Strike-slip terranes and a model for the evolution of the British and Irish Caledonides

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Abstract
Evidence is presented that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period. Some, such as the Highland Boundary Fault, moved in this way at an earlier stage in the Ordovician. These data allow the Caledonian rocks lying between the Laurentian miogeocline (whose basement is represented by the Lewisian, Moine and possibly the Dalradian) and the Gondwanaland miogeocline (Midland Platform and Welsh Basin) to be re-analysed as a group of disorganized terranes which originated to the southwest in North America and southwest Europe/Africa prior to the Silurian.The Highland Border TerraneandNorthern Belt Terraneare interpreted as duplicated pieces of a mid-Ordovician sequence which was a back are to northwest subduction. TheMidland Valley Terraneis interpreted as a slice of Laurentian foreland onto which ophiolites were obducted in the lower Ordovician but which became the basement of a continental margin arc to northwest subduction in the mid-Ordovician. TheCockburnland Terraneis inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip. TheConnemara Terraneis regarded as an allochthonous piece of the Dalradian miogeocline and theSouth Mayo Terraneas a remnant of an early Ordovician arc and fore arc which in mid-Ordovician times became a back arc/marginal basin to northwest subduction. TheLake District-Wexford Terraneis part of an arc to southeast subduction under Gondwanaland whose activity climaxed in the mid-Ordovician. TheCentral Terraneis interpreted as a Silurian overstep assemblage which blankets the junction between Laurentian- and Gondwanaland-derived oceanic terranes, and therefore Iapetus is regarded as an Ordovician ocean which closed prior to the Silurian. The model suggests that at the end of the Silurian, a clockwise-rotating Gondwanaland, having carried Laurentia into collision with Baltica, broke free and created a major sinistral strike-slip zone which disrupted the Ordovician palaeogeography in the British Isles/North American sector of Iapetus.