The San Andreas' secret helpers.

Residents of the East Coast and Midwest who view Los Angeles as a foreign land can find geologic support for their bias. The westernmost section of California is attached to the Pacific Ocean floor and moves in a completely different direction from the rest of North America. For 30 years, researchers have puzzled over what happens at the border, where the Pacific territory scrapes against North America.

Now, a team of geologists has an answer.

From the earliest days of plate tectonic theory, geophysicists have realized that the famous San Andreas fault forms the junction between the Pacific tectonic plate and the North American plate. The two great blocks slide by each other like passing trains, carrying the Los Angeles side of the fault northwest, while the North American side moves southeast. Since the birth of the San Andreas fault 16 million years ago, the two sides have shifted 315 kilometers.

Although that may seem a substantial journey, it's too short for geologists. Over that same 16 million years, the Pacific plate as a whole has moved 737 km relative to North America. How can the two plates have shifted more than twice as far as the two sides of the San Andreas fault? Geologists have proposed several mechanisms, but none has explained the entire discrepancy.

Until now. In the July Geology, William R. Dickinson of the University of Arizona in Tucson and Brian P. Wernicke of the California Institute of Technology in Pasadena show that a broad swath of North America has warped enough to account for the missing motion between the two plates.

Part of the answer lies in California's east-west mountain ranges, such as the Santa Monica and Santa Ynez Mountains. Over the past 16 million years, these ranges have rotated their orientation from almost north-south to east-west, serving as giant gears between the Pacific plate and North America.

The other half of the missing plate motion has disappeared in the Basin and Range geologic province east of the San Andreas, the geologists say. Faults there helped shift all of California northwest relative to the rest of North America.

The rotating mountains, the extending Basin and Range, and the observed San Andreas motion together explain the entire motion between the Pacific and North American plates. "There has always been some uncertainty as to whether the plate tectonic paradigm would explain what is going on. We're pleased to see it all shake out," says Dickinson.