Sammendrag
The pre-Caledonian Baltican rifted margin has been outlined as a tapering wedge with increasing magmatism towards the ocean-continent transition (OCT), and with a mostly similar structure along its length. It is, however, well-known that passive margins can be complex, with different and diachronous segment-evolutions along and across strike. The Baltican nappes in the Scandes preserve a complexity akin to modern margins, including an OCT, a micro-continent and continental slivers, hyperextended-to-embryonic oceanic basins as well as a major magma-rich segment. The margin was several hundred-, probably up to 1500 km wide, before distal parts were affected by plate-convergence between ~480 and 450 Ma. The margin vestiges, overprinted by Scandian and earlier fabrics as well as the late/post-orogenic extension, now occur at low to middle structural levels in the mountain belt. Proximal parts comprise continental to marine rift-basins with syn- to post-rift deposits of Late Cryogenian, Ediacaran and Lower Palaeozoic age. Its vestiges also comprise continental slivers, coarse- to fine-grained sediments as well as deep-marine deposits. A major change in the nappe-structure occurs across a NW-SE transverse zone parallel to a fundamental basement structure within Baltica, the Sveconorwegian (Grenville) deformation front. The most prominent changes across this lineament are: 1) the coincidence with the NE-termination of the Jotun-Lindås-Dalsfjord nappe complexes, interpreted as the Jotun-Microcontinent (JMC) and 2) the transition from a magma-rich segment in the NE to a hyperextended, magma-poor segment in the SW. The latter has numerous (>100) solitary meta-peridotites and detrital serpentinites (some fossiliferous). The mantle-fragments w/ophicalcite breccias were emplaced tectonically and are covered mostly by deep-basin sediments and local breccias and conglomerates. This ‘mixed’ (mélange) unit was locally affected by pre-Caledonian metasomatism and intruded by gabbros and granitoids (at ~487±1 to 471±2 Ma); also reflected by clastic zircons (>468 Ma) present in the sediments. Other important features are Baltican basement slivers (up to 40 x 1.5 km). The magma-poor SW segment is overlain by a huge basement/cover nappe complex, which after rifting, but prior to Scandian collision, was positioned outboard the hyperextended domain as the Jotun-Microcontinent (JMC). JMC´s most distal parts have mafic dykes and lavas and were affected by early-Caledonian deformation/metamorphism. The SW margin segment was ~400 km long, hyperextended- and magma-poor, and received sediments as late as the Middle Ordovician (and possibly until the onset of the mid-Silurian Scandian orogeny?). The NE magma-rich segment also has mantle peridotites and detrital serpentinites (locally w/fossils), but its most prominent characteristic is the Scandinavian Dyke Complex (SDC) forming parts of a ~615-595 Ma Large Igneous Province that probably assisted opening of the Iapetus Ocean. In the SW segment there is so far no evidence of Late Proterozoic magmatism, but Baltican basement was locally truncated by mafic dykes at ~850 to 830Ma and 615 Ma. The magma-poor SW segment, inboard the JMC, formed a transitional-crust basin opening to an ocean basin, similar to the present North Atlantic rift system between Ireland and the Hatton-Rockall ribbons. The margin of Baltica probably has its best modern analogue in the North-Atlantic and Norwegian-Greenland Sea margins.
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