L-S shape fabrics in the Mazinaw domain and the issue of northwest-directed thrusting in the Composite Arc Belt, southeastern Ontario
Published:January 01, 2004
Walfried M. Schwerdtner, Matthew W. Downey, Sharyn A. Alexander, 2004. "L-S shape fabrics in the Mazinaw domain and the issue of northwest-directed thrusting in the Composite Arc Belt, southeastern Ontario", Proterozoic Tectonic Evolution of the Grenville Orogen in North America, Richard P. Tollo, James McLelland, Louise Corriveau, Mervin J. Bartholomew
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Many vertical sections across deeper parts of collisional orogens contain gently dipping thrusts that have the geometric style of shallow-crustal, dip-slip faults. In the southwestern Grenville Province (central and southeastern Ontario), however, structural geologists have found it difficult to document northwest-directed thrusting within large, polydeformed complexes of medium- to high-grade metamorphic rocks. This applies, in particular, to the Mazinaw domain of the eastern Composite Arc Belt, where narrow north- to east-northeast-striking (“longitudinal”) zones of high strain have been linked tentatively to ductile thrusting at the sole of Elzevirian (1230–1190 Ma) or Early Ottawan (1085–1015 Ma) fold nappes. During subsequent northwest-southeast shortening, the apparent thrusts behaved as structural detachment horizons or were subhorizontally folded, together with their wallrocks.
Gravity modeling and reflection-seismic profiling have shown that in the present shallow crust most tabular rock bodies and coplanar structural surfaces (including high-strain zones and ductile faults) are horizontal or dip <45°SE. This seems incompatible with the idea that Elzevirian and Early Ottawan thrusts acquired strong curvature in the northwest-southeast vertical plane during upright buckling of fold nappes and other southeast-dipping lithotectonic units.
The longitudinal high-strain zones and their ductilely deformed wallrocks are characterized by steeply inclined to vertical planar fabrics (S) such as geometric systems of strained mineral constituents, pebbles, and volcanic fragments. Linear fabrics (L) coexist with the planar fabrics at many localities, but are commonly weak or seemingly absent. The prevalence of planar fabrics is difficult to explain by shallow-crustal-style ductile thrusting, but these fabrics may have been produced by large-scale strains associated with the formation of midcrustal stretching thrusts.
For a variety of thrust-dip options, we use L < S fabrics to derive the local sense of tangential shear strain within and adjacent to six high-strain zones. The sense of accumulated shear strain proves to be reverse for most realistic options, even if one presumes that the zones rotated during superimposed buckle folding. Mesoscopic Z-folds are common in one high-strain zone, and attest to dextral-reverse shearing and shortening of thrust walls during Mid- to Late Ottawan deformation (1015–980 Ma). The combined results of structural measurement and modeling of shear-strain sense suggest that the high-strain zones and their walls were stretched or shortened progressively in the direction of simultaneous tangential shearing and/or irrotational slip. This means that the longitudinal dislocations of the Mazinaw domain qualify as stretching thrusts that were active in a midcrustal regime of northwest-directed distributed shearing, northwest-southeast pervasive shortening, and associated vertical thickening.