PLUTON EMPLACEMENT WITHIN AN EXTENSIONAL TRANSFER ZONE DURING DEXTRALSTRIKE-SLIP FAULTING - AN EXAMPLE FROM THE LATE ARCHEAN ABITIBI GREENSTONE-BELT

The Lake Abitibi area within the late Archaean Abitibi Greenstone Bell exhibits an interlinked plutonic, structural and metamorphic evolutio n that may characterize segmented strike-slip faults at upper-to-mid-c rustal levels. Along the major, southeastward propagating Macamic D-2 dextral strike-slip fault, rheological and preexisting D-1 structural heterogeneities induced the development of NNW-trending dextral-obliqu e splays which evolved into an extensional trailing fan and created an extensional, NNW-dipping stepover. Magma flowing upwards from deeper parts of the Macamic Fault spread towards the southeast at upper crust al levels along both the oblique-slip and extensional D-2 splays, and built several plutons in a pull-apart domain between 2696 and 2690 Ma. Different emplacement and material transfer mechanisms operated simul taneously in different parts of the system, including fault dilation a nd wedging, lateral expansion, wall-rock ductile flow and stoping. Tra nsfer of movement between D-2 splays occurred under ductile conditions during syn-emplacement, amphibolite-grade metamorphism (500-700 degre es C). During cooling (<2690 Ma), narrower brittle-ductile zones of gr eenschist-grade shearing were concentrated along the pluton-wall rock contacts, but the extensional stepover locked since both normal and re verse movements occurred along NNW-dipping faults. Pluton emplacement, contact metamorphism and propagation of D-2 faults appear to have bee n closely linked during the Superior Province-wide late transpressiona l event. (C) 1998 Elsevier Science Ltd.