Integrated analyses and interpretations of regional scale magnetotelluric and potential field data from the Precambrian crust of northern Sweden
Abstract: The Norrbotten region in northern Sweden, as a part of the Fennoscandian (Baltic) Shield, is one of the most active mining areas in Europe. It is acknowledged that geological structures such as faults, shear zones, and associated fracture systems play a key role in providing a physical pathway connecting metal sources and the sites of mineral precipitation. In particular, several magnetotelluric (MT) surveys around the world have revealed a spatial relationship and correlation between mineral occurrences and regional scale conductive structures. Previous lithospheric studies in the Fennoscandian Shield have indicated enhanced conductivities in the Norrbotten region, and they served as a basis for the proposal to test the hypothesis of this genetic connection. Therefore, the current project was initiated with the aim of obtaining more detailed information on crustal and upper mantle structures in the Paleoproterozoic Norrbotten ore province by utilising a combination of regional geophysical data including, MT and potential field data. Potential field data (airborne magnetic, and ground gravity) are extensively used as the first choice in geophysical/geological mapping due to good areal coverage, whereas MT surveys are typically designed to answer more specific geological and geophysical questions. In order to improve utilisation of the geophysical methods, quantitative analyses and integration approaches have been developed and applied to the geophysical data.This thesis covers four topics: (i) a regional-scale study of Norrbotten County using 3D inversion of newly measured MT data to reveal crustal geoelectric structures, (ii) integration of the resistivity model with regional-scale magnetic susceptibility and density models using an unsupervised classification approach to extract and analyse the possible correlation between modelled petrophysical properties and provide a geophysical domain classification as input to geological interpretations, (iii) identification of a regional-scale tectonic feature in northern Sweden by processing and analysing the potential field data, (iv) the development and evaluation of the feasibility of a classification approach designed to identify patterns in potential field data that are indicative of the type of geological environment.The conductivity model of inverted new MT data reveals the presence of strong crustal electrical conductors. The conductance of thousands of Siemens within a generally resistive crust is modelled. Some of the conductors in the model have near-surface expressions and are spatially correlated with the location of known mineralisation. A significant part of middle crust conductors is elongated in a direction that coincides with parts of major deformation zones. The derived conductivity model provides a new insight into the tectonic unit boundaries in the area.The magnetic susceptibility and density models were subsequently integrated with the results obtained from the MT data. The integration approach is based on a joint analysis of the three modelled physical properties by using an unsupervised classification approach referred to as Self-Organising Maps (SOM). The depth variations of the properties were included in the classification. The obtained domain classification is discussed with respect to the previously mapped/interpreted geological units and compositions. Some discrepancies between existing geological maps and the domain classification are noted for some areas. The discrepancies may partly be related to the fact that the surface geological features are compared with geophysical models that also include information at depth.A detailed analysis of high-resolution potential field data in northern Sweden led to the discovery of a regional-scale strike-slip fault, which we refer to as the Norrbotten Mega Fault (NMF). The presence of a fault cutting through the entire area in a roughly N5E direction with horizontal displacement of 51.2 km can be traced from Karesuando at the Swedish-Finnish border in the north to the Archaean-Proterozoic boundary, which is marked by the Luleå-Jokkmokk Zone roughly 250 km towards the south. Altogether, the length, apparent displacement, and straight character of the proposed fault suggest that it represents a late-orogenic, brittle fault. The initial identification of the location of the fault was primarily based on visual inspection of potential field data. The estimation of the displacement and validity of the proposed NMF is supported by analyses of several higher-order spatial derivatives of the potential field data. The analyses included application of a neural network SOM classification and a visualisation approach with subsequent matching of well-defined anomalies and anomaly patterns.The application of SOM for pattern recognition and classification were developed further with respect to analysis of potential field data. This involved various experiments on data feature definitions and extraction. Data features that capture information about spatial variations, both depth-wise and horizontally, are used as input to the SOM for domain classification. The feasibility of the methodology and the limitations are discussed with respect to the results derived from a synthetic model. The proposed approach is furthermore applied to measured data from the entire Norrbotten area.The models derived from the geophysical data, as well as the new integration and classification approaches presented in this study, may be applied as a guidance for future exploration and investigations in the area, such as for planning or expanding geophysical surveys in high prospective areas, mapping and interpretation of the geology, mapping the mineral prospectivity or other prediction tasks, or used as a guidance for follow-up on geological and geochemical work.
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