A coming together: The juxtaposition of the Rae and Hearne cratons along the Virgin River shear zone (Snowbird tectonic zone), Saskatchewan, Canada, and the implications for proto-Laurentia
The Snowbird tectonic zone of western Laurentia remains an enigmatic feature; some researchers have suggested that it represents a Paleoproterozoic suture, whereas others have proposed that it represents a reactivated Archean intracratonic fault zone. The Virgin River shear zone is the most southwesterly exposure of the Snowbird tectonic zone, along which rocks of the southern Rae and southern Hearne provinces meet. The Rae craton is the hanging wall to the northwest-dipping structure. Rocks of the southwest Rae are an extension of the Taltson basement complex and the intrusive Taltson magmatic zone. The Taltson basement complex contains ca. 2460 Ma diorites and granites, and granulite-facies metasedimentary rocks, the Careen Lake group. The ca. 2110 Ma Clearwater anorthosite complex and a swarm of mafic dykes intruded the Taltson basement complex near the Virgin River shear zone. These rocks were intruded by continental volcanic arc plutons between 1.99 and 1.97 Ga, indicating a convergent tectonic setting along the southwestern margin of the Rae. The metamorphic culmination of the Taltson orogeny developed at 1.94-1.93 Ga, independently of the Thelon orogeny, which culminated some 10-20 million years earlier. The footwall of the Virgin River shear zone comprises rocks of the southern Hearne Province, which is dominated by a felsic orthogneiss complex that contains ca. 2809 Ma tonalites, ca. 2664 Ma leucotonalites, ca. 2554 Ma granodiorites and quartz diorites, and ca. 2.63-2.60 Ga porphyritic granites. These rocks are unconformably overlain by the <2600 Ma Black Birch Lake assemblage, a quartzite- and amphibolitebearing supracrustal succession that may be correlative with the lower part of the Wollaston Supergroup. Although all of these rocks contain a fabric that predates the Virgin River shear zone, the bulk of the deformation and the metamorphic assemblages derive from the Trans-Hudson orogeny. The rocks of the felsic orthogneiss complex, which have dominantly evolved Sm-Nd signatures, contrast those of juvenile felsic gneisses of the Central Hearne Province. A probable tectonic boundary between the Central and southern Hearne provinces corresponds with a change in rock units and negative aeromagnetic and Bouger gravity anomalies in northeastern Saskatchewan. The Virgin schist group, a predominantly metasedimentary succession in the core of the Virgin River shear zone, has a detrital zircon signature consistent with derivation from the western Rae craton. A leucotonalite that returned a 3129 Ma crystallization age, also resides in the core of the Virgin River shear zone. These Mesoarchean rocks likely represent an extension of the Taltson basement complex from Alberta to the Virgin River shear zone. The Taltson basement complex apparently extends to the northeast implying that Mesoarchean rocks of the Taltson basement complex mantle the core of the Rae craton. A ca. 1940 Ma, arc-type diorite pluton from the hanging wall (Rae) side of the Virgin River shear zone implies a convergent tectonic environment along the Rae margin during the Taltson orogeny and before 1.90 Ga metamorphism along the structure. This, in addition to the presence of an apparent rifted margin in the hanging wall, a profound change in rock units, and the internal structural characteristics of the shear zone, suggests that the Virgin River shear zone is a suture that formed during the Snowbird orogeny at ca. 1.90 Ga. The Rae craton was thrust over the Hearne at this time and that uplift continued during dextral – oblique, strike-slip displacement during the early parts of the Trans-Hudson orogeny.
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Faculty of Graduate Studies and Research, University of Regina