The quantitative link between fold geometry, mineral fabric and mechanical anisotropy: as exemplified by the deformation of amphibolites across a regional metamorphic gradient

Abstract

This work shows lateral variations in fold geometry within an amphibolite unit of constant mineralogical composition under increasing metamorphic grade. Analysis of the fold geometries indicate: (1) medium amplification associated with low post-buckle flattening in the garnet zone, (2) high amplification coupled with medium post-buckle flattening in the staurolite zone, and (3) passive amplification dominated by intense post-buckle flattening in the sillimanite zone. A systematic decrease in the mechanical anisotropy of the folded fabric is observed with increase in metamorphic grade. Quantitative microstructural study shows contrasting deformation micro-mechanisms associated with folding manifested by: (1) brittle dominated deformation of amphiboles that form a stress supporting network with a high competence contrast with respect to plagioclase in the garnet zone, (2) ductile dominated heterogeneous deformation of an interconnected weak layer structure with low competence contrast in the staurolite zone, and (3) homogeneous deformation of a stress supporting framework with low competence contrast in the sillimanite zone. The difference in the folding style between the garnet and staurolite zones is associated with the lateral variations in microstructure of the amphibolites inherited from a pre-folding metamorphic zonation and with different deformation mechanisms in the hinge zones. However, the change in fold style observed as one moves into the sillimanite zone is controlled by the recrystallization associated with an important syn-folding heat input from an adjacent granite intrusion.