Late orogenic, plastic to brittle extension along the Robertson Lake shear zone: implications for the style of deep-crustal extension in the Grenville orogen, Canada

Abstract

Rocks regionally metamorphosed and deformed at middle- to lower-crustal levels in the contraction-dominated Mesoproterozoic Grenville orogen are exposed in southern Ontario, Canada. Investigation of the Robertson Lake shear zone (RLSZ) indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen. The RLSZ is a discrete zone 0.5–1.0 km thick that can be traced for nearly 100 km, cross-cutting regional structures and lithologic units. The zone is composed of a thick mylonite zone and a narrow overlying brecciated zone. Mylonite foliation shallowly dips east-southeast and contains a down-dip lineation. Mylonites are characterized by microstructures indicative of crystal-plastic deformation and contain a variety of shear-sense indicators, including SC and CC′ composite fabrics, sigma porphyroclasts, mica fish and other shape fabrics oblique to shear planes, consistently indicating a normal (down-to-the-east) sense of shear. A zone of cataclastic structures overlies the mylonite zone, and brittle deformation overprints the mylonitic fabrics. Slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips. Sense of slip indicators and brittle fault orientations also indicate down-to-the-east displacement during east-west extension. Juxtaposition of brittle and crystal-plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone. Metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall. The regional variations in metamorphic grade and the low-angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension. Combined with the crystal-plastic to cataclastic nature of the RLSZ, new 40Ar/ 39Ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 ± 1 Ma, late in the evolution of the Grenville orogen.