Minor Structures 



87 



Figure 74. Character of record obtained on shelf of southern Cahfornia by "L" spread of geophones. Note that beds 

 on one side of the shot point appear to be horizontal, whereas on the other side they dip and diverge from each other. 

 Courtesy International Geophysics, Inc. 



32 km below sea level near the shore at San 

 Diego to 18 km at the top of the continental 

 slope to 13 km on the abyssal sea floor. 

 This seaward decrease in depth corresponds 

 well with gravity data. Below the discon- 

 tinuity the rocks have a seismic velocity of 

 about 8.3 km/sec and above it about 6.7 

 km/sec. Above the latter zone is a less 

 widespread zone, having a velocity of 5.9 to 

 6.3 km/sec, that may consist of intrusive 

 igneous or metamorphic rocks that prob- 

 ably form the basement series of the region. 

 Its top is at a fairly uniform depth of about 

 7 km throughout the borderland and be- 

 neath the abyssal sea floor beyond. Within 

 the basins is a zone having a seismic velo- 

 city of 4.3 to 5.1 km/sec that rises under the 

 islands and presumably consists of sedimen- 

 tary and volcanic rocks (Shor and Raitt, 

 1956, 1958). Lastly, the basins contain a 

 variable thickness of material having a seis- 

 mic velocity of between 1.8 and 2.8 km/sec 

 and believed to be unconsolidated sedi- 

 ments. Maximum thicknesses of unconsoli- 

 dated sediments were estimated as 11,000 

 (?) feet for the Santa Cruz Basin (Fig. 75), 

 9000 feet for San Nicolas Basin, and 



7000 to 9000 feet for both Santa CataUna 

 Basin and San Diego Trough. Estimated 

 depths to basement for the same basins are 

 11,000 (?), 23,000 (?), 10,000 to 31,000, and 

 10,000 to 13,000 feet, respectively. About 

 4500 feet of sediment was found on the sea 

 floor near the base of the continental slope, 

 in contrast to typical thicknesses of 1500 

 feet on the abyssal sea floor farther west. 

 Unfortunately, at present the seismic refrac- 

 tion tool is too coarse to yield results as 

 precise as desired for making geological 

 studies of the basins. 



Minor Structures 



Superimposed atop the major structural 

 units of the continental borderland are many 

 secondary structures that are minor in size, 

 although some are important as oil traps. 

 Truncation of some structures by wave 

 erosion in very shallow water has resulted 

 in a slight topographic relief of harder layers 

 which then serve as places for attachment 

 of kelp, so that hnear patterns of kelp often 

 reflect the underlying structure (Fig. 76). 



