Nya litologiska-tektoniska modeller för regionen Västerbotten - södra Norrbotten

Sammanfattning: High quality bedrock and tectonic maps are the platform for locating natural resources such as ore, oil, gas, and water. Bedrock maps are also used in the design of large constructions, including highways, bridges, dams, power plants, tunnels, nuclear and other waste repositories. The main objective of the thesis is to improve and develop analysis and techniques for bedrock mapping mainly based on regional geophysics. The value of the improved approaches is demonstrated by constructing a lithologic-tectonic map for the Västerbotten - South Norrbotten region in northern Sweden. A fractal model has been adopted for the design and performance of the study. Instead of investigating an area of 50 km x 50 kin or smaller, a whole region or an area of 300 km x 250 has been studied. According to the fractal model, there are genetic connections between small and large geologic structures. The application of this model made possible: (a) the detection of structures, (b) establishing a genetic link between geologic structures and put them into large lithologic-tectonic relations and (c) a better understanding of the initiation, formation and development of different structures. The bedrock mapping is performed by integrating information provided by various sources: as a geoscientific multiand interdisciplinary analysis concept (MIA). Nine different main data sets have been used: (1) aeromagnetic data, (2) gravity data, (3) radiometric data, (4) satellite photo, (5) topography, (6) drainage system, (7) structural geologic map, (8) seismicity map and (9) esker map. For the interpretation a number of different visualizations of gravity data, aeromagnetic data and topographic data have been used. Besides the nine main data sets used in the neotectonic analysis, information on end moraines, highest coastline, isostatic uplift, seismotectonics, in situ stress measurements and documented late- to postglacial faults have been included. The enclosed sketch map shows the new tectonic "backbone" for the region. Some of the zones are defined as reverse faults/thrusts, e.g. the Malå-Skellefte Tectonic Zone (MSTZ) and the Knaften-Bureå Tectonic Zone (KBTZ). A large circular structure with a diameter of about 100 km, Lycksele Ring Structure (LRS), is defined in the central part of the region. Several graben-horst structures have been outlined in the northern part of the region. The so called "Nickelzonen" is interpreted as a dextral shear zone and it is renamed the Burträsk Shear Zone (BSZ). In the eastern part of the region a large scale fault zone, Krukmakar Fault Zone (KFZ) is defined. The major part of the bedrock west of the KFZ consists of granitoids while the eastern part consists mainly of migmatites. Cauldron structures have also been identified, e.g. the Gallejaure structure and the Jörngranite structure situated just north of the Skellefte field. The area between Urneå-Skellefteå or the Lövånger Tectonic Zone (LTZ) belongs to the localities in Sweden with the highest earthquake frequencies. In this thesis, tectonic models are presented for the LTZ, the Burträsk area (BSZ, KBTZ), the Skellefte valley (MSTZ), the Storavan area (horst-graben) and the central part of the study area (LRS). Some of the defined faults coincide with earthquakes and documented late- to postglacial faults, which indicates reactivations of very old weakness zones in the crust. Yet other major findings are: (1) initiation, formation and development of kursu valleys. This has been enigmatic for the last 100 years. It is now obvious, these valleys are caused by neotectonics and erosion. (2) Glaciations/deglaciations are considered to enhance fluid circulation in the crust. A hypothesis of glacial pumping is proposed to explain this process. (3) The interpretation of the tectonics in the study area shows that neotectonic faults are much more extensive than previously documented in the region. (4) The tectonic and neotectonic features recognized by this study are also compared with corresponding features in the central part of the Baltic Shield, e.g. they show large scale connections. The new bedrock mapping technique and the new lithologic - tectonic models presented in the thesis are of utmost consequence for future ore prospecting in the region and for investigations concerning the bedrock stability.