Chapter 20 Lower thrust sheets in the Caledonide orogen, Sweden: Cryogenian–Silurian sedimentary successions and underlying, imbricated, crystalline basement

Abstract

Abstract The Jämtlandian Nappes and their equivalents further north, belonging to the lower thrust sheets in the Caledonide orogen of Sweden, comprise a mega-duplex of Cryogenian–Silurian sedimentary rocks sandwiched between structurally higher allochthons and a basal décollement. Further west towards the hinterland, crystalline basement is increasingly involved in this thrusting, imbricate stacking occurring beneath the décollement in antiformal windows. The sedimentary successions were derived from the Cryogenian rifted margin of Baltica, the Ediacaran–Cambrian drifted margin, and Ordovician and Silurian foreland basins. During the Early–Late Ordovician (Floian–Sandbian), hinterland-derived turbidites were deposited in response to early Caledonian accretion of subducted complexes belonging to the outermost margin of Baltica, now preserved in the higher allochthons. Following a quiescent period during the Late Ordovician (Hirnantian) and early part of the Llandovery, collision of Laurentia and Baltica reactivated the foreland basins, with flysch and molasse deposition during the Llandovery–Wenlock. Collisional shortening during this Scandian orogenic episode continued into the Devonian. High- and ultrahigh-pressure (HP/UHP) metamorphism accompanied Baltica's underthrusting of Laurentia in the deep hinterland, and prominent basement-cored antiforms developed towards the foreland during the advance of the orogenic wedge over the foreland basin onto the Baltoscandian platform.