Iberian and Welsh Variscan Oroclines: A record of Permian subduction within the heart of Pangea and the case against a long-lived stable Pangea A

Thursday, 18 December 2014: 11:05 AM
Stephen T Johnston, University of Victoria, Victoria, BC, Canada
Arguments against Pangea B are that the Intra-Pangean Paleomagnetic anomaly was based on poor data, and that there is no geological evidence for the megashear required to bring about Pangea A prior to the Triassic. I address the implications of map-view bends (oroclines) of the Carboniferous Variscan orogen, the orogen inferred to record the Pangea-forming Gondwana – Laurussia continental collision upon closure of an intervening Rheic ocean.

The Variscan orogen of Iberia has been depicted as a convex to the west arcuate feature (the Ibero-Armorican Arc). The IAA assumes continuity of geological belts of southern Great Britain, and is cored by the Cantabrian orocline, an isoclinal orocline affecting the Gondwana foreland fold and thrust belt. We demonstrated that the Cantabrian orocline is coupled to a more southerly convex to the west Central Iberian Orocline. Together these coupled oroclines (1) define a continental scale S-shaped fold of the Variscan orogen; and (2) require that the continuation of the Rheic suture and Variscan geological belts lies to the west and south. Palinspastic restoration of the oroclines yields a 2300 km long previously linear orogen. Paleomagnetic, structural and stratigraphic data constrain orocline formation to a 10 Ma interval starting at ~ 300 Ma, involving 1100 km of relative translation. The Rheic suture and geological belts in southeastern England similarly define a convex to the west ‘Welsh’ orocline: a NNE-striking serpentinite belt in SE Ireland is a continuation of the Lizard ophiolite around the orocline. This geometry implies that the Rheic suture and more internal geological belts cross back into Britain to the north, and explains paleomagnetic data that suggest 180° of rotation of southern Britain. Orocline formation is probably coeval with Cornubian magmatism and requires subduction and large-scale translations as late as 270 Ma.

The Iberian and Welsh oroclines require thousands of kms of translation between 310 and 270 Ma at rates of as high as 10 cm/a. The creation of large-scale oroclines, the implied rates of translation, and the voluminous syn-kinematic magmatism all points to the need for subduction of oceanic lithosphere within the heart of Pangea well into the Permian and suggests that the ‘megashear’ may have consisted of one or more subduction zones.