Tectonic evolution of the South Tazin Lake shear zone; a major Arrowsmith-age (ca. 2.36 Ga) structure in the SW Rae craton, Laurentia

Abstract

The Rae Province records the effects of multiple, staggered orogenies along both margins, leaving a complex record of tectonism within major crustal-scale shear zones. While the recently recognized 2.5–2.3 Ga Arrowsmith orogeny is now known to have affected a large tract of the western Rae, few related structures have been described. In this paper, we present the first detailed investigation of such a structure, the ‘South Tazin Lake shear zone’, localized along the Nolan-Zemlak lithotectonic domain boundary in the SW Rae. Multi-scale structural analysis undertaken herein documents the lithostructural zonation and strain progression across this structure and evaluates its context in the regional tectonic evolution. Strain intensity increases dramatically from north to south, with onset of thick, continuous intervals of protomylonite to ultramylonite toward and into a 3–5 km wide corridor that straddles the Nolan-Zemlak domain boundary and constitutes the core of the shear zone. New U-Pb zircon igneous crystallization ages are consistent with previous results for units across the Nolan-Zemlak domain boundary while ⁴⁰Ar-³⁹Ar thermochronology of hornblende and biotite provides temporal constraints on deformation and a more detailed thermal evolution than previous studies. An ⁴⁰Ar-³⁹Ar date of 2361 ± 12 Ma for hornblende overgrowing the sheared and recrystallized matrix in blastomylonitic rocks places a minimum age on D₂ mylonitization. This, together with similar ages in the core of the shear zone from foliation-parallel hornblende, definitively links ductile shearing and the related S₂ fabric along the South Tazin Lake shear zone with Arrowsmith orogenesis. Cooling rates suggest an initial accelerated phase (∼2360–2350 Ma) of post-Arrowsmith exhumation, but the absence of extensional shear-sense indicators implies control by structures outside of the immediate area; or masking by younger (Taltson-age) structures. This was likely followed by a much slower erosion-related (≤2.35 Ga) phase. All-told, the South Tazin Lake shear zone represents a significant Arrowsmith-age tectonic front that was reactivated only along its southern margin under ductile–brittle conditions during the ca. 1.93 Ga Taltson orogeny. These dual fronts were later transposed to the northeast in concert with deformation along the Black Bay fault during the ca. 1.90 Ga Snowbird orogeny.