Image:Hercynian/Variscan unconformity in Torbay
The reconstruction of tectonic plates over the last 300Ma is based primarily on magnetic evidence. As Paleozoic oceans have all been destroyed and as paleomagnetism then picks up huge errors, the reconstruction of a different set of plates for these older times must resort to geological and paleobiological studies. This includes the identification of ‘Wilson Cycles’, a term named after the great Canadian geologist J. Tuzo Wilson, who proposed in the 1960s that different oceans separated America and Africa/Europe twice over the last 500Ma. ‘Wilson Cycles’ may be long or short , applying to wide and narrow oceans, with cycles commencing with periods of rifting and ending in the compression associated with plate collision. Key aspects in such work include identification of rifts, differing magmatic types, ophiolites, folding and metamorphism Prior to their eventual closure in the Variscan/Hercynian orogeny in Devonian to Carboniferous times, such evidence suggests that at least 6 Paleozoic oceans separated the stable cratonic regions of Scotland (Laurentia) and Africa (Gondwana), some opening after others had closed. The Wilson Cycle that can be most clearly identified is that associated with the Devonian ‘Rheno-Hercynian’ ocean in the type area of Devon itself. This talk will therefore set the tectonic frame for an RGS field trip to the UNESCO English Riviera Geopark around Torbay in 2Q 2027.


Image:South Devon Chronostratigraphy: Duncan Macgregor

Duncan Macgregor is a retired petroleum geologist, with a specialism in the tectonic history of Africa. He now works on his African website www.africageologicalatlas.com. His work has recently expanded back to Paleozoic times, which necessarily includes the interplay of Gondwana with areas closer to home, as e.g. Avalonia (England and Newfoundland) was a plate detached from northwest Africa as part of Tuzo Wilsons’ first ‘Wilson Cycle’.