Transtensional origin of multi-order cross-folds in a high-grade gneiss complex, southwestern Grenville Province: formation during post-peak gravitational collapse

Abstract

The well-mapped western part of the Ottawa River Gneiss Complex (ORGC; new name), a large metamorphic core complex, hosts a system of gently plunging cross-folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high-grade core of the complex. The cross-folds are buckle structures that deform the attenuated gneissic layering, plunge parallel to the regional elongation lineation, range from upright to recumbent, and exhibit distinctive hinge-parallel elongation lineations, all features of extension-dominated ductile transtension. Our L–S fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds, and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes. On the basis of petrologic data, we show that cross-folding postdated the peak Ottawan metamorphism and took place during retrogression and exhumation of the thrust-sheet stack. The cross-folds form an inclined system that is principally developed in retrograde melt-weakened amphibolite-facies rocks, with the transtensional origin implying that exhumation and retrogression of the high-grade core of the complex took place in an oblique extensional setting. A transtensional origin for the cross-folds removes the need to appeal to orogen-parallel regional shortening, an implausible requirement of previous interpretations, and is compatible with data indicating that much of the visible fabric and structure of amphibolite-facies domains of the ORGC developed during post-peak exhumation, retrogression, and gravitational collapse of the thrust-sheet stack.