Extension across the Eocene Okanagan crystal shear in southern British Columbia

Okanagan Valley follows a gently west-dipping crustal shear, across which the upper plate moved west above the lower plate during the middle Eocene. Matching the lower- and upper-plate rocks indicates about 90 km of offset. Mylonitic gneiss at the top of the lower plate records the Eocene extensional strain, registering the zone's evolution through progressively shallower depths and cooler temperatures. Middle Eocene volcanics, normal faults, and slide blocks demonstrate the upper-plate extension.