KONTINENTALSOKKEL

Detailed study of the gravity field and the isostatic state of the Barents Sea Region shows that the Eastern Barents Sea basins are not typical rift basins. They show distinctive features as large wavelengths, high lithospheric mantle density, thick sequence of sediments, a flat Hoho and high elastic thickness, which are normally associated with cratonic or intracratonic basins. To understand the underlying cause of the Eastern Barents Sea basins, we make a comparison to well studied cratonic basins. For these basins, the structure, subsidence history and temperature evolution is relatively well known. We study the West Siberian basin, as an adjacent large-scale basin, the Michigan basin in North America, the Solimoes, Amazon, Parnaiba and Parana basins in South America, the Tarim basin in Central Asia and the Congo basin in Africa. The analysis includes the characterization in terms of gravity signal, geoid undulations, isostatic state, age and igneous activity.

A high sensitivity aeromagnetic survey, BAS-06, was carried out in an area of 46530 km2 from the Finnmark Platform to the Bjarmeland Platform in the Barents Sea. Data processing comprised spike removal and data editing, systematic (IGRF and lag) corrections, statistical, median and micro levelling. Several potential field maps were produced along the survey area. Examples of various filters applied to the magnetic field have been illustrated. Trend enhancement and a preliminary interpretation of the survey have been carried out.

The West Siberian Basin covers an area of ~3.2 x10(6) sq.km and is among the most extended basins in the world. Recent investigations have revealed that the basin contrains and extensive layer of flood basalts of late Permian-Triassic age, which have been set into relation to the basalts of the Siverian traps. In the northern part of the basin, the basalts overly older sediments that reach locally over 15 km in thickness. Our work aims at reducing the observed gravity field to the basement level, estimating the contribution of the sediments and of the basalt layer to the gravity field. Published seismic sections with well-calibration are used for constraining the sediment isopachs and for estimating the density-depth functions. We also make use of published models on crustal thickness and besement depth and the gravity field derived from the integration of the satellite mission GRACE with terrestrial gravity measurements. The resulting 3D-density model is used for inferring density anomalies in the lower crust and upper mantle and allows calculating the total load acting on the crust and estimating the isostatic state of the region. A key question related to the formation of the basin is, whether a high density anomaly in the crust or upper mantle has contributed to the large scale subsidence of the basin, as has been postulated for other large scale basins. The lower crust shows considerable density variations, that allow making a segmentation of the basin into four blocks, the southern, mid, northern and north-western segments. We identify several rift structures and estimate the amount of basalt-filling. The eatern part of the basin towards the Siberian Platform shows an evident arch-shaped density increase in the lower crust, which is coincident with a rift extending for more than 1500 km length and bending into the Yenisey-Khatanga trough.

For the recently awarded Statoil block 02/06, offshore West Ireland, gravity and magnetic data of the area from Statoil's DRAGON database were assorted to give a comprehensive overview of what data are already on hand. For the area of interest eight folders with gravity and 12 folders with magnetic data were extracted and displayed.
For both gravity and magnetic, the different surveys were evaluated in terms of data quality and resolution. For gravity data, a compilation of the requested surveys already exists, whereas for the magnetic the corresponding datasets were merged in one map considering the evaluation results.
The datasets on hand already cover almost the entire Statoil block 02/06 and its surroundings to the west. Towards the east and for the eastern most part of the blocks themselves, new data need to be purchased or acquired. Therefore, an overview of additional available data is presented and evaluated based on the available information. An outlining of the eastern basin edges is not permitted on the base of the high-resolution data sets.
All maps and compilations are produced with Geosoft Oasis Montaj and presented in an Esri ArcGIS project. Grids are provided in Gosoft Oasis Montaj format.

The objective of this study was to investigate methods to resolve the problem of mapping basalts and basement in a continental margin setting. It is well known that basalts may trap considerable amounts og hydrocarbons accumulated in sediments below the basalts. A 3D model of the Møre margin offshore Norway was constructed by a forward modelling employing the gravity and magnetic fields. Numerous independent data were used to diminish the nonuniqueness of the method. The resulting model consists of several horizons that divide the main geological bodies in the study area. In addition, two horizins are based on reflection seismic interpretation only. Based on gravity/magnetic modelling and seismic interpretation, a layer model was constructed in RMS. (...)

The KONTIKI final report summarises the results from the three-year KONTIKI project (CONTInental Crust and Heat Generation In 3D) established by NGU and StatoilHydro to improve the understanding of heat flow along the Norwegian continental margin. A compilation of c. 4000 new and old geochemical data is used to produce the first map of radiogenic heat production in bedrock, covering large tracts of Norway. In general granitic rocks have higher heat production than intermediate and mafic rocks. Sedimentary rocks display variation in heat production that can be ascribed to sedimentary processes. Age and metamorphic grade do not appear to affect heat production significantly, whereas tectonic setting appears to impart some effect in that extensional or within-plate rocks have higher heat production than similar rocks formed along plate margins. A total of 11 deep wells (c. 350-1000 m) have been established for temperature logging and heat flow calculations in mainland Norway. Calculated palaeoclimatic corrections range fro +2 to +9 mW/m2 (i.e. 5-20% of original heat flow values). The new heat flow map shows that heat flow values in Norway are much higher than previously proposed. The two most robust features on the map are the relatively high and low heat flows in the Oslo Region and Nordland respectively. New geochronological data from basement samples from the North Sea and Norwegian Sea show that the upper basement to a large degree consists of magmatic and metasedimentary rocks that can be correlated with rocks in tectonostratigraphically high (i.e. Upper and Uppermost Allochthons) nappes within the Scandinavian Caledonides. Both 3D crustal and thermal models have been produced from the Mid-Norwegian continental margin. Our 3D modelling suggests that substantial heat flow variations between 35 and 65 mW/m2 occur at the base of the sedimentary basins in the inner parts of the mid-Norwegian Margin. We show that the structure and nature of the basement underlying these basins are the most likely factors influencing heat flow variations at typical wavelengths of 10s to 100s of km. One of the major outcomes of the modelling and the heat flow and heat production data collected on land, is that the Moho heat flow should range in between 30 to 40 mW/m2, a value in the lower part of this range being the preferred one. The temperature of the outer Nid-Norwegian margin is lower than previously assumed and the petroleum potential of the area should be considered.

The Barents Sea is characterised by deep basins in the western and eastern Barent Sea, and large-scale differences in the lithospheric structure, which reflect the different tectonic history and basin forming processes. The western Barents Sea is associated with typical rift basins, while the eastern Barents Sea features large-scale megabasins, which are not typical rift basins. Isostatic as well as seismological studies point towards a heterogeneous upper mantle with high-density material underlying the megabasins. A global study of large-scale basins shows that isostatic balance is often achieved by densification of the lower crust or upper mantle. These structures are expressed in geoid anomalies, but are associated with small density contrasts, which make detailed imaging difficult. However, the high-density structures in the upper mantle appear to have a generic link to the basin formation.
The anomalies in the Barent Sea point towards the presence if an intra-crustal intrusive along the transition zone between the rift basin setting to the megabasin setting, which is also visible in the gravity signal and isostatic results. While the eastern Barents Sea appears to be under Timanian influence, the western Barents Sea is clearly influenced by Caledonian tectonic events and the Mesozoic break-up of the North Atlantic region, as also evident from detailed basement models based on seismic and potential field interpretation. Studies of regional isostatsy are as yet not conclusive, but show the importance of the intra-crustal density distribution in models at the lithospheric and basin scale.

Integrated 3D density and isostatic modelling show that a large difference exist between the Eastern and Western Barents Sea in terms of physical properties in the crust and underlying mantle. To constrain our analysis we make use of a 3D density model based on the velocity model BARENTS50. The density model provides information about the crustal configuration, e.g. the Moho and the loading of the crust including all internal density variation. The calculated gravity anomalies (computed from these density variations) cannot be adjusted to the observed gravity field. Therefore the effect of the Earth's curvature on the gravity calculation was investigated by a coordinate transformation and projection of the 3D model into a spherical 3D model. The error between the modeled and observed gravity remains significantly large. The missing masses, which are needed to minimize the difference, are supposed to be located not only in the crust but also in the mantle. High density material (>3300 kg/m3) is needed below the Eastern Barents Sea in order to isostatically balance the masses from the thick crust and also to fit the observed gravity field. The isostatically calculated mantle densities correlate well with other results and confirm the large difference between the Eastern and Western Barents Sea.
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A high-resolution aeromagnetic survey (RAP-07) was carried out in an area of ca. 158 km2 over the northeastern part of the island Andøya in Vesterålen, northern Norway. Data processing comprised spike removal and data editing, IGRF correction and diurnal variation correction as well as microlevelling. Four maps were produced for the survey area. One of these shows magnetic total field anomalies after correction for the IGRF of 2005. Two maps show residual magnetic values after application of two different high-pass filters. A map showing magnetic TILT derivative is also presented. All maps were evaluated in terms of geological relevant information and interpreted.