COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



469 



tensive parts of the continental shelves to have been emergent and how 

 quickly the geography can change. 



SAN ANDREAS FAULT SYSTEM 



Aspects of Controversy 



Perhaps the most discussed and widely known structural feature of the 

 western United States is the San Andreas fault. See index map, Fig. 29.2, 

 for location. It may be traced with ease and certainty from Tomales Bay, 

 40 miles northwest of San Francisco, to Cajon Pass, 50 miles east of Los 

 Angeles. It has also been traced with a little doubt and difficulty for 

 some scores of miles northwest and southeast of these limits. Its total 

 known length is, therefore, more than 600 miles. This fault is so con- 

 spicuous that it was well known even before April 18, 1908. On that 

 date, it was the site of a violent earthquake in the vicinity of San Fran- 

 cisco. 



There is much conflicting literature written about the age of the San 

 Andreas fault, its movement, and its relation to the compressional folds 

 and faults. Some believe it came into existence first in pre-Cretaceous 

 time and moved recurrently through the Cenozoic to the present. Some 

 view the movement to have been mostly vertical, others mostly horizontal. 

 The vertical movement is said to be great, around 20,000 feet by some; 

 and only a few feet, by others. Those who recognize horizontal movement 

 are divided in their opinions. Some think the movement has been a few 

 thousand feet, others 300 miles or more. The most perplexing problem 

 about the San Andreas fault in the central ranges is its setting in typical 

 compressional structures running parallel or at an acute angle to it. The 

 f great fault seems at odds with the geomorphic provinces. 



Those who have studied the fault north of the Garlock fault commonly 

 [interpret it differently from those who have studied it southward. Dib- 

 blee, however, who has studied the fault system both north and south 

 of the Garlock fault probably more extensively than any other geologist, 

 sees right-lateral movement predominantly throughout the entire length 

 (Hill and Dibblee, 1953). 



Main Faults and Relations of the System 



The master fault of the system is considered the San Andreas, and the 

 Big Pine and Garlock faults principal conjugate sheers (Hill and Dib- 

 blee, 1953). See Fig. 29.2. 



In the San Francisco Bay area the Hayward fault passes through 

 Berkeley and the site of the University of California stadium. A little to 

 the east is the parallel Calaveras fault. Branches of the San Andreas ex- 

 tend up the peninsula on the west side of the bay. No long faults have 

 been mapped in the northern Coast Ranges except some just north of 

 San Francisco Bay. 



The Garlock fault is conspicuous from its position at the boundary of 

 a region of strong relief on the north and subdued relief on the south 

 in the Mojave Desert. 



The San Jacinto and Elsinor faults are major ones in the Peninsular 

 Ranges and most probably shared the horizontal movement with the 

 San Andreas. In fact, most all the faults shown on the map of Fig. 29.2 

 are large, and probably parts of the system. 



In studying displacements and ages of the faults the following rock 

 types, as far as manner of response to deformation, have been distin- 

 guished (Hill and Dibblee, 1953): 



1. Sierran basement complex (pre-Cretaceous): metasedimentary and meta- 

 volcanic rocks, intensely deformed and widely invaded by granitic rocks. Be- 

 cause of physical similarity, the Santa Lucia granitics and metamorphics of the 

 southern Coast Ranges and the complexes of the Transverse and Peninsular 

 ranges belong in this group. These are relatively rigid rocks which fail locally by 

 fracturing and, since they or rocks like them are extensively exposed and are 

 presumably of state-wide occurrence at depth, their mechanical behavior is 

 tectonically important. 



2. Franciscan basement (pre-Cretaceous): sedimentary and volcanic rocks, 

 regionally unmetamorphosed but highly indurated, commonlv intruded bv basic 

 igneous rocks which are usually altered to serpentine and have caused local 

 metamorphism. These rocks are exposed in large areas in the Coast Ranges; on 

 the northeast side of the San Andreas fault, and also on the west side of the 

 Nacimiento fault zone. They presumably underlie a much greater area but are 

 probably in turn underlain by granitic rocks. The Franciscan, unlike the granitic 

 basement, is typically incompetent. Although in places intensely fractured, 

 often before being covered by later Jurassic or Cretaceous strata, and usually in 



