Transformation of Archean structural inheritance at the Grenvillian Foreland Parautochthon Transition Zone, Chibougamau, Quebec

Abstract

The easternmost portion of the Archean Abitibi greenstone belt adjacent to the Grenville Front in the Chibougamau area displays distinctive structural features related to crustal-scale contraction during the Grenvillian orogeny. The "Foreland parautochthon transition zone," (FPTZ) a 3–15 km wide zone of Archean rocks, shows a Grenvillian deformation and metamorphic history that overprint fabrics inherited from the Archean Kenoran orogeny. The deformation history of this zone can be divided into two stages. During stage 1, a major crustal-scale deformation zone is responsible for a first uplift episode of deep Archean crust. During this stage, the Grenvillian deformation reactivated and emphasized the Archean fabrics. Amphibolite-grade mylonitic fabrics are interpreted to be an enhancement of the Archean regional schistosity, which remains axial planar to the regional folds. Archean rocks with well-developed former fabrics are strongly affected by Grenvillian non-coaxial flow associated with southeast-dipping stretching lineations. The northwestward transport direction, deduced from shear-sense indicators, is compatible with the signature of the Grenvillian orogeny. Stage 2 is responsible for the formation of the N015°-trending, late Grenvillian faults that have dissected the stage 1 deformation zone. Steep east-dipping narrow zones of retrograde low-temperature mylonites indicate a reverse, east-side-up movement that has contributed to the uplift of deeper Archean structural levels. In a later stage of brittle deformation, pseudotachylite injections and cataclasis, resulting from sinistral lateral offsets in a dominantly brittle regime, are superimposed on the mylonitic structures in response to a modification of the transport direction at the end of the Grenvillian orogeny. Even if high-grade metamorphism can be partly explained by the uplift of deep Archean crust, the structural signature associated with amphibolite mylonites within the FPTZ is interpreted as a product of the Grenvillian orogeny.