REGIONAL SETTING AND KINEMATIC FEATURES OF THE NEEDLE FALLS SHEAR ZONE, TRANS-HUDSON OROGEN

Needle Falls Shear Zone is the southern part of a major northeast-tren ding ductile shear system within the Paleoproterozoic Trans-Hudson Oro gen in Saskatchewan. Throughout its exposed length of approximately 40 0 km, the shear zone separates reworked Archean continental crust and infolded Paleoproterozoic supracrustals of the Cree Lake Zone, to the northwest, from mainly juvenile Paleoproterozoic arc terrains and gran itoid plutons of the Reindeer Zone, to the southeast. It also defines the northwest margin of the ca. 1855 Ma Wathaman Batholith, which form s the main protolith to shear zone mylonites. Although not precisely d ated, available age constraints suggest that the shear zone formed bet ween ca. 1855 and 1800 Ma, toward the end of peak thermotectonism in t his part of the orogen. In the Needle Falls study area, shear zone myl onites exhibit varied, sequentially developed, ductile to brittle fabr ic features, including C - S fabrics, winged porphyroclasts (especiall y delta type), small-scale compressional and extensional microfaults r anging from thin ductile shear zones to late brittle faults, early iso clinal and sheath folds, later asymmetric folds related to compression al microfaults, and variably rotated and (or) folded quartz veins. All ductile shear-sense indicators suggest dextral displacement, as do mo st later ductile-brittle transition and brittle features. In conjuncti on with a gently north-northeast-plunging extension lineation, such da ta indicate oblique east-side-up dextral movement across the shear zon e. However, preexisting structures in country rock protoliths rotate i nto the shear zone in a sense contrary to that predicted by ideal dext ral simple shear, a feature thought to reflect significant flattening across the shear zone. Other ductile to brittle fabric elements in the mylonites are consistent with general noncoaxial strain, rather than ideal simple shear. Amount of displacement cannot be measured but indi rect estimates suggest approximately 40 +/- 20 km. The Needle Falls Sh ear Zone is too small and has developed too late in regional tectonic history to be considered a crustal suture. Rather, it is interpreted a s either a late-tectonic oblique collisional structure or as the resul t of counterclockwise oroclinal rotation of the southern part of the o rogen.