THE SOUTH TAZIN LAKE SHEAR ZONE, A FUNDAMENTAL TECTONIC BREAK AND ARROWSMITH-AGE (CA. 2.37 GA) HIGH-STRAIN ZONE IN THE SOUTHWEST RAE PROVINCE OF LAURENTIA

Once formed, crustal-scale shear zones are typically reactivated multiple times over their history. The South Tazin Lake shear zone (STLsz) of the WSW Rae Province is an archetypal example. Localized along the Nolan-Zemlak domain boundary of NW Saskatchewan, this 7-8 km wide zone of progressive strain is interpreted as a reactivated cryptic early-Arrowsmith suture. It extends from weakly foliated granites of the Nolan domain, southward through proto- to ultramylonitic equivalents, into equally deformed quartz dioritic orthogneisses of the Zemlak domain. Most strain is focused in a 3-5 km wide zone straddling the domain boundary, comprising a series of gneissic to protomylonitic lenses in an anastomosing network of mylonite-ultramylonite. Lower-grade ductile-brittle deformation is expressed by several younger, NE-trending fault zones of chloritized phyllonite/cataclasite, while younger faults record extensive brecciation. ⁴⁰Ar-³⁹Ar thermochronology on hornblende and biotite along a N-S transect provides temporal constraints on deformation. Hornblende yields variably older ages of 2500-2420 Ma in the Nolan interior, with consistently younger ages of ca. 2370 Ma attained within 5 km of the boundary. In contrast, biotite ages are 2370-2330 Ma across the entire transect, except proximal to the boundary. Two tectonothermal events can therefore be linked to the boundary zone, one at ca. 2370 Ma and another at ≤ 2180 Ma. Combined field, structural-geochronological study indicates the first involved ductile shearing at the ca. 2370 Ma peak of Arrowsmith orogenesis. The recorded sinistral strike-slip kinematics may mask an earlier thrust history that caused burial-heating of the Nolan domain to temperatures in excess of the ~530 ^oC closure of hornblende, and consistent with recrystallization microstructures. Lack of robust plateaux ages in variably reset biotite renders the timing of the second event problematic, but it may correlate with low-grade faulting that pre-dated the ≤ 1.92 Ga Thluicho Lake group. The STLsz therefore represents a significant Arrowsmith-age tectonic front that was reactivated under lower grade conditions during the ca. 1.93 Ga Taltson orogeny. These dual fronts were later transposed to the NE in concert with deformation along the Black Bay fault during the ca. 1.9 Ga Snowbird orogeny.