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Browsing by study line "Fasta jordens fysik"

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  • Tuikka, Leevi (2023)
    On the hotter Paleoproterozoic Earth, the regime of plate tectonics was likely in transition, including features from the early Earth but having e.g. subduction already established. The different mode of tectonics affected also on the deformation on plate boundaries, on orogenesis, for example. Due to the increased lithospheric temperatures, the Paleoproterozoic orogens were hotter and lower. Such conditions made also HP-LT metamorphism rare. In addition to the different temperature conditions, the past tectonics pose another challenge. Largely the Paleoproterozoic rocks that can be accessed today, are the remnants of ancient middle or lower crustal layers, which have exhumed due to deep levels of erosion. Hence, a great amount of evidence of past orogens is erased. To overcome this issue, geodynamic modeling is used to build set of 19 continent-continent collision models, with the temperature conditions ranging from the Paleoproterozoic to the Phanerozoic. Additionally, P-T-t paths are recorded in the models for comparison of pressure-temperature conditions using pseudosection diagrams. Another significant quantity governing the deformation on collisional continental margins, is the angle of convergence obliquity. With roughly 60° obliquity, full strain partitioning should be triggered in a orogen, forming a strike-slip fault. In the models, different temperature cases with the obliquity angle are varied. The geodynamic modeling software to produce the models was DOUAR, a 3D thermo-mechanical code coupled with erosion model FastScape. The initial models with 35 km thick crust were not able to produce proper strain partitioning due to low resolution, so another set of models with 45 km thick crust was run. On top of that, the lower crustal strength was varied. Outcome of the thicker crust was wider orogens, and hence more space for strain partitioning to develop. However, strain partitioning was not able to be preserved that well for the whole 40 Ma, which was the runtime of the models. Though in terms of strain partitioning, the results were not ideal, the angle of obliquity affected on the crustal shear zones as well, in a interesting way.
  • Lakshika, Palamakumbure (2022)
    Space weathering can be defined as the combination of physical and chemical changes that occur in material exposed to an interplanetary environment on the surface of airless bodies. This process produces amorphous surface layers often containing small opaque particles such as nanophase metallic iron (npFe0). This darkens the topmost layer resulting in alterations in material spectroscopic features.Eventually it can lead to misinterpretation of remotely sensed data in the visible- near-infrared (VIS-NIR) spectrum. The goal of this research is to simulate solar wind effects on asteroid spectra through low energy 1 keV hydrogen ion irradiation of meteorite powder samples and measure the changes in their reflectance spectra. This allows to understand how space weathering depends on the mineralogy of the material. We used Bjurböle (L/LL4), Avanhandava (H6) and Luotolax (Howardite) meteorites. H+ ion irradiation was carried out on powdered samples compressed into pellets. The pellets were placed into a vacuum chamber with pressure between 1.2 x 10 -7-2.4 x 10 -7 mbar for the whole experiment. To simulate solar wind irradiation, H+ ions were used with 1 keV under three fluences; 1 x 1017, 2 x 1017 and 5 x 1017 ions/cm2. Subsequently reflectance spectra of the samples were measured and processes using Modified Gaussian Model (MGM) to derive key spectral parameters. Both chondrites show significant reddening in the VIS region. Bjurböle being an LL, it is more oxidized than Avanhandava. The reddening in the NIR region is more significant in Avanhandava than in Bjurböle. My work indicates that even for low-energy solar wind conditions, the chondritic materials (Q/S-type asteroids) with high olivine content and/or higher fayalite (Fa) compositions are more susceptible to silicate absorption bands reduction. Luotolax meteorite being howardite rich in orthopyroxene and clinopyroxene, shows VIS reddening but not observable band depth changes with increasing exposure to H+ ion irradiation. The smaller change in Luotolax may be due to higher pyroxene resistance to low-energy ion irradiation. Overall, at short timescales and typical solar wind energies, VIS slope reddening is the most dominant factor in all three material compositions.
  • Kolehmainen, Kauri (2023)
    A new three-dimensional crustal and upper mantle P-wave velocity model and a Moho depth map of Finland and the surrounding area were constructed using kriging interpolation. The models are based on the latest wide-angle reflection and refraction (WARR) data from the Fennoscandian shield. The Moho depth map agrees with previous Moho maps but also shows new details in the large Moho depression in central Finland compared to the previous Moho maps. The new Moho features include a new Moho depth local maximum near the center of the depression and increased Moho depths extending to the northwest and south of the depression. The three-dimensional crustal and upper mantle P-wave velocity model differs from previous models by showing a non-existent high-velocity lower crust beneath the Wiborg Rapakivi Batholith. Both the Moho map and the velocity model exhibit distinct features within the tectonic provinces in the Fennoscandian Shield. The uppermost velocity model layer is shown to roughly correlate with general features of surface geology. Statistical analysis of Moho depth and P-wave velocity data was performed, and variogram models were fitted to capture spatial autocorrelation. Ordinary kriging was used to generate a Moho model with a 25 × 25 km grid cell size. The three-dimensional P-wave velocity model was constructed in two parts, with separate universal kriging schemes for the crustal and upper mantle velocities. The velocity model has a grid cell size of 50 × 50 × 1 km in the uppermost 4 km and a lower resolution of 50 × 50 × 2 km below 4 km depth. The presented models can be utilized for a variety of applications, including seismic source location, crustal effect correction for seismic tomography and teleseismic studies, and general modeling of large-scale tectonic processes.