Abstract

The Highland Border–Fair Head–Clew Bay–Baie Verte–Brompton Suture (Clew Bay Suture) on Achill Island, western Ireland is an E–W-trending zone of vertical, ductile dextral transpression bounding the southern edge of the Dalradian terrane. The Achill Dalradian, a sequence of coarse-grained psammite overlain by conglomerate, quartzite, schist and limestone, disposed in recumbent curvilinear folds, represents Laurentian continental margin deposits which were translated westward during deformation, parallel to the suture trace. Studies of fabric development and albite porphyroblast growth indicate that E–W dextral transpression and west-directed recumbent folding were synchronous, and are interpreted as separate expressions of the same NW-directed terrane impingement event. This process of terrane-docking was marked by a c.4kbar phase of decompression

Within the suture, four separate terrane-slivers are recognized. The presence of pyrope-garnet and crossite-amphibole within two imply the Clew Bay Zone experienced sufficient displacement to exhume rocks from deep lithospheric-levels. In addition, the extensive retrogression and destruction suffered by these remnant high pressure mineral assemblages during the albite-generating transpressional phase, implies that ductile deformation was enhanced by the introduction of fluid into and along the suture zone during impingement.

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