THE EAST IRISH SEA AND ADJACENT BASINS - NEW FAULTS OR OLD

The East Irish basin is characterized by three sets of faults, strikin g N-S, NE-SW and E-W. The Menai Strait fault system bounds the basin t o the south and shows pre-Caledonian to Cenozoic movement. This span o f activity contrasts with similarly oriented faults, to the north of t he basin, which did not exist before the early Carboniferous. N-S faul ts have a similarly diverse history. Some may have influenced Silurian sedimentation and controlled the disposition of Ordovician volcanic c entres in Snowdonia, a contrast with N-S-striking faults in the north of the area which originated as steep strike-slip faults during the la te-Silurian. E-W faults control Dinantian and Permo-Triassic thickness variations although, unlike the other fault trends, pre-Carboniferous precursors are nor apparent. Some of the basin-bounding faults develo ped as wholly new faults during Permo-Triassic extension.