Low‐angle extensional faulting and the emplacement of the Connemara Dalradian, Ireland

A breccia mapped at the contact of the Connemara Dalradian and the unconformably overlying Silurian succession of north Galway shows evidence of having formed from strains developed at the brittle‐ductile deformation transition. This breccia was surficially exposed just prior to the onset of an Upper Llandovery marine transgression. The geometry of the breccia outcrop is consistent with its having developed along a regional low‐angle extensional fault. Two possible regimes of formation are proposed. In the first the breccia represents the remnant of a significant low‐angle strike‐slip extensional fault along which the Connemara Dalradian was laterally emplaced. In the second the extensional faulting may be a slightly later event, related to the rapid uplift of the Connemara Massif. Both models involve rapid footwall uplift, an event already inferred from metamorphic criteria. The fault may be compared to major extensional detachments such as those in the Basin and Range Province of the United States. However, this is the first description of an early Caledonian fault of this nature and may have important consequences for future tectonic studies in this (and other) orogen(s).