78 



Structure 



oif southern California, citing as one exam- 

 ple the scarp bordering San Clemente Island 

 (Fig. 67). Some additional details for Santa 

 Catalina Basin were given by Taber (1927), 

 and in 1941 Shepard and Emery mapped 

 some of the more definite sea floor faults. 

 A better-defined relationship to known faults 

 on land was given by Corey (1954). A new 

 map (Fig. 68) extending farther south than 

 Corey's map contains more detail and addi- 

 tional faults drawn on the assumption that 

 the topography is basically of structural ori- 

 gin. Interpretations by other geologists 

 would be diff'erent in detail but probably 

 closely similar in general. Even casual ex- 

 amination of the contours of Chart I shows 

 a remarkable linear series of elongate fea- 

 tures for which an origin other than by 

 faulting or folding is difficult to conceive. 

 For example, when we draw a possible fault 

 along the west side of Santa Rosa-Cortes 

 Ridge, it can easily be continued southward 

 from scarp to scarp along noses, "sag- 

 ponds," and saddles. Another example is 

 the northeast side of San Clemente Island, 

 and another is the east side of the San Diego 

 Trough. 



Seven long primary faults are believed to 

 be present. It seems quite possible that these 

 are the result of large strike-slip movements. 

 A movement of 25 miles was suggested by 



Shepard and Emery (1941, pp. 24-25) to ex- 

 plain the topographic relationships of San 

 Clemente Island to Fortymile Bank and of 

 Santa Catalina Island to Thirtymile Bank. 

 Alternately, the topographic relationships 

 might have been caused by rotation of a 

 block on a horizontal axis extending north- 

 easterly from the southeastern end of San 

 Clemente Island. However, large strike-slip 

 movements are known in the region. Ac- 

 cording to W. H. Corey (personal communi- 

 cation), there are four lines of evidence for 

 at least 10 miles of left lateral movement on 

 the Santa Rosa Island fault. Even larger 

 horizontal movements have been suggested 

 for land faults of the region by Hill and 

 Dibblee (1953). Data from earthquakes are 

 also strongly suggestive of strike-slip move- 

 ments, as discussed in the next section. 



Between each of the seven primary faults 

 are blocks containing other scarps which are 

 usually shorter and less straight than those 

 along the primary faults. Many of these 

 scarps lie at an angle of 20 to 40° from the 

 trend of the primary faults. Good examples 

 exist in the block that contains Cortes and 

 Tanner Banks and in the block bordering the 

 continental slope. In the former, San Nico- 

 las Island is known to be an anticline from 

 subaerial geology and also from the finding 

 of progressively younger rocks on either side 



Figure 67. Long straight steep fault scarp along northeast side of San Clemente Island viewed from near the south- 

 east end of the island. Photograph from elevation of 2000 feet by Navy Fleet Air Wing Fourteen. 



