The Pacific and North American Plates

The Pacific-North American plate boundary has evolved over time. Before 30 million years ago, the entire west coast of California was a subduction zone (where one plate is forced under another), resembling the present-day Oregon-Washington coast line. Since 30 million years ago, the San Andreas fault has been cutting its way northward through the California coastal margin.

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From Bohannon, R.G., and Parsons, T., 1995, Tectonic implications of post-30 Ma Pacific and North American relative plate motions: Geological Society of America Bulletin, 107, p. 937-959