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

Abstract

S U M M A R YEarly 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 = 095"C, 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: 13"N, 271"E, 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 (10-20"s). 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.

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

“…Paleomagnetic studies from southern Britain have yielded declinations that support major Paleozoic rotations (e.g. Trench and Torsvik, 1991;Torsvik et al, 1993;Piper, 1997). Notably, declination data reported by Vizan et al (2003) suggest a 165°clockwise rotation of rocks in the Midland Platform between Late Ordovician and Carboniferous, entirely consistent with the geometry shown in Fig.…”

Precessional and half-precessional climate forcing of Mid-Devonian monsoon-like dynamics

“…This basin was limited by the Euramerican Old Red Continent to the north and by the Normannian and Mid-German highs to the south (Ziegler, 1990). The apparent polar wander paths of Avalonia and Euramerica are well established (Torsvik et al, 1993;Mac Niocaill and Smethurst, 1994). Torsvik et al (1993) estimated the paleolatitude of Southern Britain at 389 Ma to be 6.9 • S. During the Middle-Devonian, the studied section and Southern Britain were only a few hundred kilometres apart (Cocks et al, 1997) and consequently this value serves as a good estimate for the paleolatitude of the section.…”

The Trans-European Fault: a critical reassessment

“…Recent ideas favour a terrane accretional model rather than fault movement to interpret the observed geology of the region (Ziegler, 1990;Berthelsen, 1992a;Torsvik et al 1993;Meissner, Sadowiak & Thomas, 1994;Tanner & Meissner, 1996). Such a model envisages the northward convergence and accretion of Gondwana-derived continental fragments to the southern margin of the newly-forming Laurussian super-continent (Ziegler, 1989).…”