The quest for primary Devonian magnetism in the Pyrenees and the implications for the Paleozoic paleogeographical position of Iberia

Abstract

The paleogeography of Iberia during the Devonian is still under debate. Paleomagnetic data suggests an episode of rifting between Iberia and the rest of Gondwana during the Middle Paleozoic whereas results from paleontology, geochronology and geochemistry argue that such a departure is unlikely. To shed more light on this intricate matter extensive paleomagnetic fieldwork has been undertaken in the Pyrenees, sampling 18 Devonian limestones and 1 Permian granite. Stepwise demagnetization and end-member modeling indicate magnetite and pyrrhotite as responsible for the magnetization. The abundance of pyrrhotite and several negative fold tests reflect on the magnetism’s secondary character. Of these 19 sites, 7 record the counterclockwise rotation of Iberia during the Cretaceous with a combined direction of 326.9°/38.1°, k=7.7 and a95=5.5°. The other sites either have scattered directions or lack consistency; trying to link those to known vertical axis rotations in the Pyrenees have proven unsuccessful. The directions recorded in the granite (217.8°/40.5°, k=24.7 and a95=11.4) are somewhat similar to a component found in another study (Tait & Bachtadse, 2000) (224°/51°, k=24.5 and a95=8.5°) that is used to justify the separation of Iberia and the rest of Gondwana during the Devonian. It is likely that both these directions are the result of a remagnetizion event with an as of yet unknown timing since it is impossible to record a Devonian direction in rocks from the Permian. Combining these results with the counterarguments provided by other fields of geology and the scenario in which Iberia drifted away from Gondwana during the Paleozoic is no longer supported.