Tectonic controls of Ordovician arc and marginal basin volcanism in Wales

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Abstract
Ensialic destructive plate margin volcanism persisted in Wales from late Tremadoc to Caradoc times. Late Tremadoc uplift and erosion in NW Wales is attributable to onset of subduction and development of a frontal ridge system (fore-arc). South-east of this, in the vicinity of the Harlech Dome and in SW Wales, more localized uplift was associated with the development of arc volcanoes at a volcanic front. In North Wales the arc volcanism occurred in a regime of E-W crustal extension and was centred on a deep crustal discontinuity. During Arenig times the arc terrane was transgressed. Subsequently, marine conditions, with local and temporary emergence, persisted through Arenig to Caradoc episodes of marginal basin volcanism. While there was widespread and persistent subsidence in Wales during development of the marginal basin, the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence. The grabens mark the sites of deep and steep crustal discontinuities (‘fractures’), one of which controlled the earlier arc volcanism. Changing sites of volcanism in Wales can now be seen as due to transfer of active extension from one fracture to another. There is evidence that the fractures were present by Tremadoc times, and they almost certainly formed before development of the Cambrian marine rift-basin. They are most likely to have been strike-slip faults or shear zones, and their pattern in Wales indicates that they may represent part of a complicated deep-crustal strike-slip duplex system. The volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults (flower structure) which channelled magmas through the cover during extension across the fractures. The fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during Late Caledonian compression and transpression. In North Wales the volcanically active grabens trended broadly N-S, oblique to the exposed NE-SW basement tectonic grain, and dominantly E-W extension seems to have persisted throughout the Ordovician volcanism. While there is considerable uncertainty concerning Ordovìcian plate tectonic configurations and displacements, the extension is best explained by invoking a N-S convergence vector between the oceanic and over-riding microcontinental plates, with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin. The model highlights the suspect nature of the relationship between the Welsh Ordovician terrane and others to the north.