Geology of British Columbia. Sydney Cannings

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       The Latest Collisions

      About 55 million years ago, the relative direction of the Kula Plate changed, and it began to slide more northward along a fault similar to today’s San Andreas Fault in California. Since the Olympic Peninsula did not yet exist, Vancouver Island projected out into the Pacific and formed a trap for northwesterly slipping terranes. During this period, two terranes were brought up the coast on the Kula Plate and pushed into southern Vancouver Island. The Pacific Rim terrane, which arrived about 55 million years ago, consists of sedimentary and volcanic rocks that now form the southwest coast of the island (Map 9, page 41). After it made contact, the fault between the Kula Plate and North America jammed and a new fault formed slightly farther out in the Pacific. The Crescent terrane, made up of former marine volcanoes that had formed along the fault, was brought alongside sometime later, but before 40 million years ago. In British Columbia, the Crescent terrane’s oceanic lavas form the rocks of Victoria’s Western Communities; to the south, they make up the Coast Mountains of Washington and Oregon (the rocks of the Olympic Mountains came later). The Crescent terrane is separated from the Pacific Rim terrane by a fault along Loss Creek. The force of this collision did not create big mountain ranges in British Columbia, but it did fold and thrust-fault the sedimentary rocks along the Strait of Georgia to form the ridges and bays of the Gulf Islands.