Deformation of layered rocks near the Wawa-Quetico subprovince boundary / by David Lawrence Nicol
Nicol, David Lawrence
Petrology Ontario, Northwestern
Petrology Ontario Faries Lake Region
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A supracrustal sequence of rocks near Manitouwadge, Ontario consists of metavolcanlc rocks, Including pillow lavas, banded iron formation, amphibolite, and a quartzofe1dspathic hornblende-blotite gneiss. This rock has been given the name bladed gneiss to reflect the blade-like appearance of its mafic components. Results of a detailed geological investigation of the entire supracrustal sequence suggests that the apparent fragmental appearance of the gneiss, and to a lesser degree the other rock types, is not a primary feature, but rather, the product of deformation. The rocks were initially part of a layered sequence which became variably fragmented. The blades are in part the result of the transposition of layers and the variations in blade morphology are attributable to the response of the layers to strain during folding. Individual layers deform by the development of cuspate-1obate folds, buckle folds, and boudinage. The extent of fragmentation during deformation is controlled by competency contrasts between adjacent layers, absolute and relative layer thickness, and layer orientation with respect to principal finite strain directions. Planar and linear structural elements in the metasedimentary and metavolcanic rocks suggest a defomational history which Includes two episodes of folding accompanied by medium grade metamorphism and recrystallization. Similarities in lithology and structural elements between the rocks of the study area and those of the nearby Manitouwadge synform suggest that parent rock assemblages were closely related and that structures present in both locations developed contemporaneously in response to regional tectonic activity. "Mobilist" tectonic models for the development of the Superior Province entail northward directed subduction and accretion, and are popular with many workers. The recumbent nature of Fi folds, the shallow plunging hinge lines of Fa folds, and their coaxial relationship may be related to low angle thrust faults and nappe structures which would be likely consequences in the proposed subduction model. Many workers have attempted to map subprovince boundaries. In the region of the present study, the boundary between the Wawa and Quetico Subprovinces is traditionally placed a few miles north of Man 11ouwadge. If this is correct, the rocks studied form part of the Wawa Subprovince. Results of this Investigation suggest that, on the basis of lithology and structure, such a placement of the boundary is inappropriate. It appears more appropriate to suggest that the terrain discussed in this thesis, as well as the rocks of the Manitouwadge synform, is best considered to form a zone of transition between the two subprovinces.