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.

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