The Nonacho Lake area is located along the western margin of the Rae craton in the southeastern Northwest Territories. This region is composed of the ca. 1.91-1.82 Ga siliciclastic Nonacho Group, previously interpreted to have been deposited in a fault-controlled basin, unconformably overlying Archean to Paleoproterozoic granitic and gneissic basement rocks. Detailed mapping during the 2019 and 2020 field seasons revealed two major sinistral, SW-striking, steeply-dipping, ductile to brittle shear zones, and several minor dextral, steeply-dipping shear zones. The major shear zones, King and Magrum shear zones, flank the western and eastern edges of the Nonacho basin respectively. Shear strain was accommodated in the Hjalmar, Chief Nataway, Tronka Chua and Thekulthili formations of the Nonacho Group and within underlying basement units. The minor dextral shear zones are located east of the Nonacho basin and deform Archean to Paleoproterozoic granite and gneiss.
A variety of geochronometers were applied to determine the timing of deformation of the shear zones in this region. Titanite grains elongated in the shear fabric of the King and Magrum shear zones were analyzed by U-Pb in situ LA-ICPMS and record dominantly ca. 2.3-2.2 Ga dates. Two CA-TIMS U-Pb zircon dates of 1.825 ± 0.0006 Ga were obtained from syn-tectonic granitic dykes in the Magrum shear zone and an 40Ar/39Ar date of ca. 1.78 Ga was obtained from sinistrally-deformed biotite in the King shear zone. In the minor dextral shear zones, U-Pb titanite dates range from 1.87-1.73 Ga.
The obtained results indicate at least two periods of deformation within the two major shear zones flanking the Nonacho basin: 1) an early shearing event at ca. 2.3-2.2 Ga and 2) a sinistral, trans-tensional, greenschist-facies reactivation at ca. 1.83-1.78 Ga. The earlier phase of shearing is coeval with the late stages of the Arrowsmith Orogeny and the later phase of reactivation is coeval with the extensional collapse of the Trans Hudson orogeny proposed for the South Rae craton during this time. The exact role of these shear zones in the exhumation of this region will be evaluated by a regional transect of 40Ar/39Ar data, that is currently being processed. Furthermore, given the two recorded periods of deformation, the King and Magrum shear zones were likely not a main control in the development of the Nonacho basin but rather deformed the basin following its development. This is corroborated by observed geologic relationships and recently published stratigraphic analyses and basin revaluation.