Listric normal faulting during postorogenic extension revealed by 40Ar/39Ar thermochronology near the Robertson Lake shear zone, Grenville orogen, Canada

Abstract

The Robertson Lake shear zone is a major plastic to brittle extensional shear zone in the Grenville orogen that bounds the Mazinaw and Sharbot Lake domains and provides information on the style of late extension and the unroofing history of the orogen. Argon isotope data were collected from hornblende and micas to determine 40 Ar/39 Ar ages, constrain the temperature‐time histories of these two domains, and infer the unroofing history of the region. Hornblende cooling ages across the Mazinaw domain (footwall) show little variation, indicating uniform unroofing of the footwall since 950 Ma. Phlogopite, muscovite, and biotite cooling ages of the footwall are 924 to 890 Ma. The cooling history of the Mazinaw domain is characterized by slow cooling after peak metamorphism (circa 1000 Ma), accelerated cooling (4°–5°C/m.y.) from 950 Ma to 890 Ma, and an average cooling rate of ∼1°C/m.y. to circa 590 Ma, when these rocks were at or near the surface. The cooling history of Sharbot Lake (hanging wall) domain is drastically different than that of the Mazinaw domain. Hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 Ma, respectively, indicating a cooling rate of 5°C/m.y. after slow cooling from metamorphic temperatures. Biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve, indicating that the cooling history varied across the domain. Cooling rates in the hanging wall adjacent to the shear zone are low (2°C/m.y.). A biotite cooling age (1029 Ma) and preservation of an amphibole growth age (1205 Ma) in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 Ma. These data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone. The varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement.