THE RIFT-INHERITED ARCHITECTURE OF THE SCANDINAVIAN CALEDONIDES AND THE SIGNIFICANCE OF 437 MA MAGMATISM WITHIN THE DISTAL DOMAIN OF THE PRE-CALEDONIAN RIFTED MARGIN OF BALTICA

The Scandinavian Caledonides were finally assembled during the Silurian Devonian continental collision of Laurentia with Baltica/Avalonia. During the collision an intricate stack of nappes was thrust over the partly subducted continental margin of Baltica. From bottom to top, these nappes contain 1) Proterozoic continental margin successions and discontinuous sheets of Baltican basement; 2) orthogneisses of Baltican affinity; 3) Proterozoic continental margin successions with locally high volumes of plume-related mafic magmas that were emplaced during continental break-up; 4) Alpine-type metaperidotite-bearing metasedimentary complexes that host an island-type, lower Middle Ordovician fauna; 5) (in South Norway) large, mainly crystalline nappes of Baltican affinity (Jotun, Lindås, Dalsfjord nappes) 6) ophiolite and volcanic arc assemblages, locally with fossils of Laurentian affinity.

We suggest that the differences in the lithological compositions as well as the structural sequence of the Baltican-derived allochthons is rift-inherited and originates from the highly irregular architecture of the pre-Caledonian rifted margin of Baltica. The rift-inherited elements include a change from an Ediacaran magma-poor to a magma-rich rifted margin along strike of the orogen, as well as a stack of proximal to distal rift domains. The transition from magma-poor to magma-rich occurs over a distance of about 200 km and coincides with the termination of the large crystalline nappes in Southern Norway.

By unstacking the Caledonian nappe stack, the pre-Caledonian rifted margin of Baltica can be reconstructed. We suggest that the pre-Caledonian rifted margin of Baltica may have been very wide (≥ 1000 km) and may have included a magma-poor hyperextended domain as well as a microcontinent (Jotun Microcontinent). A ≥1000 km wide margin, however, would suggest that the initial collision between Baltica and Laurentia may had occurred considerably earlier than in traditional plate tectonic models. Here we show new geochronological evidence that suggest that the dyked Neoproterozoic continental margin successions as well as the metaperidotite-bearing units were in an upper plate-position before ~437 Ma.