inngeo

CALIFORNIA


Text excerpt from log report for Day 3:

The San Andreas Fault is the most striking physical feature to traverse California. This fault line, which stretches from Cape Mendocino in the north, past San Francisco, down through the Coast Ranges into the Gulf of California (approx. 1,200 kilometers long), separates two large lithospheric plates which are passing each other - the Pacific Plate and the North American Continental Plate. The Pacific Plate, extending right in front of us as we stood on our vista point, and the segment of California forming part of it, and situated west of the San Andreas Fault, has been moving about 6 centimeters annually in a northward direction for millions of years (i.e. "shift to the right" - if one stands on the eastern side of the fault and observes the other side then this moves to the right). Future scenarios envision Los Angeles drifting past San Francisco within a period of several million years from now - a result of the ongoing plate drift (Alt /Hyndman 1995, 404).

Picture: Water reservoir in the San Andreas Rift Valley
Picture: Earthquake fault on the highway between Mammoth Lakes and Mammoth Mountain
Picture: Berkeley's stadium was built on the the Hayward Fault. Cracks show the effects of earthquakes.

As the rock on both sides of the tectonic lines is very resistant, the major part of this movement is held back for a long period of time and is, instead, conserved in the form of elastic tension. Once the limit of elastic tension is surpassed, this form of energy, conserved for decades, is released in the form of a tectonic earthquake.

This dextral movement began about 20 - 30 million years ago, after the Pacific Plate had come into contact with the North American Plate. The Farallon Plate, which originally separated these two plates, was increasingly subducted and today can only be found in non-subducted areas such as the Rivera-Cocos Plate in the south and the Juan de Fuca-Garda Plate in the north (cf. fig 3/3). As a result of the constant subduction of the Farallon Plate, a deep rift was formed which was filled up with eroded sediments from the eastern continent over a period of several million of years. These sediments and "terranes" (i.e. foreign lumps moved by the drift of the plate) attached themselves and were eventually "welded" to the North American Plate. This way the mainland was extended in a westerly direction. Thus, the older northern part of the San Andreas Fault (today's California) had been fully formed while the southern part (Baja California) was waiting to take shape.


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