The three-dimensional crustal structure of the Faroe-Shetland region

A three-dimensional model of the Faroe-Shetland region has been constructed, which incorporates new seismic mapping, rock property models derived from a large database of geophysical well logs, and deeper structure based on gravity inversion. New gravity, magnetic and topographic compilations were produced which integrate BGS and released Faroese data. A large seismic database spanning the Faroese and UK sectors of the Faroe-Shetland Basin was used to map seabed, the top of the Palaeogene volcanic sequence and, where possible, base volcanics, base Cenozoic and top basement. Velocity logs and VSPs were used to derive velocity models that were employed in the depth conversion of these data. Published sources were used to extend the model into neighbouring basins (the North-east Rockall Basin, Møre Basin and Northern North Sea). Analysis of density logs was used to define compaction trends in sedimentary sequences and departures from these relating to overcompaction and lithological variation. A density model was derived for the volcanic rocks which reflected the differences between the massive flows observed on the Faroe Islands and volcanic sequences elsewhere which include a higher content of relatively porous material (weathered layers, hyaloclastites, volcaniclastics) and do display compaction trends. A method was developed for quantifying the influence of sills on the average density of the sequence into which they were intruded. The cover sequence geometries and rock densities were incorporated into a regional gravity model and inversion of short- and long-wavelength gravity anomalies was used to refine the forms of the top basement and Moho interfaces respectively. Density anomalies associated with the intrusive components of volcanic centres were incorporated in the model. Forward modelling was used to predict the magnetic signatures generated by the volcanic rocks and crystalline basement on the basis of simple magnetic property assumptions. The modelling results were assessed by comparison with wide-angle and normal incidence seismic results. The modelled top basement depths were converted into two-way travel time and integrated into the seismic workstation environment so that it was possible to interrogate the seismic database for evidence of any indications of structures that might have been missed without the guidance provided by the gravity model. The geophysical imaging and modelling results were analysed in a GIS and loaded into a customised viewer that facilitates the detailed comparison of multiple mapped and modelled layers, and the linkage of these to descriptive text. The modelling results provide a new view of the compartmentalisation of the Faroe-Shetland Basin, resolving the forms of a complex set of sub-basins and structural highs. On the western side of the basin its components are strongly influenced by a north-north-east structural grain whereas, in the east, north-east to east-north-east trends are more evident. The gravity signatures over the Munkagrunnur Ridge indicate a requirement for low density rocks beneath the volcanic sequence proven by the Lopra borehole. There is little evidence for deep basins beneath the Fugloy Ridge but basins are indicated further to the north-east beneath the Pilot Whale Anticline and the Møre Marginal High. The relatively subdued gravity expression of the Faroe Bank Channel Basin is attributed to the presence within it of a substantial volume of dense igneous rocks, including both extrusive and intrusive components. The results provide new insights into the igneous history of the region, identifying a set of normally magnetised, low density intrusions north of the Faroe Islands and beneath the Faroe Bank, which may be analogues of the post-breakup felsic intrusions observed in East Greenland. Although not a primary target, the successful resolution of basement structure beneath the Northern North Sea provides a useful test of the metholodology. This project has established a structural framework which should form a useful foundation