Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales—a review

Abstract

The Mona Complex of Anglesey and Llŷn can be divided into: (1) gneisses and granites; (2) Penmynydd schists; (3) Monian Supergroup (= Bedded Succession of Greenly, 1919). Monian stratigraphy has been repeatedly revised yet important problems remain in determining relative and absolute ages of rocks within the Complex. Some Monian rocks long believed to be Precambrian have recently yielded Cambrian palynomorphs (Gwna Group) and radiometric dates as young as 518 ± 17 Ma (Sarn granite). Given the uncertainty which exists over the age of the earliest Cambrian sedimentation in the Welsh Basin (somewhere between 590–510 Ma), there need be no conflict between Monian age data and the field evidence which indicates the Mona Complex to be older than the Welsh Basin. The occurrence in Anglesey of blueschist, melange and ‘ophiolites’ (?), with nearby ignimbrites, has resulted in Monian geology being explained in terms of palaeo-subduction. The evidence for this is weak. No-one has yet proved the existence of a Monian accretionary prism, or forearc basin, or volcanic are. All previous models invoking Monian subduction are rejected as oversimplistic. Like the Ganderian of Newfoundland, Monian rocks once lay on the NW margin of the Avalonian plate. Most previous subduction models assume this location to be synonymous with the SE margin of the Iapetus Ocean. Yet the Monian Supergroup, with its enigmatic provenance, may have been deposited before Iapetus opened. The source area could even have been the ‘vanished continent’ possibly removed by transcurrent faulting from a position S of the Dalradian Basin. The potential importance of early transcurrent movement to Monian evolution has been overlooked. Internally, much of the complex appears as a tectonic collage with steeply dipping mylonitic ‘Penmynydd’ schist zones of probably early Cambrian age separating strips of deformed low grade melange from continental crustal basement. The Complex has little in common with adjacent areas and may represent part of an exotic terrain moved to approximately its present position during the Cambrian.