3D geophysical and geological modelling of the Karasjok Greenstone Belt
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The Karasjok Greenstone Belt (KGB) is a linear segment of rift-related metavolcanic and metasedimentary rocks and forms the northeastern prolongation of the Paleoproterozoic Central Lapland Greenstone Belt in the Fennonscandian shield. Due to the few exposures and lack of surface data data in the Finnmarksvidda area, the internal structures within the KGB are poorly understood. This thesis is therefore a part of the project Mineral Resources in Northern Norway (MINN), governed by the Geological Survey of Norway (NGU). Its purpose is to integrate geophysical and geological data in order to present a new 3D crustal model of the northern part of the KGB, to better understand the crustal architecture. The new 3D model presented in this thesis covers an area of 20 x 30 km, to depths of 10 km, just north of Karasjok. It is based on 3D density modelling of newly acquired high resolution Airborne Gravity Gradient data, integrated with new geological field observations and supplemental petrophysical data. As a first step in developing the new 3D model, a qualitative structural interpretation of curvature analysis of gravity gradient and aeromagnetic data, was followed up by field mapping of key structures. The results from these investigations were used as inputs to constrain the 3D model. This multiscale approach has allowed establishing a link between near-surfaces and deeper, regional structure of the belt. By utilizing the second and third order invariants of the gravity tensor during the modelling, it has been possible to constrain the 3D model shapes and geometries of the main structures within the KGB. The results of the study show that the KGB makes up an east-dipping crustal feature, positioned between the Archean Jergul Gneiss Complex (JGC) and the Paleoproterozoic collisional mélange, the Tanaelv Migmatite Belt (TMB). The model suggests that the KGB is rather shallow, making up a 3 km deep structure. The first order structures were caused by west-vergent thrusting, formed during a regional $D_1$ deformation phase. These structures reflect imbricate stacking and internal folding, reassembling a fold-and-thrust belt. Later deformation phases reactivated these thrust structures, initiating transpressional and strike-slip movement, in addition to folding. Steep NE-SW trending faults have been identified as important structures, indicating a prolonged deformation history. These faults are attributed to complex dextral rotation, explaining the arcuate structural grain towards the southern part of the study area. The developed 3D crustal model suggests that the volcanic units within the Bakkilvarri Fm. are less voluminous than previously interpreted. The volcanics are concentrated in the eastern part of the belt, forming a NW-SE linear structure. The 3D model shows that outcropping tonalities in the northeastern parts of the area are recognized as crustal scale structures, either representing thrust emplaced basement complex, or a deformed intrusive unit. This complex explains the circular-shaped negative gravity and magnetic anomaly. The mafic and ultramafic intrusions are generally shallow features, and were most likely emplaced before, or during an early stage of the main thrusting. This study shows that careful integration of geological and geophysical data can vastly improve the 3D understanding of complex, poorly exposed terranes. Integrated 3D density modelling using airborne gravity gradient data and rotational invariants helps to refine and establish realistic 3D subsurface models.