Ductile transpression and localization of deformation along tectonic boundaries in the Caledonides of northwestern Ireland

Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D-1 was not associated with major structures, D-2 is related to north-northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D-2 deformation subsequently became the locus of intense D-3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S-2 fabrics being transposed by S-3 resulting in a composite foliation over large areas. Similarly, the L-2 mineral lineation is abruptly transposed by L-3 over relatively small distances indicating high D-3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D-4) which was increasingly localized and focused into earlier (D-2-D-3) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F-4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.