone defined at the level of the western volcanic sheared margin and proto-breakup axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west

Geophysical expression of the Leka Ophiolite, Norway modeled from integrated gravity, magnetic and petrophysical data

The ca. 497 Ma Leka Ophiolite Complex (LOC) comprises oceanic lithosphere that formed near the margin of Laurentia in a suprasubductionzone setting. The LOC was obducted onto Laurentia in the Early Ordovician and later thrust onto Baltica during the Scandian continent-continent collisional orogeny (ca. 430 Ma), and now forms part of the Uppermost Allochthon. The LOC contains superb exposures of partially serpentinized mantle rocks, crustal cumulate layered series and wehrlites, sheeted dykes and pillow basalts, including exposures of the petrologic and geophysical paleo-Moho. 564 specimens were collected and measured for density, magnetic susceptibility and natural remanent magnetization. These form the constraints for three profiles forming a new 2.5D gravity and magnetic model. This is the first study of the LOC that models both gravity and magnetic data, whereas previous models were based on gravity data alone. The Bouguer-corrected anomalies express a distinct high correlating with the topographic highs in the center of the island, as well as an additional high in the southern part of the island, indicating an increasing depth of the LOC southwards. New high-resolution aeromagnetic data were used to characterize the nature of the contacts between the rock units, and the orientation of the major normal fault between the ultramafic units and the gabbro to the east. We suggest that this major fault is serpentinized, which accounts for the distinct magnetic anomaly along the trace of the fault (magnetic anomaly up to 2800 nT). Three model sections that transect the island from east to west were created. These sections indicate that the LOC has a bowl-shaped, synformal structure. Extensive serpentinization was found in samples in the uppermost portion of the LOC. The new models suggest that the deepest extent of the complex is approximately 4 km and that the volume of the LOC is approximately 200 km3

Magnetic anomalies of the mafic/ultramafic seiland igneous province

The Ediacaran Seiland Igneous Province (SIP) is the largest complex of mafic-ultramafic intrusions in northern Fennoscandia and one of the few examples of a well preserved deep-seated magmatic plumbing system. The major gravity anomaly caused by the dense rocks of the SIP has been recently modelled indicating multiple deep roots located north of the Øksfjord peninsula. Magnetic forward modelling is applied to estimate the geometry and the magnetisation of the magnetic sources. The largest magnetic anomaly is located at the eastern side of the Øksfjord peninsula and far from the deep ultramafic roots of the complex. The modelled sources of the magnetic anomalies reach a maximum depth of 3 km and are related both to gabbroic bodies and to a lesser extent to the contacts of the ultramafic intrusions with country rock. Rock properties were analysed using the petrophysical database of the Geological Survey of Norway. Generally, SIP rocks have low natural remanent magnetisations (NRM) and Konigsberger ratios (Q) below 2. However, high NRM values are observed at the eastern side of the Øksfjord peninsula, where the NRM direction will strongly affect the magnetic anomalies and the modelling results. Due to the lack of NRM directional information, we modelled the effect of different NRM directions. Comparison with the magnetic anomalies indicated steep NRM inclinations. Most of the ultramafic rocks have high densities and low susceptibilities, with a few exceptions on the island of Seiland where tectonic processes and later alteration likely affected the magnetic properties. Modelling suggests the alteration at these locations is within a depth of 400 m. The occurrence of numerous metal deposits on the island of Stjernøya, and particularly around one of the roots of the SIP, suggests that the root could have acted as a preferential pathway for the fluids accommodating the precipitation of metal-bearing minerals

Magnetic mapping of fault zones in the Leka Ophiolite Complex, Norway

The island of Leka and surrounding skerries expose a complete suite of ophiolitic rocks, which are heavily faulted. Large areas consist of ultramafic rocks, which are locally hydrated and form serpentinites. Faults are commonly fluid pathways and can be areas of increased serpentinization. Because magnetite is a common product of serpentinization such fault zones add to the local magnetic response of the rocks. Here, ground-magnetic data were used in combination with aeromagnetic data to develop models over the major faults across the island. A mapping workflow was developed which uses tilted slabs to represent different zones of magnetization. The magnetic properties of surface-rock samples provided the constraints for the magnetic modeling. Sensitivity tests on the model showed the detection of magnetic fault zones to be limited to depths shallower than one km. Magnetic modeling allowed for an estimation of the magnetization of several major faults. The magnetic zone of one of the largest faults, which forms the boundary between gabbro and ultramafic rocks, had an enhanced magnetization over a width of approximately 200 m. The model is supported by both ground and aeromagnetic data: the first helped in refining the magneti- zation distribution at shallower depth, the latter allowed for modeling of the deeper part of the fault, indicating the geometry of a listric fault. The total magnetizations of the modeled slabs are well above the background magnetization of the Leka Ophiolite Complex (LOC) determined by modeling and on magnetic property data on >500 samples. This shift towards higher values indicates that serpentinization in some of the fault zones contributes significantly to the magnetic anomalies of the LOC