Carboniferous North Atlantic palaeogeography: stratigraphic evidence for rifting, not megashear or subduction

Abstract

AbstractThree theories have been proposed for the origin of Carboniferous basins in Britain: megashear; tension from Rheic Ocean subduction; tension from rifting of the North Atlantic. The first two hypotheses are rejected because they do not explain the Carboniferous volcanism, Stephanian dyke swarm, Boreal marine transgressions during the late Carboniferous and Permian, Carboniferous sedimentary basin histories, basin types or basin orientations.Carboniferous volcanics were rift-related due to crustal thinning, which also resulted in the formation of sedimentary basins in the British Isles and a marine transgression of Tethyan faunas. Newly formed Carboniferous fracture lines and basin orientations showed that tension varied between east–west and northwest–southeast. Crustal fracturing in the latest Dinantian, possibly due to collision of microcontinents in the closing Rheic and Phoibic oceans with North America, led to the synchronous initiation of rift basins in East Greenland, elevation of source areas for the Millstone Grit of the British Isles, and formation of transform fault zones near Svalbard and North Spain. The narrow, rapidly subsiding, quickly changing ‘fosse’ basins which formed in these transcurrent fault zones contrast with the coeval stable, slower subsiding rift-parallel ‘saucer’ basins of the British Isles. Variations of subsidence rates in all these basins allow interpretations of crustal stress history. Brittle fracturing in the Westphalian C formed the first oceanic crust, but free oceanic spreading from Spain to southwest of the Faeroes only occurred after Stephanian dyke intrusion and crustal thinning at the Faeroes. The Norwegian Sea underwent continental crustal thinning. Ocean spreading and crustal thinning ceased in latest Stephanian times.Boreal marine transgressions advanced down the newly thinned Norwegian Sea, firstly along its northern part and then reaching the North Sea basins and Germany in the Rotliegendes and Zechstein. Synsedimetary ore deposits formed during early tensional fracturing of the crust; thick coal sequences formed in rift-parallel basins during clastic source decay of elevated rift margins; oil source rocks formed along narrow oceanic or continental rifts from the Westphalian C onwards.