A geological and tectonic cross-section of the Caledonides of western Ireland

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Abstract
Geological and geophysical criteria allow division of the Caledonides of western Ireland into four principal zones. Zone 1, in the north, contains Grenvillian basement with late Proterozoic supra-crustals. Zone 2 contains the Westport Complex, the Deer Park Complex and the South Mayo Trough. Connemara and the Delany Dome Formation occupy Zone 3. Zone 4 contains the South Connemara Group and mid- to late-Ordovician and Silurian sediments and volcanic rocks. The various tectonic models proposed for these zones are discussed and a ‘minimum’ tectonic model is presented, which involves early Ordovician collision of the northern part of Zone 2 with the Laurentian margin (Zone 1) followed by subduction flip (northern part of Zone 4). The site of early collision between Zones 1 and 2 subsequently underwent orogenic collapse and strike-slip dismemberment leading to the sinistral transpressive emplacement of Zone 3. Assemblages in Zone 4 were accreted to the now south-facing arc. Northwards stress transfer consequent on suturing deformed the thinned crust and overlying Silurian sequences in Zones 2 and 3.