Is the Welsh Borderland Fault System a terrane boundary?

Abstract

The Welsh Borderland Fault System forms one of the most prominent tectonic lines in Southern Britain and, along with the Menai Straits and Malverns Fault Systems, is a candidate for a terrane boundry. A review of stratigraphic linkage across the system reveals significant contrasts in pre-Ashgill sequences. These contrasts occur particularly across its north-west edge, the Pontesford and Tywi Lineaments, rather than across the Church Stretton Lineament. The Ashgill event produced strike-slip dominated structure and an unconformity that expands from the Welsh Basin towards the platform. An important linkage sequence was produced by the subsequent late Ashgill to Llandovery marine transgression across the fault system. Silurian facies changes are marked but gradational, and no more than expected across a basin margin from stable to thinned crust. Devonian (Acadian) sinistral transpression, and later Phanerozoic movements, reactivated the fault system but did not engineer major terrane amalgamation across it. This reactivation has obscured the early history of the fault system. The remaining evidence admits, though does not prove, major Ashgill or earlier transcurrent movements. Broader considerations suggest that the Welsh Borderland Fault System originated either as a Cambro-Ordovician extensional system on the south-eastern margin of the Welsh Basin or during Late Precambrian-Early Cambrian transcurrent movements occurring within Avalonia, inboard from the Menai Strait line.