The North Pacific and the West Coast of North America

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

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Most diagrams on this page are modified from original diagrams in scientific papers by Tanya Atwater of Stanford University, who first worked out the ideas presented here.

Sea-floor spreading creates symmetrical patterns on both plates. Thus, if one plate is entirely subducted, it is still possible to reconstruct some of its history from the surviving plate. This is the case off the west coast of North America.

Reconstructing Past Plate History

The magnetic stripes and age patterns off the west coast of North America are strange in several respects:

• Oceanic crust increases in age away from the coast
• Magnetic anomalies bend in the Gulf of Alaska
• The East Pacific Rise terminates in the Gulf of California. (The ridge segment shown on land is under the Imperial Valley of California. This area is below sea level and underlain by oceanic crust, but is cut off from the sea by the Colorado River delta.)
• There is a small ridge off the Pacific Northwest and a subduction zone along the coast. The Cascade Range volcanoes are due to the subduction zone. The small plate between the Pacific and North American Plates is called the Gorda Plate.

These data suggest:

• There was once a continuous ridge system off the West Coast.
• The West Coast was once occupied by a continuous subduction zone.
• Most of the ridge and all the crust on the other side of it has been subducted beneath North America. The small ridge and subduction zone in the Pacific Northwest are the last remnants of these former plate boundaries.
• There was once a junction of three ridges in the North Pacific. The anomalies off California tell of east-west spreading, but those off Alaska imply north-south spreading. There must have been an additional ridge to accomodate the differences in plate motion.

 

Basically, the Pacific Plate is sliding northwest parallel to the coast of North America. If we assume an average of 5-6 cm per year and eliminate all recent oceanic crust, 20 m.y. ago the West Coast looked like this. The ridge system is much more continuous, but a short segment of the plate boundary has Pacific crust in direct contact with North America. This is a transform fault, the ancestor of the San Andreas.

 

Here we see the last remnants of the third ridge. The plate north of the Pacific Plate is inferred only from the magnetic patterns on the Pacific Plate. This vanished plate is called the Kula Plate, from an Indian word meaning "all gone".

 

The Pacific Plate is now in contact with North America for the first time - the birth of the San Andreas Fault

 

Here we see the ancestral plate configuration. The plate between North America and the Pacific Plate is called the Farallon Plate. The present Cocos, Nazca and Gorda Plates are its descendants. Once the Gorda Plate became sufficiently detached from the rest of the Farallon Plate, say 5 m.y. ago, there was no longer any reason why it need move in the same direction. Thus the present ridge off Oregon and Washington, and the recent magnetic anomalies, are quite different in orientation than the ancestral East Pacific Rise.

The Sequence of Events

On the left of each figure is a geologic map showing the age of the ocean crust. On the right is a figure showing the plate configuration. Blue lines represent subduction zones, red represent spreading centers, and purple represent transform faults. 

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Created 18 Nov 1997, Last Update 6 February 2001