Devonian-Carboniferous paleopoles for Africa: Consequences for Hercynian geodynamics

Abstract

Paleomagnetic analysis of marine formations in the northwestern Sahara (Algeria) yields the following results: three sites in the lower Devonian Orthoceras-bearing limestones of Béni-Abbès (29.67° N, 2.15° W) indicate a mean direction of remanence: D m = 149.6°; I m = 1.6°; α 95 = 6.8°. This typical Permian direction implies a complete remagnetization of the sites. The Famennian griotte limestones of the Ben-Zireg anticline (31.92° N, 1.82° W) show high remanent magnetization intensities. In this structure, which has undergone polyphase tectonism, multicomponent analysis evidences a high unblocking temperature (HTb) component carried by haematite leading to a positive fold test (after partial unfolding) at the 95% confidence level with a mean direction D = 156.4°, I = 47.6°, α 95 = 0.9° and attributed to the Early Carboniferous (355 Ma) with a paleopole at 25.3°S, 21.1°E, dp = 0.8°, dm = 1.2°. An intermediate unblocking temperature (ITb) component of magnetization carried probably by magnetite, also present, satisfies a positive fold test (after tilt correction), with a mean direction D = 154.9°, I = 54.3°, α 95 = 3.7°. This pre-folding direction of magnetization is attributed to the Famennian. It gives a paleopole at 19.2°S, 19.8°E, dp = 3.7°, dm = 5.3°. Comparison with previous results shows a significant S-N crustal shortening of about 2500 km during Carboniferous times accompanied by a clockwise rotation of the Saharan region of about 25°. These new results give information about the tectonic phenomena involved in the collision between Gondwanaland and Laurasia: they argue for a Pangea A 2 dominating model, and define mean values for shortening rate and plate velocity comparable to the present ones (3 cm/yr).