RETROGRESSED BANDED GNEISSES AS INDICATORS OF INVERTED DUCTILE THRUSTS, WESTERN OTTAWA RIVER GNEISS COMPLEX, GRENVILLE PROVINCE OF ONTARIO

It has been known since at least the 1960s that banded orthogneisses (BOs) form by intense ductile strain and recrystallization of xenolith-bearing, fragmental, or irregularly veined igneous rocks. In the Ottawa River Gneiss Complex (ORGC), the lower plate of a large metamorphic core complex with peak Ottawan granulite-facies assemblages, BOs with partially retrogressed granulite-facies or completely retrogressed amphibolite-facies assemblages comprise large portions of the allochthonous Algonquin domain and predominate in the Muskoka domain, the southeastern detachment zone of the metamorphic core complex. BOs in both domains were affected by late-orogenic cross-folding. Examples of amphibolite-facies BOs that mark the upper boundary of the Muskoka domain in the Carnarvon-Norland region, and comparable BO zones elsewhere in the ORGC, are described and illustrated.

At their borders, km-scale zones of amphibolite-facies BOs exhibit tight to isoclinal asymmetric buckle folds (TIAFs), and their interiors contain relict S-Z structures (RSZSs), boudin-like inclusions resulting from the partial dismemberment and retrogression of TIAFs. Others have simulated the formation of TIAFs in two-dimensional, steady-state, simple-shear experiments with single passive layers or with rheologically contrasting multilayers. However, steady-state simple shearing cannot transform ductile igneous rocks replete with igneous megacrysts, equant xenoliths or vein systems into coplanar compositional gneissic bands and then throw them into buckle folds. TIAFS and RSZSs of BOs thus signal departures from steady-state flow conditions, indicating either a change of the shear sense from reverse to normal, perhaps following a brief hiatus, or local perturbations in the laminar flow that shifted the glide plane to an orientation conducive to buckle folding. The former mechanism, activated at the onset of extensional collapse, explains the common occurrence of granulite-facies cores mantled by amphibolite-facies rims in disrupted mafic layers defining m-scale RSZSs in the Algonquin and lower Muskoka domains.