Seismic imaging of deep crustal reflectivity in Sweden and Iceland

Sammanfattning: Seismic reflection profiling, originally developed by the petroleum industry, was adapted to systematic imaging of deep structures some 25 years ago. In continental areas, these measurements often show weakly reflective upper crust and strongly reflective lower crust. Swedish land measurements have showed weak reflections from the lower crust in several areas, and in some areas, strong reflections from the upper crust. The apparent weak reflectivity in the lower crust in Sweden could be an artefact, due to insufficient source energy. The first study in this thesis analyses deep reflectivity and signal penetration depth, using data from the Siljan area. We observe discontinuous reflections from the lower crust in this area, indicating a crustal thickness of ~45 km. The faint appearance of some reflections is explained by insufficient charge weights. We also observe some lateral variation in the deep reflectivity. In the second study, we analyse the deep reflectivity underneath Jämtland. The analysis indicates a crustal thickness of ~50 km, locally thinning to ~45 km. Dipping deep structures suggests that the thinning is related to crustal extension. The reflection Moho, defined by the cessation of lower crustal reflectivity, appears to be more diffuse in the area of thinner crust. The areas of weak deep reflectivity, or diffuse reflection Moho, in the first two studies, are both located within the Transscandinavian Igneous Belt. In the third study, we analyse data acquired in southwest Iceland. We find that the observed reflections are diffuse, and that these often indicate dipping structures, although we cannot define a systematic dip direction. A decrease in the number of reflections suggests a crustal thickness of 26-27 km. Velocity tomography shows that the shallow crustal velocity is increasing with distance from the rift zone. Positive velocity anomalies are observed, and the most pronounced one coincides with exposed acid vulcanite. In the fourth study, we attempt to image the reflection Moho in southern Iceland, using earthquake recordings. The source energy is believed to be sufficient, although location uncertainties and complex radiation patterns are problematic. Preliminary results indicate Moho reflections, although the crustal thickness cannot be constrained with accuracy.

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