Paper No. 7
Presentation Time: 2:55 PM


SCHOFIELD, David I.1, LESLIE, Graham2, WILBY, Philip R.3, DARTNALL, Rosemary4, WALDRON, John W.F.5 and POTHIER, Hayley5, (1)British Geological Survey, Columbus House, Tongwynlais, Cardiff, CF15 7NE, United Kingdom, (2)British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, United Kingdom, (3)British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom, (4)School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom, (5)Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada,

The island of Anglesey, North Wales, comprises a collage of late Neoproterozoic and Lower Palaeozoic successions that together illustrate the complexity of protracted accretionary margin evolution. In this case spanning Palaeo-Pacific subduction, terrane translation into Iapetus during Rodinia breakup and rift-drift and subduction-accretion tectonics during dispersal and reassembly of Laurentia-Baltica.

The earliest accretionary events are recorded in penetrative, largely non-coaxial deformation preserved in siliciclastic metasediments of the Aberffraw Formation that record south-facing thrust shortening of the late Neoproterozoic continental margin of West Gondwana. Development of this structure is considered contemporary with 666 Ma metamorphism recorded in gneisses in the structurally lower part of the formation.

Renewed arc development is recorded by intrusion of the 615 Ma Coedana Granite and development of a metamorphic aureole overprinting penetrative fabrics in the Aberffraw Formation. These are unconformably overlain by mass flow deposits of the Bodorgan Formation, comprising excavated mature continental margin sandstones and carbonates with a matrix of arc-derived feldspathic sandstone. The mega-breccias pass up into pillow basalts and are interpreted to record rifting of the upper plate in response to subduction zone roll back.

Final shallowing and stalling of Palaeo-Pacific subduction is constrained to younger than c. 560 Ma blueschist metamorphism in the Penmynydd Zone. We interpret this to have been emplaced into the thinned composite upper plate, outwith its subduction channel, during the onset of Early Cambrian Iapetus rifting.

Development of the passive continental margin of Iapetus is recorded by deposition of mature basinal successions. In north Wales these include the Monian Supergroup, of Amazonian origin, and Harlech Dome of West African origin. Detrital zircon provenance evidence suggests contiguity of these by Late Cambrian times, closely pre-dating Early Ordovician Penbscot accretionary tectonics and emplacement of the Monian Supergroup onto the composite late Neoproterozoic foreland.

Middle Ordovician evolution is dominated by foreland basin subsidence and subsequent Salinic and Acadian thrust reactivation during terminal closure of Iapetus.