PALEOGEOGRAPHIC SIGNIFICANCE OF MID-SILURIAN PALEOMAGNETIC RESULTS FROM SOUTHERN BRITAIN - MAJOR REVISION OF THE APPARENT POLAR WANDER PATHFOR EASTERN AVALONIA

Early 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 (Ty pe 2) during stepwise thermal demagnetization. The type of demagnetiza tion behaviour is related to changes in magnetic petrology within the lava pile. The lower and intermediate unblocking temperature component s of Type 2 represent magnetic overprints of Tertiary/Recent and Hercy nian (mid-Carboniferous) origin respectively. The primary nature of th e high unblocking temperature magnetization component, Types 1 and 2 ( Dec = 095-degrees-C, Inc = -24-degrees, alpha95 = 8.8-degrees), is dem onstrated by positive agglomerate tests. Conversely, an overlying Uppe r Old Red Sandstone sequence did not provide primary magnetizations; t he magnetic signature is governed by syn-tectonic Hercynian remagnetiz ations. The primary Silurian data demonstrate that the East Mendips In lier experienced considerable clockwise rotation, probably of the orde r of 80-degrees, prior to partial remagnetization during Hercynian def ormation and folding. The Silurian palaeopole (VGP: 13-degrees-N, 271- degrees-E, dp/dm = 5/9) cannot therefore be used for apparent polar wa nder path construction. Nevertheless, the palaeolatitude estimate (13- degrees +/- 5S) suggests that southern Britain (eastern Avalonia) was at comparable mid-Silurian latitudes to Baltica and northern Britain ( 10-20-degrees-S). Both the Iapetus Ocean and Tornquist Sea therefore, which separated southern Britain from Laurentia and Baltica respective ly in Ordovician times, were closed by Wenlock time. As Ordovician and Silurian palacomagnetic data from Baltica and Eastern Avalonia now re cord a comparable northwards translation from high southerly to near-e quatorial latitudes during this time, we postulate pre-Wenlock collisi on of these blocks to form BALONIA (Baltica + eastern Avalonia). A new apparent polar wander path for eastern Avalonia constructed on the ba sis of this palaeogeographic model is presented.