ISOSTASI

Neotectonic crustal deformations have been reported at a large number of locations in Norway (both on local and regional scales). The NEONOR Project represents a national effort by several national research and mapping institutions to investigate these phenomena through a multidiciplinary approach. Both the industry and the Norwegian Research Council contribute with major financial support. The main objectives of the NEONOR. Project are to systematically collect data, and to provide answers to the questions: 1) How can recent crustal deformation be characterised in time and space? 2) What processes cause the neotectonic crustal deformations? 3) What are the implications for migration and occurrence of fluids (especi- ally hydrocarbons and groundwater) in bedrock? 4) What are the implications for geohazard related to constructing sensitive installations like pipelines, gas-terminals and hydropower-plants. The initial phase of the project includes a systematic collection and charac- terisation of known deformations. New geological, geodetic and seismological data are, however, also acquired.

The crustal structrue in the outer Vøring Basin is investigated by forward modelling of gravity data and interpretation of the magnetic field and the isostatic state. In addition all available magentic and gravity data for the Vøring Basin have been reprocessed. BP delivered depth-converted interpretation along six seismic profiles. These, together with the available results of OBS arrays, constrain the geometry of the structural models. The modelling has been done in 2D and where needed in 3D, Especially in the central and northern Vøring Basin, lateral changes in the density structures perpendicular to the seismic profiles had to be regarded to adjust the models.
The 2D and 3D models show that the gravity fields is mainly influenced by the thickness of the sediments, the depth of the Moho and the occurrence of magmatic underplating. The high density magmatic underplated material is probably related to the intrusion of sills from the Vøring Escarpment into the Vøring Basin and stops westwards of the Fles Fault Complex. To the east of the Fles Fault Complex normal lower crustal material is present instead.
Seismic results show in the whole area deep crustal reflectors, which have been previously interpreted to be connected to magmatic underplating or top basement. The modelling results indicate that these reflections are connected to magmatic underplated structures and intracrustal reflectors. The modelling results are subsequently used to produce new maps of the depth to Basement and Moho.
The shape of the magnetic field in the outer Vøring area, suggests, in contrast to the gravity field, shallow sources, which are probably connected to igneous rocks. But in general the magnetic signature in the outer Vøring Basin shows no prominent anomalies. The main magnetic anomalies are caused by the transition to oceanic domains.

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.

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.

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.

The main objective of the project was to model isostatic response of Cenozoic glaciations, sedimentation and erosion on the Norwegian and Barents Sea continental shelves, and to constrain consequences for petroleum systems. In order to do so, a wealth of new data from Russia, from the Barents Sea and from the North Sea regarding glacier distribution in time and space, and glacier erosion and sedimentation has been collected and analyzed. These data together with other relevant available data, provided input to the modeling task. The main results ofthe project are given in the executive summary below (p. 2).