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The Wilson Cycle, the repeated opening and closing of oceans, commonly along roughly the same lines, is here attributed to the presence of eclogite-facies roots of partially collapsed orogens. We present a finite-element thermal model that suggests that such roots will weaken the orogenic lithosphere relative to that of the adjacent foreland for hundreds of millions of years and make it a preferred site for later rifting. Keywords: Wilson Cycle, continental margin, orogenic belts, eclogite.

Early Ordovician volcanic rocks exposed in the South Mayo region of western Ireland document the history of a volcanic arc complex, produced following the initiation of south-dipping subduction within the Iapetus Ocean in the Late Cambrian or Early Ordovician. Lavas of the Lough Nafooey Group (Tremadoc - Arenig) show an eruption history marked by an initial period of tholeiitic basaltic volcanism, followed by a shift to increasing silica compositions of calc-alkaline affinity. Subsequent volcanism of the Tourmakeady Group (Arenig) shows a mixture of rhyolitic and andesitic compositions, which are in turn succeeded by tuffs of the Rosroe and Mweelrea Formations of the Murrisk Group (Llanvirn). These are also rhyolitic and andesitic but are generally more siliceous. The overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening. Trace element studies of the basaltic members of the Lough Nafooey Group show a strong subduction zone influence. While absolute levels of immobile trace and rare earth elements decrease upsection, relative depletion of the most incompatible elements decreases over the same time period. These trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection. The basaltic Bohaun Volcanic Formation (Tremadoc - Arenig ?) shows greater depletion in its incompatible trace elements than the Lough Nafooey Group, so indicating a more depleted mantle source. Comparison with modern arc systems suggests that the Bohaun Volcanic Formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the Lough Nafooey Group (i.e. in the forearc). This confirms a south-dipping polarity to the subduction zone, and suggests an age of initiation during the Late Cambrian or earliest Ordovician.

New palaeomagnetic data from the Caledonides of western Ireland indicate that the Silurian rocks of South Mayo underwent oroclinal bending, following folding, in Siluro-Devonian time. Bending was accommodated on faults cutting the Silurian sequence, and was driven by strike slip motion across the Antrim - Galway Line, a recently recognized major curvilinear lineament. We propose that the Silurian rocks in the east of the region suffered up to 30 degrees clockwise rotation, increasing towards the west to approximately 70 degrees. This newly inferred differential rotation of the Silurian rocks, about a sub-vertical axis, implies that the underlying Dalradian rocks of the Connemara Massif to the south must also have undergone clockwise rotation in late Silurian/early Devonian time. This is consistent with published palaeomagnetic data from the Connemara Gabbro and accounts for the swing in strike of the Caledonian orogen in this part of western Ireland. From this we deduce that the swing in strike of the Caledonian rocks elsewhere on the west coast of Ireland might have a similar explanation.

An analysis describing the geological historical structure of southern Ireland is presented. The analysis combines BIRPS deep seismic reflection data with other geological and geophysical data to examine the tectonic character of Ireland's crust. It is shown that the reflector called the 'Variscan Front' only draws a map line across the British Isles but has no consistent tectonic character.

Iapetus suture zone crustal structure is examined via deep seismic profiles of the Irish and North seas. Interpretation of such data is often problematic in that the suture reflectors chosen project to the basement well top, south ofthe Solway Line. A revised method is proposed, involving the identification of the leading end of imbricated Avalonian crust as a common reflectivity boundary. This makes invoking northward suture backthrusting unnecessary.