480 TERTIARY TO PRESENT/Miocene Figure Palaeogeographical reconstruction of Miocene plate configurations and continent dispersions Miocene, most of the remaining Farallon Plate had been subducted under the North American Plate, bringing the Pacific Plate into contact with the North American Plate in southern California The remaining two sections could no longer be considered a single plate by this time, and are henceforth regarded as independent plates with unique motions The Juan de Fuca Plate is found to the north off the coast of Oregon and Washington and the Cocos Plate to the south off the coast of Mexico Both continue to be subducted under the North American Plate The contact of the Pacific, North American, and Juan de Fuca plates formed a triple junction at Mendocino, California by 20.0 Ma During this transition, the subducting plate margin disappeared and the contact between the Pacific and North American plates developed into a transform fault system because the Pacific Plate is moving to the north, while the subducting Farallon Plate was moving more directly eastward This transform fault system, which separates Baja California from the rest of California, runs under the continent between San Francisco and Los Angeles The transform faulting formed along the Central Valley of California is known today as the San Andreas Fault Zone, and was in place by about 10.0 Ma Rocky Mountains The subduction of both the Farallon Plate and the more northerly Kula Plate are partially responsible for the character of the Rocky Mountains, which began their final uplift phase during the Miocene The Farallon Plate was subducted under the North American Plate at an unusually shallow angle This caused uplift of the Rockies in the western US and Mexico by compression, and probably caused some extension in the crust as the slab sank By contrast, the Juan de Fuca and Kula plates were subducted at a much steeper angle (closer to 45 ), resulting in more typical thrusted sedimentary sheets through the Canadian Rockies Basin and Range Changes in the continental crust to the west of the developing Rocky Mountains are poorly understood, but the thin crust in this region is evidence that this area became extended between 15.0 and 8.0 Ma Crustal extension resulted in a series of faults that trend north–south, with dropped fault blocks (grabens) forming flat valley floors in between raised blocks (horsts) Erosion of the nearly 400 alternating mountain blocks formed the characteristic basin and range topography of the modern western United States Columbia River Flood Basalts From 17.0 to 15.5 Ma, a tremendous volume of volcanic rock was deposited in north-eastern Oregon, south-eastern Washington, and western Idaho, from at least 300 lava flows, some of which reached as far as the Pacific Ocean The Columbia River Flood Basalts cover 164 000 km2 and are situated between the Rocky Mountain and Cascade Ranges The cause of these volcanic eruptions is complex and is tied to: (i) extension and thinning of the crust; (ii) formation of the Cascade Range; and (iii) the position of the Yellowstone hotspot (a mantle plume) Most flood basalt provinces are directly tied to a mantle hotspot, but the Yellowstone