Abstract

Structural mapping of Upper Ordovician (Caradoc) volcaniclastic rocks around Llyn Ogwen, North Wales has revealed a hitherto unrecorded, kilometre-scale, Ramsay 'type-3' refold pattern. Each phase of folding is associated with an axial planar low grade, metamorphic fabric. The Caradoc aged, sub-volcanic Ogwen microgranite, contains two fabrics which are contiguous with those in the metasedimentary rocks. Recognition of two tectono-metamorphic events may simply explain the discrete Late Silurian and Early Devonian isotopic ages for deformation in North Wales and has implications for models of Acadian tectonics and the use of K/Ar and 40Ar/39Ar dating techniques in slate belts.

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