THE PALEOPROTEROZOIC EVOLUTION OF THE ATHABASCA GRANULITE TERRANE (SNOWBIRD TECTONIC ZONE), WESTERN CHURCHILL PROVINCE, CANADA: DEFORMATION DUE TO FAR-FIELD OROGENIC PROCESSES

High P granulites within the Athabasca granulite terrane (AGT), western Churchill Province, record several distinct episodes of Paleoproterozoic deformation. The study region is situated between two major Paleoproterozoic orogenic belts (Trans-Hudson and Taltson-Thelon), and far field driving stresses for deformation could be related to either margin. We have used U-Th-total Pb monazite geochronology to constrain the age of deformational phases and to relate them to the craton bounding orogens. Four main phases of Paleoproterozoic deformation have been identified within the AGT. The earliest of which is the most pervasive and represents the Paleoproterozoic regional metamorphic peak. Large amplitude folding, and dextral shearing occurred at ca. 1900 Ma, synchronous with the extensive Chipman dike swarm. Sinistral strike-slip shear associated with the 3-5 km wide Cora Lake shear zone occurred from ca. 1885 to 1870 Ma. Dextral shearing and thrusting occurred at ca. 1850 Ma along the Legs Lake shear zone. Punctuated episodes of dextral shear lasted until ca. 1800 Ma within the Grease River shear zone. Thus, the region records a phase of dextral shearing, followed by sinistral shearing, and then another phase of dextral shearing with similar foliation and lineation orientations. The change in kinematics can be explained by evolving boundary conditions associated with craton bounding orogens. Dextral shear at ca. 1900 Ma corresponds to the latter phases of Slave craton indentation into the western margin of the Churchill Province. Sinistral shear within the Cora Lake shear zone at ca. 1885-1870 Ma coincides with the accretion of Lynn-Lake and La Ronge arcs along the southern flank of the Churchill Province, and a phase of quiescence along the western margin of the Slave and Churchill Provinces. Convergence was active again along the western margin of the contiguous Churchill/Slave craton by 1865 Ma (Wopmay orogen) promoting a new phase of dextral shearing localized along the Legs Lake thrust/shear zone. Thus, localized deformation within the AGT is interpreted to reflect far field changes in the stress field. The AGT may have acted as a long term lithospheric weak zone, and as such, a sensitive monitor to evolving boundary conditions during the initial phases of Laurentian amalgamation.