Deep structure of the Mid-Norwegian shelf and onshore-offshore correlations: Insight from potential field data

Abstract

Maps of the depth to the crystalline basement and to the Base Cretaceous, and of the pre-Cretaceous sedimentary thickness have been constructed. These maps illustrate the deep geology of the study area. In addition to showing the total thickness of sedimentary basins (>12 km in the Møre and Vøring basins), the maps also show a lucid picture of structural relief, both at basement depth levels and in the pre-Cretaceous sediment thickness map. Buried rift valleys are seen in the maps. In the platform area, a picture of horsts and grabens emerges that, in most areas, is probably related to the combined late Palaeozoic to early-middle Mesozoic structuring of the area. The geometry of structural highs and lows shown in our maps is the combined result of several rifting events that were superimposed on the Devono-Carboniferous, late- to post-orogenic extensional collapse structure. We think that the NW SE structural grain on the shelf reflects the offshore continuations of Devono-Carboniferous shear zones and detachments that are observed onshore. Locally, these detachments were reactivated as transfer zones. Movements along these shear zones have determined the position of boundaries between the magnetic basement terranes (primarily Precambrian high-grade rocks) and the less magnetic Caledonian allochthons. From our maps, it can be seen that ‘old’ sediments must occur within Permo-Triassic basins and Jurassic basins, not only on the Trøndelag Platform (previously well documented), but also farther west, beneath the Cretaceous and Cenozoic sediments of the Møre and Vøring basins. The basement topography reveals a narrow, deeply buried, rift-related relief (Jurassic rift valleys), and a wide basinal area recording prolonged subsidence in the mid and late Cretaceous, as well as the Tertiary and Quaternary subsidence. The difference between the basement map and the Base Cretaceous map supports the idea that the Cretaceous sedimentation represents a post-rift thermal and isostatic subsidence stage resulting in the infill of a pre-existing rift topography. Comparision of the Base Cretaceous and the basement maps shows a close correspondence of trend features, demonstrating a basement influence through to the Cretaceous. The topography and the offshore basement map demonstrate similar tectonic trends, probably indicating that a basement similar to that in western Scandinavia underlies the marine areas, and that several tectonic events have affected both the land and the sea areas. Highly magnetic, granulite-facies, felsic rocks give rise to strong magnetic anomalies along the Precambrian gneiss terranes of the coastal zone in central-northern Norway. The occurrence of these magnetic rocks is structurally related to the Devonian shear zones and the basement antiforms. We recognise these antiforms and synforms in the offshore, thereby providing a model for the distribution of low-magnetic Caledonian rocks and high-grade Precambrian intermediate rocks in the shelf areas. The rhomboid-shaped geometry seen in the basement map is partly due to the Devonian extensional collase structures and partly to N-S and NE-SW oriented faults active during post-Devonian rifting. Major trends responsible for the rhomboid-shaped geometry (NW-SE, N-S and ENE-WSW to NE SW) are also found in the topography of the Precambrian and the Cambro-Silurian basement units of western Scandinavia, suggesting that Precambrian faults have been reactivated both on land and in the offshore. The Møre-Trøndelag Fault Complex (MTFC) is suggested to consist of two branches in the offshore. One branch is the extensional fault, located east of the Slørebotn Sub-basin sub-parallel to the coastlines, and the other one is the fault alignment from Hitra towards Shetland.