Hyperextension in magma-poor and magma-rich domains along the pre-Caledonian passive margin of Baltica

Passive continental margins are formed when continents rift and break apart at the start of so called Wilson cycles. After rifting and break-up the continents drift apart by seafloor spreading, before the ocean formed eventually closes by plate convergence. When the continents collide mountain belts are formed, and the Wilson cycle is completed. In this project we have re-mapped and studied in detail by various methods a large part of the remnants of the pre-Caledonian continental margin of the ancient continent Baltica that are preserved within the Caledonian mountain belt of Scandinavia. We have used present-day analogues to compare and learn more about the geological processes that takes place at depth when continents rift apart. In the project we have shown that the pre-Caledonian margin of Baltica had a magma-rich segment and a magma-poor segment, separated by a wide transition zone. In addition we have shown that the margin also had a number of continental slivers and fragments as well a large sliver of continental crust known as the Jotun microcontinent (JMC). Today, this is found within the mountain belt as the well-known Jotun nappe and related large nappes of Precambrian rocks in southern Scandinavian. The research shows that the sea-way, which was formed between the Jotun microcontinent and Baltica was an area with intense crustal thinning known as hyperextension. Hyperextended domains are found in areas where the crust is stretched to less than 10 km in thickness and in some cases stretched completely off. In such domains that the upper mantle may be exposed at the sea-floor. In other areas, particularly in the present northern Scandinavia, the stretching of the lithosphere resulted in large-scale magmatic activity associated with a mantle plume that rose from great depth in the Earth, at the core-mantle boundary. This formed the Scandinavian dyke complex (SDC) covering a length of more than 1000 km along the margin. The volume of basaltic magma was so large that we call it a large igneous province (LIP). The age dating shows that it was intruded over a relatively short period of time in less than 20 million years, at approximately ~605±10 million years ago. In this project we have remapped the remnants of the ancient continental margin in order to understand the processes that forms passive continental margins. We have dated the SDC rocks by radiometric methods, analysed the geochemistry of the SDC and presented models for how the margin formed and evolved both along- and across strike of the mountain belt in Scandinavia. We have also presented models of how the architecture of the margin controlled the later sequence of events during the final, collision stages of the Caledonian Wilson cycle that eventually produced the mountain belt in Scandinavia approximately 400 million years ago. Important components in this project have been education and outreach. Several PhD- and Master-students from Norway, Germany, France and Brazil have been affiliated with the project. The project has contributed to outreach via the production of a Video-series on Wilson-cycle tectonics, which presently is under editing. The video series will be available for teaching and outreach for general public via the CEED Youtube channel early in 2020.

1) New understanding of the structure along and across strike of the Pre-Caledonian margin of Baltica 2) Results from the project funding published in articles (#11) ans several (#3) in prep for highly-ranked international journals. 3) Geoscientists educated: 3 Master Sci, 1 PhD and 1 Postdoc. 4) Postdoc J. Jakob, starts as permanent Reseracher at the Norwegian Geological Survey, spring 2020. 5) PhD candidate, Dr H.J. Kjøll starts as Postdoc at the Dept. Geosciences UiO, 2020. 6) Results from the project presented in number (>50) of presentations, many invited/keynotes, at international/national symposia and meetings. 7) The project group are invited members of a specialists working-group, "ImaginRifting" from more than 10 nations, working to understand rifting processes and architectures

Distal portions and highly extended passive continental margins are frontiers for petroleum exploration and for understanding the ocean continent transition. These cannot be easily studied by direct field observations except within orogenic belts, where parts of such margins have been exposed by orogenic collision and exhumation. The best know examples are from the European Alps and the Pyrenees. Recently, members of this project research group have suggested that large parts of the the Scandinavian Caledonides have many characteristics which point to an origin in a hyperextended margin similar to the present-day examples, and those recently described from the Alps. The most characteristic feature of hyperextension is that continental crust locally is thinned to the extreme so that the mantle is exhumed to- or close to the syn-extensional surface. In this process, mantle peridotites are strongly affected by extensional deformation, and subjected to direct contact with seawater and syn- to post-rift sediments. Hence, mantle culminations may undergo hydration, brecciation and carbonatization. Diapiric rise of serpentinites, formation of soap-stones and talc-schists as well as development of ophiocalcites may be associated with a mature stage or early contraction. Mantle- rocks may also be reworked as serpentine conglomerate and sandstone. Several of these features have already been identified in the Caledonides of Scandinavia. In addition this orogen also preserves a unique transition from a magma-poor in the South to a magma-rich segment in the North. The magma-rich northern segment has unique exposures and an intensity of the magmatism is similar to a LIP. The project will describe the nature of the segmentations as well as investigate the role of the large volumes of magma in the continental break-up. This project will establish the Scandinavian example as an analogue for modern margins available for direct observation, particularly of the deeper parts of margins.