Major northwest-directed Caledonian thrusting and folding in Precambrian rocks, northwest Mayo, Ireland

Abstract

AbstractStructural mapping of the North West Mayo Inlier has investigated the kinematics of ductile fault and shear zones within the Dalradian and pre-Dalradian rocks. The area is characterized by an intense east to southeast trending elongation direction. Pre-Caledonian deformation is preserved as west-northwest trending sinistral mylonite zones. These are cross-cut by Grenville pegmatites and by undated mafic dykes. Subsequent deformation has been influenced by the majorbasement structures. During the Caledonian deformation, the pre-existing sinistral structures were reactivated coaxially as steep sinistral shear zones, deforming the mafic dykes. Major northwesterly verging thrusts are widespread in flat-lying zones which occur over much of the inlier. These are interpreted as D2 in age. The S2 fabric is the dominant fabric and contains an east to northeast trending mineral lineation. The southeastern part of the inlier is characterized by very large scale (wavelengths of kilometres) reclined D2 folding. This is interpreted as a crustal scale ramp zone. In the vicinity of this ramp there is evidence of earlier (F1) folds which are possibly the product of ramprelated deformation. All of these structures are interpreted as the product of northwest verging thrusting with crustal scale frontal and lateral ramps.