Palaeomagnetism of Devonian ring complexes from the Bayuda Desert, Sudan-new constraints on the apparent polar wander path for Gondwanaland

Abstract

SUMMARY The approach of Gondwana, and/or microplates derived from its northern margins, towards Laurentia and the closure of the intervening Palaeo-Tethys is a controlling factor in the geodynamic evolution of the Atlantic bordering continents during the early and mid-Palaeozoic. However the currently available palaeomagnetic data for Gondwana, especially for the geodynamically critical Silurian to Devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios. In order to address this issue 114 samples (18 sites) have been collected from a series of high level acidic intrusive rocks, rhyolitic lavas and tuffs of mid-Devonian age (377 ± 5 Ma, Rb/Sr) along the southern escarpment of the Gilif Hills, Sudan (17.83°N, 32.67°E). Detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization, labelled A, B and C. The overall mean direction of component A (Dec. 0.9°, Inc. 29.8°, α95= 4.6°, k= 91.1, N= 12 sites) is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern Sudan. Mixed polarities suggest a Recent or Pleistocene age of this component. Component B also displays mixed polarities. The site mean directions for this component average to 6.5° declination, -40.2° inclination with α95= 7.0° and k= 48.1 (based on 10 sites) and yield a palaeomagnetic pole at 48.8°S, 23.5°E (dp= 5.1°1, dm= 8.4°). Maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component A and/or B. Both A and B are the probable results of low-temperature oxidation of magnetite. Although the age of component B can not be directly determined, it is inferred by correlation with published data to reflect a late Carboniferous overprint. Component C, identified in 11 sites (39 samples), is characterized by a very steep downward magnetization (Dec. 296.2°, Inc. 79.3°, α95= 10.8°, k= 18.7, n= 11 sites) which corresponds to a palaeomagnetic pole position in the southwestern corner of Libya (25.9°N, 11.6°E, dp= 19.6°, dm= 20.6°). Maximum blocking temperatures of component C, generally below 580°C, are indicative of magnetite as carrier of this remanence. The results of thermal remanent magnetization experiments demonstrate that component C is a thermo-remanent magnetization, probably primary, and suggest this component to be representative for the direction of the Earth's magnetic field during emplacement of the Gilif Hills volcanic rocks in mid-Devonian times. The position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide (approx. 5000 km) Pre-Hercynian ocean during Devonian times. The simplest reconciliation of component B with the reference apparent polar wander path is achieved by assuming it to be of Carboniferous age and by invoking a westward cusp in the Carboniferous part of the apparent polar wander path. Taken at face value, the age of this cusp coincides with large-scale post-orogenic right lateral strike slip movements in Hercynian Europe and North Africa. This interpretation does not require the apparent polar wander (APW) loop and very high rates of plate motion implied by some alternative models. However, if it is valid it will require reinterpretation of the remanence-age of one West African intrusive suite and of some eastern Australian Palaeozoic rocks.