MAJOR LENTICULAR STRUCTURES IN THE OTTAWA RIVER GNEISS COMPLEX, SW GRENVILLE PROVINCE: MEGABOUDINS FORMED DURING 2-STAGE DUCTILE OROGENIC COLLAPSE

In the SW Grenville Province, early-Ottawan ductile thrusting of granulite-facies orthogneisses created zones of fine-grained, gently SE-dipping banded gneisses that played a prominent role in the ensuing post-peak evolution. Exhumation of the Ottawa River Gneiss Complex (ORGC), the core of a large metamorphic core complex, involved two kinematically distinct phases of extensional reworking of banded gneiss zones: (i) disharmonic, transtensional cross-buckling; and (ii) transverse extension centred in the detachment zone of the ORGC, the Muskoka domain, in which reworked banded gneisses enclose an array of SE-plunging disk- or lozenge-shaped major lenticular structures (MLSs) up to 50 km in long dimension. The focus of this poster is on the latter phase.

The cores of MLSs consist of weakly strained and partly retrogressed granulite-facies orthogneisses, but their margins and tails are much finer-grained, intensely strained and retrogressed amphibolite-facies banded gneisses. We previously interpreted them as parts of anastomosing shear-zone systems formed during extensional collapse, but the MLSs lack the second- and third-order anastomosing shear zones that characterize such systems. Moreover, the presence of a km-scale necked zone in a major upright cross-fold, the Bracebridge antiform, and the extensional origin of an adjacent bending fold, the Camel Lake synform, adjacent to and in the northern Muskoka domain, attest to a later, kinematically distinct phase of late-Ottawan extension. In summary, the evolution of banded gneiss zones in and adjacent to the extensional detachment zone of the ORGC indicates that: (i) they were the sites of ductile décollement during km-scale cross-folding, and (ii) they formed incompetent layers during subsequent orogen-perpendicular extension that led to formation of MLSs by megaboudinage.