A palaeomagnetic study of the Builth Wells‐Llandrindod Wells Ordovician Inlier, Wales: palaeogeographic and structural implications

Abstract

Discrepancies exist in the Southern British Ordovician palaeomagnetic data set with individual studies indicating either high southerly or temperate southerly palaeolatitudes. Although previous palaeomagnetic investigations from the Builth inlier have delineated a three‐component remanence structure, doubts have been raised concerning (i) the magnetization age of the supposed ‘primary’component (P); (ii) the structural setting of the sampling area (Llanelwedd quarry) within a zone of strike‐slip deformation; and (iii) whether detailed step‐wise demagnetization has fully separated the various magnetization components. In addressing these problems, we present new palaeomagnetic results as follows: (i) a positive palaeomagnetic conglomerate test establishing a pre‐Late Llanvirn age for component (P); (ii) an enlarged geographic sampling embracing several tectonic domains indicating that relative rotations linked to strike–slip tectonism are minor within the Builth sampling area; and (iii) evidence that component (P) is uncontaminated by overprints at some sites suggesting the resulting palaeolatitude of 35°S to be representative for the Builth inlier. We also present evidence for local syn‐volcanic deformation within Llanvirn volcanics and for intrusion‐related deformation of Llanvirn shales. When the revised Llanvirn pole from the inlier (3°S, 4°E) is combined with other data from Southern Britain in a statistical spline analysis, a mean palaeolatitude of 44.5°S is calculated for Builth in Llanvirn times. A palaeoreconstruction for 468–474 Ma (c. Mid‐Llanvirn) is presented utilizing this analysis. Copyright © 1991, Wiley Blackwell. All rights reserved