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Abstract:

<p>Zones of mylonites are a prominent feature of rocks that occur at the Grenville Front in central Ontario. They represent periods of northwest-directed thrusting during the Mesoproterozoic, the latest period due to the Grenville Orogeny at ?1.0 Ga. We describe the strain and folding of a 300&nbsp;m wide zone of mylonites southeast of Coniston, Ontario that occur in the foreland to the Grenville Province. They are greenschist facies quartz mylonites and quartz feldspar mylonites formed in the upper crust from sandstones and granites in the Southern Province. They are bounded to the north and south by the Murray Fault and Grenville Front Boundary Fault, respectively, steeply dipping reverse faults, which are post-mylonitization in age. The latter fault marks the southern boundary of the foreland mylonites with zones of mylonite in high grade, lower crustal schists, and gneisses of the Grenville Province. The Murray Fault marks their northwestern boundary against a middle Proterozoic metamorphic terrane. Shape fabric measurements of quartz indicate that strain intensity is random across the mylonite zone. Open angle quartz c-axis fabrics, the dominance of subgrain rotation recrystallization of quartz, and the metamorphic paragenesis quartz?muscovite?chlorite indicate that the mylonites formed at low greenschist facies temperature (350?430&nbsp;°C). The measured variation in strain is attributed to variations in strain rate and water weakening. The mylonites are folded; hinge lines and axial planar stretching lineations plunge southeast perpendicular to the Grenville Front. The mylonites are similar to those that occur in the foreland of the Moine Thrust (northwest Scotland) and the Munsiari Thrust (northwest India).</p>