Grenville Front zone, Gagnon terrane, southwestern Labrador: Configuration of a midcrustal foreland fold-thrust belt

Evolution of a metamorphic fold-thrust belt in Archean and Proterozoic rocks in the Gagnon terrane, northwestern Grenville province, involved underthrusting beneath a crustal-scale, orogenic wedge andmetamorphism ranging from upper greenschist facies (similar to 600 +/- 100 MPa/450 +/- 50 degrees C) to upper amphibolite facies (similar to 1100 +/- 100 MPa/750 +/- 50 degrees C) during the terminal stages of the Grenvillian Orogeny. Structural and metamorphic data indicate that D-1 structures formed at peak pressures during basal accretion to the overriding orogenic wedge, whereas D-2 structures developed within the wedge during its displacement toward the foreland in a sinistral transpressive setting. The fold-thrust belt developed sequentially on two levels: a thin-skinned, cover-dominated thrust system preceded and overlies a thick-skinned, basement-dominated system. The D-3 cross-folds postdated normal faulting at the top of the wedge and formed during gravitational collapse. On the basis of this study, characteristic features of midcrustal, metamorphic, thrust wedges include growth by basal accretion, polyphase ductile deformation and noncoaxial strain, dual-level development, extensive basement involvement, and terminal gravitational collapse associated with cross-folding.