A NEW LOOK AT THE SCANDINAVIAN CALEDONIDES AND THE SIGNIFICANCE OF HYPER-EXTENSION IN THE PRE-CALEDONIAN PASSIVE MARGIN OF BALTICA (Invited Presentation)

The Baltican margin has been outlined as a wedge with increasing magmatism towards the ocean-continent transition (OCT), with the same structure along its length. It is now known that margins are complex, with different/diachronous segment-evolutions. The Baltican nappes preserve a complexity akin to modern margins, including an OCT, microcontinent and continental slivers, hyperextended-to-embryonic oceanic basins and magma-rich segment. The margin was up to 1500 km wide before distal parts were affected by plate-convergence at 480-450 Ma. The margin, overprinted by Scandian, earlier fabrics and post-orogenic extension, now sit at low structural level in the orogen. Proximal parts have continental to marine basins with deposits of Cryogenian, Ediacaran to Lower Palaeozoic age. Its vestiges also have continental slivers, coarse- to fine-grained clastics and deep-marine deposits. A major change occurs across a NW-SE zone parallel to a the Sveconorwegian (Grenville) front in the basement. The prominent changes across this line are: 1) coincidence with the termination of the Jotun-nappes, interpreted as the Jotun-Microcontinent (JMC) and 2) transition from a NE magma-rich, to a SW magma-poor segment with numerous solitary peridotites and detrital serpentinites (some fossiliferous). The mantle-fragments w/ophicalcites were emplaced tectonically and are covered by deep-basin sediments and local breccias and conglomerates. This unit was affected by pre-Caledonian metasomatism and intruded by gabbros and granitoids (~487-471Ma). Other important features are Baltican basement slivers (up to 40x1.5 km). The magma-poor SW segment is overlain by thehuge basement nappes, which after rifting, but prior to Scandian collision, was outboard the hyperextended domain as the JMC. JMC´distal parts were affected by early (~450 Ma) deformation/metamorphism. The ~400 km long SW segment was hyperextended, magma-poor and received sediments at least as late as the Middle Ordovician. The magma-rich segment also has mantle peridotites and detrital serpentinites, but the prominent characteristic is the Scandinavian Dyke Complex (SDC) forming a 615-595 Ma Large Igneous Province that assisted opening of Iapetus. In the SW there is little Late Proterozoic magmatism, but the basement was locally truncated by dolerites(~840 and 615 Ma). The magma-poor segment inboard the JMC, formed a OCT basin opening to an ocean basin, similar to the Atlantic margin between Ireland and the Hatton-Rockall ribbons.