Detailed and reconnaissance mapping of areas along the east and west sides of the Okanagan Valley, from the International Boundary in the south to Kamloops in the north, has revealed a similarity in structural sequence and geometry in rocks ranging in age from Pennsylvanian (Harper Ranch, Anarchist, Kobau, Old Tom, and Shoemaker Groups) through Late Triassic – Early Jurassic (Nicola, Sicamous, and Slocan Groups).Earliest recognizable folds, F1, have northerly trending axes, are isoclinal in form, and are disrupted on all scales by a strongly penetrative second phase of deformation, F2. This second phase is characterized by northerly and southerly verging isoclinal folds having east–west axes that are very nearly parallel with a well-developed stretching lineation, L2. Mylonitic lamination is developed parallel with these F2 axial surfaces. A third phase of folding, F3, comprising more unright open structures having a consistent southerly vergence, deforms the earlier F1 and F3 structures. Later phases of deformation, F4 and F5, almost coeval, have produced the present foliation configuration outlining basins and domes and associated northerly trending normal faults.Progressive metamorphism accompanied F1, F2, and F3 deformations and peaked during F2. F4 and F5 are associated with a thermal event resulting in resetting of most radiometric ages within the region.F1 deformation is probably Permo-Triassic in age and associated with lower greenschist metamorphism, whereas F2 and F3, associated with a much higher metamorphism up to amphibolite facies, affect all the sedimentary rocks within the region and apparently terminated by about 178 Ma (K–Ar on hornblende), Triassic – Early Jurassic. The latest movements, F4 and F5, involve volcanic rocks whose age of crystallization is set radiometrically at about Eocene.A plate model involving easterly obduction during the Permo-Triassic, followed by easterly dipping subduction with associated dextral transform movement during the Late Triassic – Early Jurassic, is proposed to explain the observed geometry. A mantle diapir below the region is rationalized to explain the localized high heat flow during Eocene time.

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