An Early Ordovician (Finnmarkian?) foreland basin and related lithospheric flexure in the Scandinavian Caledonides

Abstract

Early Ordovician (479 - 455 Ma) graywackes overlying Cambrian passive margin successions in the Baltica palaeocontinent-related Lower AUochthon of the Scandinavian Caledonides document early Caledonian tectonic activity. Such rocks occur from the Jamtland area (Sweden) northwards along the eastern Caledonian margin, and imply a wide regional extent of a foreland basin in Early Ordovician times. This foreland basin was subsequently incorporated into a fold-and-thrust belt during the final Caledonian, Scandian tectonic episode (c. 425 - 390 Ma). Based on published cross section data, this late deformation is restored. The results and other available information are compiled into a palaeogeographic map and sections on the geometry and evolution of the Early Ordovician foreland basin. Graywacke/turbidite sedimentation (Follinge Formation) started in western (internal) areas already in Early Arenig times with a main phase from Llanvirn to Late Caradoc (c. 472 - 461 Ma). There, the Follinge Formation rests with an erosional unconformity on older beds. Towards east, however, it overlies successively younger beds related to a carbonate domain. It is suggested that the areas of the sub-turbidite erosional unconformity represent the early location of a flexural forebulge, which subsequently migrated eastwards towards the margin of the carbonate domain of palaeocontinent Baltica. The restored foreland-basin geometry is compared with numerical models in order to derive some characteristics of the flexure of the foreland lithosphere. Time and lithospheric constraints make it possible to test the viability of the restoration and of foreland basin models. Relative to the available lithospheric strength data, the width of the basin is too large, the depth relatively small, and the large wavelength of the flexure is difficult to explain with simple orogenic loading. Graywacke sedimentation occurred after Finnmarkian (515 - 475 Ma) HP metamorphism, perhaps as a consequence of exhumation and consequent loading of the Baltican margin. Graywacke sedimentation ended at the time of Jamtland phase (460 -440 Ma) HP metamorphism and relatively deep water conditions. It is speculated that eclogitization of crustal material produced an additional load, which caused a relatively large down flexure of the lithosphere.