The Nazca-South America convergent margin is marked by the presence of the Andean mountain belt, which stretches along the 8000-km long western margin of the South American plate. The subduction zone is characterized by significant along-strike changes in both upper plate structure and slab geometry that make it an ideal region to study the relationship between the subducting slab, the surrounding mantle, and the overriding plate. This dissertation summarizes the results of three finite frequency teleseismic tomography studies of the central Nazca-South America subduction zone which improve our understanding of how along-strike variations in the Andean mountain belt and the subducting Nazca plate interact with each other and with the surrounding mantle. This is accomplished by first focusing on two smaller adjacent regions of the central Andes to explore upper mantle variations and then by using a combined dataset, which covers a larger region, to image the deeply subducted Nazca slab to investigate the fate of the slab. The first study focuses on the central Andean upper mantle under the Altiplano-Puna Plateau where normally dipping subduction of the Nazca plate is occurring (18° to 28°S). The shallow mantle under the Eastern Cordillera is generally fast, consistent with either underthrusting of the Brazilian cratonic lithosphere from the east or a localized "curtain" of delaminating material. Additional evidence for delamination is seen in the form of high amplitude low velocities under the Puna Plateau, consistent with proposed asthenospheric influx following lithospheric removal. In the second study, we explore the transition between normal and flat subduction along the north edge of the Altiplano Plateau (8° to 21°S). We find that the Peruvian flat slab extends further inland along the projection of the Nazca Ridge than was previously proposed and that when re-steepening of the slab occurs, the slab dips very steeply (~70°) down through the mantle transition zone (MTZ). We also tentatively propose a ridge parallel tear along the north edge of the Nazca Ridge. Both of these observations imply that the presence of the Nazca Ridge is at least locally influencing the geometry of the flat slab. The final study investigates along-strike variations in the deeply subducted Nazca slab along much of the central Nazca-South America subduction zone (6° to 32°S). Our results confirm that the Nazca slab continues subducting into the lower mantle rather than remaining stagnant in the MTZ. Thickening of the slab in the MTZ north of 16°S is interpreted as folding or buckling of the slab in response to the decreased slab sinking velocities in the lower mantle.

The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P-wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. Shear-wave splitting and P-travel time residual analyses also detect anisotropic mantle structure. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Joint inversion using both the P and S data reveals 3D self-consistent anisotropic models with well-defined mantle lithospheric domains. These domains of differently oriented anisotropy most probably retain fossil fabric since the domains' origin, supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. The possible disturbing effects of anisotropy on seismic tomography studies are investigated, and found to be potentially significant. P-wave arrival times were adjusted based on the estimates of mantle anisotropy, and re-inverted. The general pattern of the velocity-perturbation images was similar but changed significantly in some places, including the disappearance of a slab-like structure identified in the inversion with the original data. Thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images, and should not be ignored. If, as we believe, our estimates of anisotropy are reasonably correct, then the model based on the adjusted data should give a more robust and correct image of the mantle structure.

Abstract The crustal and upper mantle structures of the Shield on the regional scale were investigated using the data of the POLENET/LAPNET passive seismic array and the previously published models of active and passive seismic experiments in the study area. This area is centred in northern Finland and it extends to surrounding areas in Sweden, Norway and northwestern Russia. The bedrock there is mostly of the Archaean origin and the lithosphere of the region was reworked by two orogenies during Palaeoproterozoic. One of the results of the thesis was a new map of the Moho depth of the study area, for which new estimates of the crustal thickness were obtained using receiver function method and complemented by published results of receiver function studies and controlled source seismic profiles. The map differs from the previously published maps in two locations, where we found significant deepening of the Moho. The 3D structure of the upper mantle was studied using teleseismic traveltime tomography method. The resulting model shows high seismic velocities below three cratonic units of the study area, which may correspond to non-reworked fragments of cratonic lithosphere and a low velocity anomaly separating these cratonic units from each other. The regional scale studies were complemented by two smaller scale studies in upper crust level using combined interpretation of seismic profiling and gravity data. These studies were centred on Archaean Kuhmo Greenstone Belt in eastern Finland and central Lapland in northern Finland located in the crust reworked during Palaeoproterozoic. Both areas are considered as prospective ones for mineral exploration. Both studies demonstrate the advantage of gravity data inversion in studying 3D density structure of geologically interesting formations, when the Bouguer anomaly data is combined with a priori information from petrophysical and seismic datasets.