N.E. Pacific and W. North America Plate History,
38 Ma to Present.

Plate Tectonic History of the N.E. Pacific and W. North America, 38 Ma to Present (stable North America held fixed).

Drawn and animated by Tanya Atwater using Photoshop and Morph. Thanks to Bill Menard, Joann Stock, Jeff Severinghaus, Doug Wilson, Craig Nicholson, Gene Humphries, and many others. Reconstructions closely follow those presented in Atwater and Stock, 1998, Int. Geol. Rev., v. 40, p. 375.

Here are suggested narratives written to accompany the showing of the QuickTime animations (each * represents one play-through of a given animation clip)

Present Situation. At Present the Pacific Plate fills most of the northeast Pacific Ocean basin with only the small Juan de Fuca and Cocos plates remaining from the previous configuration. The various shades of blue show the ages of the sea floor, as deduced from marine magnetic anomalies. The Pacific plate is moving northwest past North America. It has captured some slivers of the continental edge and is carrying them northwestward toward Alaska. Thus, the present Pacific-NorthAmerica plate boundary lies withing the continent, along the San Andreas fault system. it is connected to othe plate boundaries at three-plate triple junctions, the so-called Mendocino and Rivera triple junctions; the locations and motions of these triple junctions help determine the on-shore geology in each time and place.

Early Cenozoic Situation 38 Ma. In the early Cenozoic, 50 Million Years Ago, other oceanic plates lay between the pacific and North American plates. They were spreading away from the pacific plate and subducting beneath the rim of the continent.

* East Pacific Rise Migration. Coming forward in time, the eastern edge of the Pacific plate moved steadily northeastward as new sea floor was accreted by sea floor spreading. Eventually, the spreading center itself reached the subduction zone and the intervening plate was destroyed. The Pacific plate began to break off pieces of North America and carry them away, creating the San Andreas-Gulf of California plate boundary inside the continent.

* Triple Junction Evolution. Watch the motions of the triple junctions. The Mendocino triple junction migrated steadily up the coast, attached to the Mendocino fracture zone on the Pacific plate. The Rivera triple junction hovered near the southern California borderland then, about 12 million years ago, it jumped to its modern position when sea-floor spreading and subduction ceased off Baja California.

* San Andreas System Evolution. The evolution of the San Andreas system was essentially a two stage process. First the Salinian and borderland pieces of the continent were gradually transferred to the Pacific plate, later Baja California was transferred. A third stage has begun: the Sierra Nevada/Great Valley block is in the early stages of being transferred and carried away.

* Pacific Plate Motion. Watch the motion of the Pacific plate. It continually moved off to the northwest.

* Basin and Range Expansion. Now watch what happened to the interior of North America. In the early Cenozoic, western North America was much narrower. (Check out narrow Nevada!). During the late Cenozoic it expanded, forming the Basin and Range province. This occurred during the time that the Pacific plate was pulling the rim of the continent away to the northwest, perhaps causing the expansion or, more likely, just making room for it. (It was already elevated, hot and weak, ready to fall apart given the chance.)

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