Abstract

The Yalakom fault strikes northwest for nearly 250 km in southern British Columbia. Immediately north of the town of Lillooet, between the Yalakom and Fraser faults, structures within Jurassic and Cretaceous clastic rocks suggest that the Yalakom fault had a history of left-lateral followed by high-angle contractional motion. North- and northeast-trending folds offset by west-northwest-trending left-lateral and normal(?) faults, plus numerous small conjugate left- and right-lateral faults with associated extension fractures fit a strain ellipse for left-lateral slip on the Yalakom fault. Younger right-lateral bedding-plane faults, a large vertically plunging open fold, and a thrust fault adjacent to the Yalakom fault zone suggest later high-angle contractional motion.

Left-lateral motion preceded, whereas contractional motion probably coincided with, 50 Ma Fraser fault deformation. To the northwest along the Yalakom fault, and on the Pasayten fault in Washington, left-lateral slip possibly occurred between about 100 and 95 Ma. Left lateral motion may prove an important element in the pre-Tertiary history of southern British Columbia.

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