Using geophysics to follow and model the Precambrian basement terranes beneath the Caledonian nappes in Finnmark, northern Norway: A case study

Abstract

The bedrock geology of Finnmark, northern Norway, consists primarily of rock complexes dating back to the Precambrian and early Mid Palaeozoic eras and comprises (1) a mid-crustal lithospheric basement which is characterised by granitoid and greenstone complexes dating from Neoarchean to late Paleoproterozoic in age; (2) the Caledonide Orogen, characterised by a series of nappes and thrust sheets. Finnmark forms part of the northern margin of the Fennoscandian Shield, which extends across northern Finland into northwestern Russia. Precambrian rock complexes are also found as thrust slices in Caledonian nappes at several places in Finnmark and Troms, as well as in tectonic windows.Recently acquired high-resolution aeromagnetic data have provided spectacular and confirmatory evidence for the continuation of the diverse Precambrian greenstone belts and granulite terranes beneath the magnetically transparent Caledonian nappes.This study aims at analysing this subsurface continuation of the terranes using potential field data and estimating the depths of the main magnetic sources beneath the nappes. In this study, several geophysical methods are used, including 3D Euler deconvolution and 2D Werner deconvolution. We have focused specifically on the depth-to-basement beneath nappes and purposefully excluded anomalies associated with shallow structures, such as dykes and faults. Different methods have produced similar results and indicate an appreciable increase in the depth of the Precambrian basement rocks from south to north. In the south, Precambrian basement rocks are either exposed on the surface southeast of the Caledonian front or reappear within tectonic windows. A prominent positive magnetic anomaly on the Porsanger Peninsula is thought to represent an extension of the anomaly ascribed to the greenstone belt of the Repparfjord Tectonic Window. A 3D model of this has also been provided. According to estimates, the depths-to-basement below nappes close to the Caledonian front range from less than 500 to 2000 m, whilst in the northern part of the study area the depths generally reach 3000 to 4000 m, and are locally estimated to range between 5000 m and 6000 m.