LATE ARCHEAN THRUSTING IN THE NORTHWESTERN PONTIAC SUBPROVINCE, CANADIAN SHIELD

In the northwestern Archaean Pontiac Subprovince, south-verging structures, such as drag folds with subhorizontal axes, stretched objects, down-dip mineral lineations, low-angle thrusts and allochthonous metavolcanic slabs were mainly produced by thrusting in response to N-S regional compression. Deformation of the dominantly metasedimentary sequences occurred under Barrovian-type metamorphism, and the horizontal shortening was accompanied by downdip stretching in the regional foliation (S1). Two nappes are assumed to be present, their boundary being marked by an abrupt change in structural style and metamorphic facies. Two phases of close to isoclinal folding are evident in the upper nappe but are absent from the lower nappe. In the upper nappe, D2 gives and E-W tectonic trend to the rock units. F2 folds range from upright to recumbent, but are most typically moderately southward inclined folds that plunge gently towards the east or west. Besides mostly retrograde recrystallization within D2 fault zones, S2 is a crenulation cleavage that did not develop sufficiently to obliterate the regional schistosity (S1) and its associated stretching and mineral lineations. This contrasts with the regionally penetrative nature of the main deformation event (D1), and suggests a narrowing, or partitioning of high-strain zones with time, possibly due to a decrease in the regional temperature. Syn-orogenic sedimentation in the northwestern Pontiac Subprovince was probably enhanced by tectonically controlled uplift resulting from early regional Kenoran compression. The sedimentary basin was subsequently deformed by further N-S compression which led to internal stacking nappe-type displacements, Barrovian metamorphism and thrusting of the southern Abitibi over the Pontiac Subprovince. The common style of folding and the same orientation of various structural elements such as cleavage and lineation in the transition zone between the Abitibi and Pontiac subprovinces indicate continuity in deformation style. Regional-scale transcurrent displacement along the northern subprovince boundary appears as a late, localized response to regional compression.