Foliated and unfoliated rhomb-porphyry dikes, genetically related to the andesitic Marron Formation, cut some westernmost exposures of the Shuswap Complex in the southern Okanagan Valley of British Columbia. Emplacement of these dikes was associated with northerly trending fractures and flexural-slip folding, localized high heat flow, re-setting of country rock K–Ar systems and hydrothermal alteration. All these events are set at 42–48 m.y. B.P. based on K–Ar measurements on dikes and country rocks.Rhomb-porphyry dikes are part of an andesitic volcanic province in south-central British Columbia that coincides in time with ductile deformation within the Cascade Fold Belt to the west and latest thrust faulting in the Rocky Mountains to the east. The volcanic province is associated in space with thinning of the crust, whereas thickening of the crust is associated with ductile shortening to the west. It is likely that the volcanic province marks the position of the ductile–brittle transition zone in the crust during late Eocene time.

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