The Okanagan subterrane: a 'Subduction Initiation Rule' ophiolite in the Canadian Cordillera




McEwen, Gerri L.

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The Okanagan subterrane in south-central British Columbia is commonly referred to as the basement to Quesnellia, a composite arc terrane in the Canadian Cordillera. Distinct geochemical, lithologic, and structural characteristics allow for interpretation of the Okanagan subterrane as a forearc ophiolite formed over a long-lived west-dipping subduction zone initiating in western Panthalassa. The forearc ophiolite model has significant impacts on existing Cordilleran models which focus on a North American origin for this terrane. The Okanagan subterrane is comprised of harzburgites, gabbros, basaltic lavas, and ocean basin sediments. The age of the subterrane spans the Middle Devonian through the Early Permian. New stratigraphic, structural, and whole rock geochemical data are presented together with a synthesis of pre-existing data from regional studies to better constrain the origins and evolution of the Okanagan subterrane. The results indicate a transition from quiet shallow ocean basin deposition to the initiation of subduction and construction of a new forearc crustal sequence and mantle wedge. The earliest crustal sequences were formed from the eruption of basalts geochemically similar to those produced at oceanic spreading centres in the Late Devonian to earliest Mississippian. A chemostratigraphic magmatic progression through the Early and Middle Mississippian resulted in lavas becoming increasingly island arc-like in composition. Uplift and erosion dominated through the Pennsylvanian and Permian, punctuated by periods of carbonate deposition and capped by a regional Permo-Triassic unconformity and a Mesozoic volcanic arc. The Okanagan subterrane was affected by two main compressional deformation events, both interpreted as products of ophiolite obduction. The first, in the Permo-Triassic, resulted in tight to isoclinal folding, uplift, and erosion yielding the Permo-Triassic unconformity and the obduction of the Okanagan subterrane onto an extinct island arc within the lower plate. The second, in the Middle Jurassic, resulted in a southwest-verging fold and thrust belt which likely records the subsequent accretion to pericratonic terranes of western Laurentia. Comparing this ophiolite to other supra-subduction zone ophiolites worldwide allows for an interpretation of the Okanagan subterrane as a forearc ophiolite constructed during Late Devonian intra-oceanic subduction initiation.



Okanagan subterrane, Canadian Cordillera, Ophiolite, Quesnellia, Subduction initiation rule ophiolite, Forearc ophiolite