Palaeogeographic significance of mid-Silurian palaeomagnetic results from southern Britain-major revision of the apparent polar wander path for eastern Avalonia

Abstract

SUMMARY Wenlock (c. 430 Ma) lavas from the East Mendips Inlier, Southern Briain, are characterized by either single-component magnetizations (Type 1) or a more complex system of three magnetization components (Type 2) during stepwise thermal demagnetization. The type of demagnetization behaviour is related to changes in magnetic petrology within the lava pile. The lower and intermediate unblocking temperature components of Type 2 represent magnetic overprints of Tertiary/Recent and Hercynian (mid-Carboniferous) origin respectively. The primary nature of the high unblocking temperature magnetization component, Types 1 and 2 (Dec = 095C, Inc = -24, ag5 = 8.8), is demonstrated by positive agglomerate tests. Conversely, an overlying Upper Old Red Sandstone sequence did not provide primary magnetizations; the magnetic signature is governed by syn-tectonic Hercynian remagnetizations. The primary Silurian data demonstrate that the East Mendips Inlier experienced considerable clockwise rotation, probably of the order of 80°, prior to partial remagnetization during Hercynian deformation and folding. The Silurian palaeopole (VGP: 13N, 271E, dp/dm=5/9) cannot therefore be used for apparent polar wander path construction. Nevertheless, the palaeolatitude estimate (13 f 5s) suggests that southern Britain (eastern Avalonia) was at comparable mid-Silurian latitudes to Baltica and northern Britain (10-20s). Both the Iapetus Ocean and Tornquist Sea therefore, which separated southern Britain from Laurentia and Baltica respectively in Ordovician times, were closed by Wenlock time. As Ordovician and Silurian palaeomagnetic data from Baltica and Eastern Avalonia now record a comparable northwards translation from high southerly to near-equatorial latitudes during this time, we postulate pre-Wenlock collision of these blocks to form BALONIA (Baltica + eastern Avalonia). A new apparent polar wander path for eastern Avalonia constructed on the basis of this palaeogeographic model is presented.