The geologic structure of Southern California is intricately tied to the convergent boundary between two lithospheric plates. The tectonic collision betwen the North American and Pacific Plates causes large masses of rock to break, bend, and travel in response to the immense forces.

   The San Andreas Fault Zone is the boundary between these two converging plates. The Pacific Plate and the North American Plate are grinding past one another along a zone of subparallel faults that is roughly one hundred kilometers wide and has shifted its location several times since the onset of this transform (strike-slip) plate boundary 25-30 million years ago.

   The rugged topography of southern California is testimony to the long-term tectonic activity associated with movement along the San Andreas Fault Zone. As the plate margins collide, broken blocks between faults are squeezed up or down, producing steep mountains and intervening basins. This exposes a bounty of geologic history, revealing past environments and the geologic activity of ancient Southern California.

   Prior to tectonic plate convergence, the plate boundary along the western edge of North America was quite different. A seafloor spreading center existed off the coast; this massive seafloor fracture allowed magma to form oceanic lithosphere as tectonic plates move apart. One of the plates born at the spreading ridge was the Pacific Plate (moving westward); the other was the Farallon Plate which moved eastward beneath the encroaching North American Plate in a process called subduction. The Farallon Plate was split into the Cocos and Juan de Fuca Plates when the spreading center was overrun by the North American Plate.

   About 29 Ma, after ramping up the flanks of the spreading ridge, the North American Plate began overrunning the spreading ridge itself, and the Farallon Plate became completely subducted at that point. Subsequently, more of the spreading ridge became overrun by the North American Plate as the San Andreas fault system grew in size and scope.

   The modern San Andreas fault system in Southern California has existed for about 8 My. Although the initial contact between the plates was around 29 Ma, several major strike-slip faults were the principle plate boundaries prior to the current San Andreas configuration.

   The San Andreas Fault is believed to be 'locked' south of San Gorgonio Pass down to the Salton Trough. No major quakes have occurred along this segment of the fault during historic time, and many hypotheses have been proposed to explain the lack of motion of the fault through this area. Plate motion may have been taken up on the San Jacinto and Elsinore Faults where 24 and 30 km of offset have been measured, respectively. Prior to the existance of the San Andreas Fault Zone, motion was taken up in the heavily-faulted continental borderland offshore of Southern California.

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