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1 Department of Geology, University of Wales, Cardiff CF1 3YE, UK
2 Department of Geology, National Museum of Wales, Cardiff CF1 3NP, UK
The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and PT conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Ll
n peninsula (Lln Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to c. 550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.
