Vestiges of the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt

Abstract

The Pre-Caledonian margin of Baltica has been outlined as a tapering wedge with increasing magmatism towards the ocean–continent transition. It is, however, well known that margins are complex, with different and diachronous evolution along and across strike. Baltica’s vestiges in the Scandes have complexities akin to modern margins. It included a microcontinent and magma-poor hyperextended and magma-rich segments. It was probably up to 1500 km wide before distal parts were affected by plate convergence. Characteristic features are exhumed mantle peridotites and their detrital equivalents, some exposed to the seafloor by the pre-orogenic hyperextension. A major change in the architecture of the mountain belt occurred across the NW–SE trending Sveconorwegian front in the Baltican basement. This coincided with the NE termination of the Jotun-Lindås-Dalsfjord basement nappes, the remains of the Jotun Microcontinent (JMC) formed by hyperextension prior to the orogeny. Mantle with ophicalcite breccias exhumed by hyperextension are covered by deep-marine sediments and local conglomerates. Baltican basement slivers are common in the transitional crust basins. Outboard the JMC, the margin was magma-rich. The main break-up magmatism at 605 ± 10 Ma was part of the vast Central Iapetus Magmatic Province. The along-strike heterogeneity of the margin controlled diachronous and contrasting tectonic evolution during the later Caledonian plate convergence and collision.

An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides

“…Sketch map (top) and vertical W–E cross section (bottom), illustrating the inferred origin of the Kråkfjord protolith at an Ordovician slow‐spreading centre near continent Baltica (palaeogeography adapted from Andersen et al, 2022). The cross section outlines an extended transition zone between the continental shelf of Baltica in the east and the Iapetus Ocean in the west.…”

“…A third scenario for the origin of the Kråkfjord complex, which we favour, is an oceanic spreading ridge near to or within a Cambro-Ordovician hyper-extended OCT zone along southern Baltica (Figure 12). Andersen et al (2012Andersen et al ( , 2022 and Jakob et al (2017Jakob et al ( , 2019 propose that ribbons or microcontinents of Baltica crust were separated from the Baltica mainland by hundreds of kilometres of hyperextended crust during the Late Cambrian-Ordovician. The crystalline thrust sheets of the Middle Allochthon of southwestern Norway, for example, the Jotun and Lindås nappes, are thought to represent these ribbons or microcontinents.…”

“…The metamorphic textures and peak-pressure assemblages in metadolerites within the Precambrian gneiss complex in Roan are different partly because metamorphic plagioclase was stable together with garnet and clinopyroxene (Möller, 1988). Stable plagioclase in mafic rocks suggests a peak pressure in the HP granulite facies, lower than eclogite facies F I G U R E 1 2 Sketch map (top) and vertical W-E cross section (bottom), illustrating the inferred origin of the Kråkfjord protolith at an Ordovician slow-spreading centre near continent Baltica (palaeogeography adapted from Andersen et al, 2022). The cross section outlines an extended transition zone between the continental shelf of Baltica in the east and the Iapetus Ocean in the west.…”

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

“…For instance, Young (2018) suggested that the study area comprised delaminated parts of the upper Baltica crust. Moreover, based on U-Pb dating of intrusive rocks, Jakob et al (2017) and Andersen et al (2022) suggested that the Baltica margin experienced extension from the Late Cambrian until the Middle Ordovician. In such a context, lithospheric thinning (and related exhumation of the mantle) would have caused a temperature increase of the crust (including the mafic lenses), potentially up to 600 C. In this case, the stacking of the allochthons from the hyperextended margin would result in compression during the first stages of shortening, perhaps up to the inferred amphibolite facies pressure conditions.…”