Modern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield

Abstract

The Intra Tantato shear zone (ITSZ), which occurs within the East Athabasca mylonite triangle in the southern portion of the ∼1.9 Ga Snowbird tectonic zone in northern Saskatchewan, Canada, is a low to high angle, curvi-planar, kilometer-thick mylonite zone hosted within granulite-facies gneiss that records a complex history with both thrust- and normal-sense motion. New field mapping, microstructural analysis, and quartz crystallographic preferred orientations (CPO) detail a composite history for the ITSZ that includes top-to-the-ENE thrusting of the hanging wall over the footwall that is overprinted by normal-sense top-to-the-WSW reactivation. Quartz CPOs further indicate constrictional deformation related to both events and quartz c-axis opening angles define deformation temperatures of ∼750 ± 50 °C for the thrusting event and ∼550 ± 50 °C for the later, normal-sense reactivation.Monazite and garnet geochronology on rocks within and around the ITSZ indicate it may have nucleated along a pre-existing 2.6–2.5 Ga sub-horizontal fabric. This appears to have occurred immediately after the ∼1.920 Ga metamorphic peak of the hanging wall and during the ∼1.905 Ga metamorphic peak of the footwall. Monazite ages from small matrix grains in interstitial positions in specimens that record reactivation kinematics indicate overprinting occurred at ∼1.860–1.830 Ga, contemporaneous with the earliest recorded activity along the nearby Grease River Shear Zone.Overall, this tectonic evolution can be attributed to a crustal thickening process that occurred in the Athabasca mylonite triangle during the Snowbird orogeny at ∼1.9 Ga and during the post-collisional exhumation processes. Moreover, these results show that the lower continental crust can actively accommodate shortening during large-scale collisional events with tectonic units piling up via nappe-stacking processes similarly to what is typically observed in the middle crust.