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TitleModern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield
AuthorGraziani, R; Larson, K; Smit, M; Cottle, J M; Lamming, J; Piette-Lauzière, N
SourcePrecambrian Research vol. 380, 106817, 2022 p. 1-21,
Alt SeriesNatural Resources Canada, Contribution Series 20220252
Mediapaper; digital; on-line
File formatpdf
NTS74O/07; 74O/08; 74P/05
Lat/Long WENS-107.0000 -105.5000 59.5000 59.2500
Subjectsgeochronology; tectonics; continental crust; shear zones; mylonites; tectonic zones; East Athabasca Mylonite Triangle; Snowbird Tectonic Zone; Canadian Shield; Precambrian
Illustrationslocation maps; photomicrographs; plots
Released2022 08 20
AbstractThe 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.
Summary(Plain Language Summary, not published)
One of the widest gaps in the current knowledge of orogenic belts is related to how the lower portions of the continental crust behave during continental collisions. The Eastern Athabasca Mylonite Triangle (eAMT) is one of the best examples of lower continental crust. The purpose of this study is to understand the structural evolution of this area by applying microstructural analysis and geochronology to the Intra Tantato shear zone (ITsz), one of the major ductile mylonitic belts of the areas associated with orogenic processes. We found a large volume of consistent evidence showing that the ITsz was activated at 1.9 Ga as a compressive structure in association with the Snowbird orogenic event and it was later reactivated as an extensional structure during the late stages of the orogeny. The eAMT underwent similar compressing and extension processes to what is usually found at higher levels of the continental crust.

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