DUCTILE TRANSPRESSION AND LOCALIZATION OF DEFORMATION ALONG TECTONIC BOUNDARIES IN THE CALEDONIDES OF NORTHWESTERN IRELAND

Orogenesis is increasingly interpreted in terms of strain focusing, lo calization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existi ng structural and stratigraphic variability providing inherent zones o f crustal weakness. Detailed structural investigation of Neoproterozoi c Dalradian metasediments in the Glencolumbkille region, northwest Ire land, enables patterns of reworking and strain localization to be asse ssed in terms of four overprinting ductile deformation episodes. A wel l-defined and intricate Dalradian stratigraphy provides readily distin guishable markers which not only focus deformation along marked rheolo gical boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationsh ips 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-amphibo lite facies metamorphism. Structures generated during D-2 deformation subsequently became the locus of intense D-3 strain and were reactivat ed in an oblique sense associated with south or southwest directed tra nslations. Local overprinting relationships clearly demonstrate S-2 fa brics being transposed by S-3 resulting in a composite foliation over large areas. Similarly, the L-2 mineral lineation is abruptly transpos ed by L-3 over relatively small distances indicating high D-3 strain g radients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D-4) which was increasingly localized an d focused into earlier (D-2-D-3) high strain zones, is marked by a pro nounced phase of sinistral transpression associated with clockwise cle avage and minor fold transection of northwest verging upright folds. S inistral shear is strongly partitioned in to the steep limbs of mesosc opic F-4 folds. The detailed investigation of structures generated wit hin such multiply deformed and reworked zones provides evidence of bot h the kinematic and tectonic evolution of regional deformation systems .