Late Devonian to Early Carboniferous palaeomagnetic poles from the Armorican Massif, France

Abstract

In order to test plate tectonic hypotheses for the Hercynian orogeny in western Europe, Late Devonian and Cambro-Ordovician redbeds and volcanics have been palaeomagnetically studied. The Late Devonian redbeds show nearly univectorial remanent magnetizations during stepwise thermal, chemical and alternating field demagnetization and yield a pole position at 19.8° N, 144.2° E. All three Cambro-Ordovician units studied yielded characteristic directions of magnetization that are interpreted as remagnetizations of Late Devonian-Early Carboniferous age on the basis of negative fold tests, similarities with the directions of the Late Devonian redbeds, reset K/Ar ages of 345 MA, or the occurrence of significant high- temperature magnetizations. A comparison of a mean Late Devonian-Early Carboniferous pole for all four formations (28.1° N, 146.4° E, dp = 3.87°, dm = 7.50°) from the Armorican Massif with contemporaneous poles from stable (‘extra-Hercynian’) Europe indicates that there was little or no separation between Hercynian and stable Europe in that time. A significant separation between the Armorican Massif and Gondwanaland, on the other hand, suggests that an intervening middle Palaeozoic ocean existed which subsequently was consumed by subduction somewhere to the south of the Armorican Massif. Those high-temperature directions from the Cambro- Ordovician redbeds and volcanics that are relatively well grouped are interpreted as original Cambro-Ordovician magnetizations. They show shallow inclinations and north-westerly declinations, but are not sufficiently substantiated to give more than the tentative interpretation that their palaeolatitudes also are roughly in agreement with the data from stable Europe for that time.