On the evolution of the North Atlantic -from continental collapse to oceanic accretion

The North Atlantic between Iceland and Svalbard and surrounding continental areas represent an ideal laboratory for studying plate tectonic processes, due to the presence of well exposed continent-continent collision terranes on the Norwegian mainland and dense coverage of offshore geophysical data documenting subsequent extensional processes, continental break-up and formation of oceanic crust. Interpretation of crustal-scale wide-angle seismic models suggests that the main Caledonian suture might be linked with bodies of lower crustal eclogites localized along the Norwegian continental margin (e.g. Mjelde et al., 2010). Since terrains of eclogites are exposed in the Western Gneiss Region, a wide-angle seismic profile was acquired across the onshore transition in this area. The main aim with the research work presented in Paper 1 (this thesis) is to identify possible lower crustal high-velocity bodies, and thus the suture, in this region. Paper 2 aims at interpreting the suture from the North Sea to Svalbard, by use of available crustal scale models. Furthermore, paper 2 discusses the back-stripping of two crustal-scale transects across the Atlantic, in order to identify the dominant tectono-magmatic processes active at various stages of the area’s geological evolution. The two transects are located on opposite sides of the Jan Mayen Fracture Zone, which assures that possible change from upper to lower plate configuration along strike can be addressed. Paper 3 focuses on interpretation of multi-channel seismic data from the Knipovich Ridge. The main aim with this study is to reveal the interplay between tectonism and magmatism along this ultra-slow oceanic spreading ridge. Furthermore, the sedimentary processes transporting huge amounts of sediments from Svalbard, and their interaction with the spreading ridge, will be addressed. The main contribution with this thesis is that it discusses a selection of tectonic, magmatic and sedimentary processes active during all stages of ocean basin formation, i.e. collapse of a mountain range, formation of a passive continental margin and active oceanic accretion. In the following chapters I will first briefly describe the tectonic setting and the geological history of the study areas. Secondly, I will explain the different types of data used, and the methods used for the interpretation and modeling. Then, the three papers will be introduced, and lastly I will give a short summary with proposal for future work

Crustal composition of the Møre Margin and compilation of a conjugate Atlantic margin transect

Highlights • The basement at the mid-Norwegian Møre Margin is dominantly felsic in composition. • A lower crustal body is interpreted as a mixture of continental blocks and eclogite. • The thickness of the outer lower crustal body is twice as thick on the East Greenland Margin. • The thinning during this first phase of post-Caledonian extension was highest for proto Norway. Abstract The inner part of the volcanic, passive Møre Margin, mid-Norway, expresses an unusual abrupt thinning from high onshore topography with a thick crust to an offshore basin with thin crystalline crust. Previous P-wave modeling of wide-angle seismic data revealed the presence of a high-velocity (7.7–8.0 km/s) body in the lower crust in this transitional region. These velocities are too high to be readily interpreted as Early Cenozoic intrusions, a model often invoked to explain lower crustal high-velocity bodies in the region. We present a Vp/Vs model, derived from the modeling of wide-angle seismic data, acquired by use of Ocean Bottom Seismograph horizontal components. The modeling suggests dominantly felsic composition of the crust. An average Vp/Vs value for the lower crustal body is modeled at 1.77, which is compatible with a mixture of continental blocks and Caledonian eclogites. The results are compiled with earlier results into a transect extending from onshore Norway to onshore Greenland. Back-stripping of the transect to Early Cenozoic indicates asymmetric conjugate magmatism related to the continental break-up. Further back-stripping to the time when most of the Caledonian mountain range had collapsed indicates that the thinning during the first phase of extension was about 25% higher for proto Norway than proto Greenland

Crustal structure across the Møre margin, mid-Norway, from wide-angle seismic and gravity data

The Møre Margin in the NE Atlantic represents a dominantly passive margin with an unusual abrupt transition from alpine morphology onshore to a deep sedimentary basin offshore. In order to study this transition in detail, three ocean bottom seismometer profiles with deep seismic reflection and refraction data were acquired in 2009; two dip-profiles which were extended by land stations, and one tie-profile parallel to the strike of the Møre–Trøndelag Fault Complex. The modeling of the wide-angle seismic data was performed with a combined inversion and forward modeling approach and validated with a 3D-density model. Modeling of the geophysical data indicates the presence of a 12–15 km thick accumulation of sedimentary rocks in the Møre Basin. The modeling of the strike profile located closer to land shows a decrease in crustal velocity from north to south. Near the coast we observe an intra-crustal reflector under the Trøndelag Platform, but not under the Slørebotn Sub-basin. Furthermore, two lower crustal high-velocity bodies are modeled, one located near the Møre Marginal High and one beneath the Slørebotn Sub-basin. While the outer lower crustal body is modeled with a density allowing an interpretation as magmatic underplating, the inner body has a density close to mantle density which might suggest an origin as an eclogized body, formed by metamorphosis of lower crustal gabbro during the Caledonian orogeny. The difference in velocity and extent of the lower crustal bodies seems to be controlled by the Jan Mayen Lineament, suggesting that the lineament represents a pre-Caledonian structural feature in the basement