Geophysical Mapping and Paleomagnetic Study of the Leka Ophiolite Complex
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This thesis presents the results of a geophysical study of the Leka Ophiolite Complex (LOC) in central Norway, carried out to refine the understanding of the LOC’s sub surface geometry, fault structure and structural evolution history. The LOC is part of the Upper Allochthon of the Norwegian Caledonides and one of the most complete as well as one of the oldest ophiolites found in Norway. The LOC has more than 50% of rock exposure and is a superb study area for the ancient oceanic crust and the effects of serpentinization. The first manuscript of this thesis revises earlier geophysical models derived from gravity modeling. The database was expanded with new aeromagnetic data and a large petrophysical database. The interpretation resulted in three profile sections derived from 2.5D gravity and magnetic modeling, which indicate that the LOC has a bowl-shaped, synformal structure. The orientation of the major normal fault between the ultramafic units and the gabbro was found to be dipping towards the southeast at approximately 45°. It is suggested that this major fault is serpentinized, which explains the distinct magnetic anomaly along the trace of the fault (magnetic anomaly of up to 2800 nT). It was concluded that the entire LOC could not be uniformly serpentinized as a surface average density between 2600 and 2900 kg/m3 for the ultramafic rocks applied for the entire complex would result in a modeled depth of 30 km by gravity modeling, which is geologically unreasonable. Consequently, the LOC needed to be denser and less serpentinized at larger depths. With the modeling, the depth to the base of the LOC was estimated at 4 km whereas the layer of enhanced serpentinization to a depth of less than 1 km. The volume of the LOC was estimated to approximately 200 km3. The second manuscript addresses the small-scale variability of magnetization associated with fault zones. Ground magnetic data was used in combination with aeromagnetic data to develop models over major fault zones. A mapping workflow was developed which uses tilted slabs as a model for different zones of magnetization in a fault zone. Local averages of magnetic properties of rock samples were used for the host-rocks outside the fault zone, and as starting values for modeling and inversions. Both magnetic susceptibility and natural magnetic remanence were modeled. The magnetic zone of one of the most significant faults, which forms the boundary between gabbro and ultramafic rocks, expressed an enhanced magnetization over a width of approximately 200 m. Comparison with aeromagnetic data documented the gain of using ground magnetic data, because the magnetized zone forms a broad aeromagnetic anomaly high without details, even after using a tilt derivative filter. Finally, the combined modeling of ground- and aeromagnetic data of this large fault allowed for the development of a model for its deeper part indicating the geometry of a listric fault. Since ground magnetic data existed only along selected pathways, the modeling results were extended into the map plane by correlating the magnetization models with lineaments on LiDAR elevation data and anomalies of aeromagnetic data. Finally, comparing magnetic attribute averages of rock samples with modeled attributes showed, that the averages of the modeled total magnetizations are clearly above the background magnetization of the Leka Ophiolite Complex. It was concluded that serpentinization in fault zones constitutes a large part of the magnetic anomalies of the Leka ophiolite complex. The third manuscript presents a paleomagnetic study, starting with the analysis of natural remanent magnetizations (NRM) of the rock samples from 108 sites. Only the ultramafic rocks carried significant NRMs, while the gabbro and basalt have weaker NRMs and no characteristic remanent magnetization. Thermomagnetic and magnetic hysteresis measurements suggest that the gabbro and basalt are mostly composed of paramagnetic minerals. Both the layered series (lower crust) and the harzburgite (mantle) have a wide range of NRM intensities and susceptibilities with mostly positive inclinations. The primary carrier of magnetization is magnetite. The Curie temperature for most of the samples is between 530 and 575°C. Eightyfour sites were collected within ultramafic rocks. However, only 46 sites yielded reliable, stable results. The paleomagnetic pole was calculated to be located at 3.5°S, 7.9°E (A95=8.3°) after tectonic correction based on the flowtops of the pillow basalts of the island of Storøya. This pole is adjacent to a Baltica pole from ~440 Ma, which suggests that the magnetization was acquired during the Caledonian orogeny as the complex was obducted and transferred onto Baltica from Laurentia, and not, when the rocks crystallized in the late Cambrian.