The effect of Late Caledonian and Variscan continental escape tectonics on basement structure, Paleozoic basin kinematics and subsequent Mesozoic basin development in NW Europe

Abstract

Caledonian structures in NW Europe formed by northwestward accretion of continental fragments and magma tic arcs onto the Proterozoic North Atlantic Craton. The main crustal fabrics were formed by Caledonian basin development, subsequent inversion, collision tectonics with thick-skinned and thin-skinned thrusts and large NW–SE strike-slip faults. These fabrics were modified and offset by late Caledonian displacement on NE–SW-trending strike-slip faults. Late Caledonian (Devonian and Early Carboniferous) tectonics in NW Europe resulted from the lateral expulsion of the English–North Sea–Baltic Block away from an Acadian indentor. The block was approximately triangular in shape bounded by (i) the Ural Ocean to the east, (ii) the left-lateral Great Glen–Midland Valley–North Atlantic shear systems to the NW and (iii) the right-lateral English Channel–South Polish Trough shear systems (in Devonian times) and the South Wales–Southern North Sea–Polish Trough shear zone (in Early Carboniferous times) to the south. During the Early Carboniferous the block expanded NW–SE, as it was released from the indentor, into space created by back-arc extension related to Variscan subduction. Pull-apart basins which developed along the shear systems include the following, (i) In the Northern North Sea, the West Orkney Basin–East Shetland Platform–Viking Graben formed as a large pull–apart basin in the left-lateral shear systems during Devonian/Early Carboniferous times, (ii) In the Southern North Sea and East Midlands, NW-trending Carboniferous basins developed in a right-lateral shear system, associated with clockwise rotation of Caledonian basement blocks. The Devonian and Carboniferous basin-fill depended on the position of the block boundary shear systems in the Caledonian and Variscan orogens and their thickened crust. The northern shear boundary affected crust c. 50 km thick following Caledonian collision and hence the syn-rift and post-rift sediment fill was dominantly intracontinental. In the south the basin developed in crust of average thickness and hence marine conditions were dominant during the extension. However, Devonian and Carboniferous basins in the Northern North Sea provided channels for sediment fill from the Caledonian mountains into the southern basins. During the Late Carboniferous, closure of the Ural Ocean and Ural plate collision reversed the sense of shear along the block margins and the wedge-shaped block was pushed back between the Acadian collision zones. This shear reversal caused inversion of the pull-apart and expansion basins in northern Britain. Subsequent reactivation of these basins during the Mesozoic was responsible for the tectonic framework of the North Sea.