470 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



fault contact with the other principal rock types, its response to deformational 

 forces has been characterized by folding. 



3. Cretaceous and Cenozoic sedimentary and volcanic formations: mainly 

 marine clastic sediments with local volcanics and nonmarine deposits, not 

 strongly lithified and of extremely variable thicknesses and facies. Deposited 

 in large and small basins; locally highly deformed, especially during the late 

 Pliocene-Pleistocene revolution in the Coast and Transverse Ranges, and in 

 uplifts in the Mojave Desert and Salton Sea region regions. These rocks form 

 a pliable mande on the above described complexes and have therefore re- 

 sponded to tectonic forces primarily by folding, particularly where the sedimen- 

 tary section is thick or where underlain by Franciscan basement. 



The San Andreas fault marks such an important contact that rarely can 

 it be crossed, except in Recent alluvium, without passing into significantly 

 different rocks. It is also a steep, if not nearly vertical fault and extends 

 to depths of at least 10 miles, according to seismological evidence. 



Evidence of Horizontal Displacement 



The following evidence of horizontal movement on the San Andreas 

 fault is presented by Hill and Dibblee ( 1953) : 



1. The trace of the San Andreas zone is typically continuous and straight. 

 There is evidence of recent activity along its entire course. Excepting a 30-mile 

 segment trending eastward in the San Emigdio Mountains, and another stretch 

 of similar trend 100 miles to the southeast, the zone is remarkably straight from 

 Point Arena southeastward nearly to Mexico. These aspects of continuity and 

 straightness are considered typical of strike-slip faults. 



2. The San Andreas is a steep fault which transects major topographic fea- 

 tures but develops all along its course one or several parallel trenches, sag 

 ponds, low ridges, saddles, and/or scarps. Its steepness is indicated by the 

 straight trace, the fact that mapped fault planes are nearly vertical, and the 

 failure of near-by drill holes to penetrate the zone. These characteristics are 

 typical of strike-slip faults. The development of fresh topographic features, 

 many of which are in unconsolidated recent sediments, and the common lack of 

 appreciable vertical or consistent vertical components of offset clearly indicate 

 the recency of lateral movements. Seismic evidence for recent right lateral 

 movements on the San Andreas, as summarized by Wallace (1949), comprises 

 the following maximum displacements at the time of earthquakes: 30 feet (San 

 Emigdio Mountains, 1857), 10 feet (San Francisco area, 1868), 21 feet (San 

 Francisco area, 1906), and 10 feet (Salton Sea area, 1940). 



3. The San Andreas fault zone ranges from a few feet to a few miles in width. 

 Locally a single recent trace may be irregular, with 15-degree variations in 

 strike within a few hundred feet, or it may disappear and be replaced, en 

 echelon, by another. Occasionally two or three parallel traces widen the zone 



of recent traces to a maximum of about half a mile. Wider segments of the 

 zone consist of several faults (not necessarily active) which are usually steep 

 and nearly parallel to the trend of the zone. These characteristics are considered 

 typical of strike-slip fault zones along which recurring movements have taken 

 place. 



4. The apparent throw is commonly reversed along the San Andreas fault 

 as indicated by topographic and geologic relationships. These throws are prob- 

 ably due to the major strike-slip component which places in juxtaposition un- 

 like topographic elevations and geologic sections, and thus the reversals of 

 dip-slip are mainly illusory. 



5. Drainage lines are consistently offset in a right lateral sense. These offsets 

 are especially clear on the southwest side of the Temblor Range where a maxi- 

 mum of 3000 feet of displacement has occurred through recent movements on 

 the fault. Wallace (1949, p. 805) reports a probable drainage offset of VA 

 miles on the north side of the San Gabriel Mountains, and Allen (1946, p. 50) 

 reports 3800-foot offsets of drainage lines near the Gabilan Range, also in a 

 right lateral sense. 



6. Recendy developed trenches which irend southward into the fault have 

 been observed in aerial reconnaissance on the southwest side of the Temblor 

 Range. These are oriented correctly to be tensional in origin and due to right 

 lateral movement on the San Andreas. 



7. Locally developed west-northwest trending folds adjacent to the San 

 Andreas are obviously drag folds resulting from the right lateral movement on 

 the San Andreas. Such drag folds are expecially clear in the Salton Sea Region, 

 and, besides indicating the right lateral sense of movement on the fault, many 

 of them show by their discordance with topographic form that the fault was 

 active before the present physiographic features were developed. 



8. Wallace (1949) reports a probable 6-mile right lateral offset of terrace 

 deposits on the north side of the San Gabriel Mountains, and L. F. Noble 

 (personal communication) describes similar late offsets in that area of several 

 miles. 



9. Between the San Emigdio Mountains and the Temblor Range, there are 

 two facies of Pleistocene gravels. On the southwest side of the San Andreas, 

 the pebbles are granite, gneiss, quartzite, limestone, black shale, and sandstone 

 which undoubtedly came from the San Emigdio Mountains. On the other side 

 of the fault, the pebbles are almost exclusively white siliceous shale which 

 probably came from the Miocene shale of the Temblor Range. These two 

 facies are in direct contact along the San Andreas for several miles. Further- 

 more, the northwest end of the crystalline clast facies is about 14 miles north- 

 west of the crystalline rocks of the San Emigdio Mountains. These relationships, 

 thus indicate a right lateral displacement of approximately 10 miles on the San 

 Andreas fault since Pleistocene deposition in this area. 



10. In the Caliente Range, marine sediments of upper and middle Miocene 

 age grade laterally eastward into continental red beds which strike into the 

 San Andreas fault, whereas strata of the same age are marine shales on the 



