Abstract

Deformation within part of the Parry Sound shear zone (PSSZ), in the southwestern Grenville structural province, has been examined at the transition from mylonitized gabbroic anorthosite and orthogneiss within the body of the PSSZ to retrograde granulites in the immediate hanging wall. Detailed structural analysis shows a framework of internally deformed imbricate domains within the hanging-wall granulites that are truncated by a major dislocation within the PSSZ mylonites. Deformation geometries within discrete hanging-wall domains and the PSSZ exhibit strong similarities ascribable to a common deformation cycle that typically preserves three generations of overprinting structures. This cycle comprises (i) formation of shear-induced recumbent isoclinal folds and stretching lineations with concomitant transposition of compositional layering; (ii) rotation of fold axes during noncoaxial shear into parallelism with the stretching lineation and formation of sheath folds; and (iii) refolding about axes nearly orthogonal to the stretching lineations, followed by another period of transposition and reorientation during continued shear. The periodicity of this deformation is specifically documented by the generation, dispersion, and reorientation of fold axes and stretching lineations toward an end orientation consistent with northwest overthrust transport. This cycle reconciles the coexistence of upright structures with large subhorizontal displacements within a regime of progressive noncoaxial shear, where there is a near orthogonality between overprinted finite strain elements, which reflects the stability of the kinematic framework at both local and regional scales.

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