The Midland Valley as a Rift, Seen in Connection with the Late Palaeozoic European Rift System

Abstract

The Midland Valley is first recognisable as a graben after the closure of Iapetus. NE-SW structural lineaments parallel to the closure then controlled deposition of early Devonian intermontane sequences and the siting of some of the calcalkaline volcanism within the rift. The phase, which included similar volcanism and related plutonism beyond the rift, was ended by a mid-Devonian orogeny. Carboniferous volcanism was alkaline and mid-continental; its location, like the distribution of sediments, indicates continuing NE-SW basement controls. These probably owed less to subduction, hot-spots or plate collision than to adjustment-fracturing and underlying partial melting in an inhomogeneous continental plate drifting northwards.

Late in the Carboniferous new stresses led to emplacement of E-W tholeiite dykes, along extensional fractures transgressing all the earlier structures. They may continue eastwards beneath the North Sea, reappearing along ESE to SE trends in southern Sweden and are possibly comagmatic with the earliest lava flows in the Oslo Graben. This orogenic phase may been initiated by rotation of a South Europe microplate during the closure of a mid-European ocean. After this brief interruption, continental alkali-basaltic magmatism became re-established and continued into early Permian times. It was more silica-undersaturated than before — a change which may represent a new stress regime, corresponding with NW or NNW basement controls recognised in the western part of the Midland Valley and correlatable with the N-S rifts of the North Sea and the Oslo Graben.

Dept. of Earth Sciences, University of Leeds, from Oct. 1977.