THE ORDOVICIAN EVOLUTION OF THE SOUTH MAYO TROUGH, WESTERN IRELAND

The South Mayo Trough, a broad synclinorium containing a 10‐km sequence of Early Ordovician turbidites passing up into shallow water sandstones, is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north‐facing arc. The arc collided with the Laurentian margin in Early Llanvirn times to deform the Dalradian Supergroup rifted margin clastic sequence. Prior to the late Llandovery, sinistral transcurrent faulting transposed the Connemara Dalradian terrane to its present position on the “outboard” ocean ward side of the South Mayo Trough. The South Mayo Trough was fed principally by arc and ophiolite‐derived elastics during the Arenig with an increasing metamorphic component during the Llanvirn. The Late Ordovician transpressional “docking” of Connemara provided the southerly source for high‐grade metamorphic clasts in the Deny veeny Formation.