Palaeomagnetic constraints on the evolution of the Caledonian-Appalachian orogen

Abstract

Summary Late Proterozoic and Palaeozoic (pre-Permian) palaeomagnetic data from all regions involved in, or adjacent to, the Caledonian-Appalachian orogenic belt are reviewed. Between about 1100 and about 800 Ma the Laurentian and Baltic shields were close together, prior to the opening phase of the Caledonian-Appalachian Wilson cycle. The problems of tectonic interpretation of Palaeozoic palaeomagnetic data from within and around the belt derive mostly from differences of typically 10°–20° between the pole positions. These can variously be interpreted in terms of (i) relative displacements between different continents or terranes, (ii) differences in ages of remanence and (iii) aberrations due to inadequacy of data or geomagnetic complexity, and it is not always easy to discriminate between these alternatives. If the Pangaea A2 reassembly of continents around the northern and central Atlantic is taken as the end-product of Caledonian-Appalachian orogenesis, the following conclusions can be drawn. 1 Lower Palaeozoic palaeolatitude differences between the N American and British-Scandinavian margins of the Caledonides are small; hence any convergence must have been mainly E-W. 2 There are additional differences which could be due to major pre-Carboniferous strike-slip (more than 1000 km), although later strike-slip on this scale is no longer considered likely. 3 The Lower Palaeozoic apparent polar wander paths for Northern Scotland and N America disagree on face value, but must be reconciled if their conventionally assumed geographic relation is correct. 4 Lower Old Red Sandstone data from Britain and Norway disagree, but this is more likely to be due to magnetic overprinting in the Norwegian rocks than to remnant oceans between the regions of Old Red Sandstone facies. 5 Armorica seems to have been far to the S, adjacent to Gondwana, in Ordovician time. The latest view is that it collided with Euramerica in early Devonian time to form the Old Red Continent. 6 The timing of Gondwana’s collision with the Old Red Continent is controversial; it is within either the late Devonian or the Carboniferous. If it occurred early in that time range, much of Hercynian-Alleghanian orogeny post-dated it.