PLUTON EMPLACEMENT BY WALL-ROCK THRUSTING, HANGING-WALL TRANSLATION AND EXTENSIONAL COLLAPSE - LATEST DEVONIAN PLUTONS OF THE COBEQUID FAULT ZONE, NOVA-SCOTIA, CANADA

Latest Devonian A-type granite-gabbro plutons, in part ductilely defor med, are spatially associated with the strike-slip Cobequid fault zone . The youngest intrusions are close to the Cobequid fault zone, which was the main conduit for magma. Two phases of deformation accompanying magma emplacement are recognized. Early magmas intruded ductile rocks during left-lateral oblique thrust movements. A second stage of right -lateral oblique slip normal faulting accommodated uplift of the pluto ns when coarse granite was emplaced in the crestal regions. Cross-cutt ing late stage porphyries, granitic clasts in marginal basins cut by g ranitic dykes, and superposition of brittle on ductile structures all indicate rapid uplift of the plutons. The geometry of the Cobequid fau lt zone shows that pluton emplacement was not the result of extension in releasing bends during transcurrent shear. Rather, flower-structure high-angle faults acted as magma conduits and space was created by tw o processes: translation of wall rocks along thrust faults at depth, d eveloping space away from the master fault zone and backward collapse of the uplifted magma chamber creating space towards the fault zone.