Occurrence and significance of inverted metamorphic gradients in the western Scandinavian Caledonides

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Abstract
In the Scandinavian Caledonides, the basement with a thin veneer of cover sediments can be traced from the frontal autochthon far westwards towards the centre of the orogen. The changing metamorphic grade can be recorded and compared with that of the overlying nappes. The metamorphic nature of these cover sediments is examined in the deepest structural levels of two windows, 200 km to the W of the Caledonian front. The emplacement of nappes above the cover caused a progressive recrystallization under greenschist facies conditions, with development of quartz + margarite + muscovite, quartz + chlorite + muscovite and quartz + white mica + graphite among parageneses defining the penetrative foliation related to the nappe displacement. The final overburden pressure is estimated to be 6–8 kb. Temperatures reached peak values around 550°C first after nappe emplacement. Despite the westerly situation and deep tectono-stratigraphic level of the metasediments, their metamorphic history is incompatible with a location close to an orogenic interior. The overlying cover sequence displays at least 2 inversions of metamorphic zonation. These developed as a result of the emplacement of high-grade nappes above low-grade or unmetamorphosed units, with incomplete subsequent restoration of the thermal profile, and in response to the catalytic effect of concentrated strain during thrusting. The imprinted saw-tooth pattern of the thermal profile in the western windows requires that tectonic discontinuities exist in this part of the western Scandes, considered by some geologists the axial zone of the orogen. It also suggests that the uplift and the formation of late regional syn- and antiforms followed soon after, or were simultaneous with, the late translation of the Caledonian front allochthon.