PLATETEKTONIKK

Confidential until December 1, 2008
A new kinematic model from the pre-breakup to present day has been developed for the Arctic-North Atlantic region. Using potential field data (magnetic and gravity), published seismic interpretation and geological records we have re-interpreted the continent ocean boundaries and transition zones. Seafloor spreading has been quantitatively determined and new palaeo-age grids have been constructed for the oceanic area. Kinematic parameters have been used in the case of a triple junction to estimate the errors of continent ocean boundary location. For the Jurassic-Cretaceous evolution of the Arctic we have explored several scenarios of oceanic basin evolution and used the predicted present day age configuration for estimation of crustal thickness. A new plate tectonic model of the North Atlantic-Barents Sea area have been used for modelling the Late Triassic-Early Jurassic compression in the Novaya Zemlya and eastern Barents Sea basins.

The African plate is affected less by subduction than other plates and, as a consequence its surface experienced less subduction-related deformation while its mantle contains fewer subducted slabs. This region therefore appears suitable to study the etfect of large-scale mantle density anomalies and flow on both present-day dynamic topography and past surface uplift and subsidence events. Dynamic topography quantifies the mantle component to Earth's surface topography and as such influences the formation of basins and natural resources. It is computed here from a mantle flow model with density variations infelTed from seismic tomography and radial mantle viscosity variations. In order to assess the robustness of tomography features, we have compiled here a total of 18 models (13 whole-mantle and 5 upper mantle). We have also computed a weighted mean tomography model that gives a fit between dynamic topography and observation-based "residual topography" substantially better than any individual model. To better understand discrepancies between dynamic and residual topography we compute their con-elations and ratios (I) regionally in caps of 30 degrees of arc and (2) spectrally as a function of spherical harmonic degree up until degree 31. We compare the observed con-elation and ratio of geoid and residual topography with the values "expected" from a geodynamic model. Correlations between residual and dynamic topography are typically high in a region centred on northeastern Africa. Ratios tend to be higher in the oceans than the continent, indicating substantial lateral viscosity variations in the asthenosphere. We compute models of past dynamic topography by backward advection of density anomalies. Combining these with models of African plate motion, we compute upl ift and subsidence of points moving with the plate. However, when interpreting these results it needs to be carefully considered at what depth the density anomalies that cause these uplift and subsidence are located. Results can only be considered if those density anomalies are not advected in or out of thennal boundary layers, such that the neglect of diffusion backward in time does not introduce a substantial error. To study the effects of including a deformable lithosphere with a pressure-and temperature-dependent rheology and a free surface, we apply pressures and velocities trom the mantle flow computations at the base of a model lithosphere. Results indicate little deformation in the African basins, such that the treatment with a pure mantle flow code appears valid there.

A compilation of published isotope geochronological data for Cenozoic volcanic / magmatic activity on the African plate and for part of the adjacent Antarctic plate was created within the African Plate project (TAP; collaboration between NGU and Statoil). This compilation covers the continental as well as the oceanic part of the African plate. Areas with documented recent or historic volcanic or magmatic activity are also included. In addition, geochronological data for selected Northern African sedimentary basins (Tadouenni, Kufra, Murzuq, Illizi, Sirte) were included. The vast majority of these data are from surface outcrops in areas immediately adjacent to these basins, rather than from the basins themselves (where few published isotope ages exist). Published data included in this database are obtained from the following isotope geochrologogical methods: Ar/Ar, K/Ar, Re/Os, Rb/Sr, Sm/Nd, Lu/Hf, FT, U/Pb, Pb/Pb and U-Th/Pb. Detailes on laboratory methodologies, age statistics, etc., are included as much as possible. For the Conozoic volcanics a subdivision into alkaline and non-alkaline rocks were made following a broad definition by Woolley (2001) as well as a much more narrow definition by Kevin C. Burke (including only nephelinite, nepheline syenite, phonolite and sövite).

The Jan Mayen microcontinent is a small, unique, crustal entity that was separated from the Norwegian and Greenland margins by two distinct rifting events. Seafloor spreading along the Aegir Ridge separated Jan Mayen (together with Greenland) from Norway by ca. 54 Ma. Cessation of seafloor spreading on the Aegir Ridge at ca. 30 Ma was more or less simultaneous with the onset of extension along the Kolbeinsey Ridge between Jan Mayen and Greenland. Break-up along the Kolbeinsey Ridge by ca. 23 or 20 Ma isolated the Jan Mayen microcontinent.

Continental fragments and microcontinents are blocks of continental crust rifted off of passive margins. These relatively unthinned regions (compared to the surrounding crust) offer a perplexing conundrum as to their tectonic history: why are these blocks relatively undeformed compared to the surrounding regions on the passive margin? In this report we review the crustal structure as revealed from deep crustal seismic studies of modern continental fragments and microcontinents. From this review, it is clear that magmatic underplating or plume and LIP emplacement are not essential to isolating continental blocks during rifting. Many continental fragments have thick crusts (> 20 km) with thin layers of overlying sediments, while other continental fragments are severely thinned (~10 km thick) and exhibit horst and graben topography in the upper crust. The wide variability in crustal thickness and structure of continental fragments and microcontinents suggests that many different tectonic processes can explain these features. Initial widespread intra-continental rifting, active upwelling from back-arc spreading, plume-induced rift jumping, inherited weaknesses in ancient suture zones, and shifting extension directions can all contribute to localizing deformation in the surrounding basins, thus separating continental fragments from the mainland.