Structural analysis of brittle deformation features along Grenvillian shear zones in southeastern Ontario

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Mitchell, Frances
Brittle features along two prominent shear zones in the Grenville Province of southeastern Ontario, the Central Metasedimentary Belt boundary thrust zone (CMBbtz) and Robertson Lake mylonite zone (RLmz), are investigated to understand the brittle deformation history of this intraplate region. Evolution of brittle deformation is interpreted from detailed mapping and characterization of joint, fault and vein patterns in Proterozoic basement rocks, Paleozoic cover rocks and Pleistocene sediments. Relationships among joint and fault orientations and maximum-horizontal stress directions associated with major tectonic events suggest a history of repeated stages of deformation along the CMBbtz and RLmz. Neoproterozoic-Cambrian opening of the Iapetus Ocean is interpreted to have produced NNE-striking Proterozoic structural highs in Paleozoic cover above the RLmz. Normal faults striking NW to WNW and ESE-striking dextral strike-slip faults in the RLmz and Paleozoic cover above both shear zones are compatible with Ordovician-Devonian Taconic and Acadian stresses. Prominent NNE- and NW-striking subvertical joint sets along the RLmz, CMBbtz and Paleozoic cover above the CMBbtz are compatible with Late Carboniferous to Permian Alleghanian stresses. Brittle features attributed to Mesozoic opening of the Atlantic Ocean are numerous and include: NNE-, ENE-, NW- and WNW-striking subvertical joint sets, WNW-striking normal faults, and NNE-striking normal faults in Paleozoic cover above the RLmz; NNW-striking dextral strike-slip faults and NW-striking normal faults in the RLmz; N- and NW-striking normal faults in Paleozoic cover above the CMBbtz; and, E- and SSW-striking normal faults in the CMBbtz. Joint sets in Pleistocene sediments above the CMBbtz and RLmz (oriented NNE, NW, NE, NNW and ENE) are compatible with both Pleistocene post-glacial flexural stress and the current in-situ stress. Strike-slip faults oriented SSW and W in the CMBbtz and a NNW-striking reverse fault in the RLmz are compatible with the current regional stress field. Further understanding of this neotectonic deformation is relevant to assess the seismic risk in the intraplate region of southern Ontario.
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