88 



Structure 



STATUTE MILES 

 10 20 30 40 50 



n \ r 



Figure 75. Seismic refraction profile from near the mid- 

 dle of Santa Cruz Basin (left) extending southeasterly 

 through the basin sill. Material of 4.3 to 5.1 km/sec 

 seismic velocity was not detected but may be present, 

 separating the 2.8 km/sec unconsolidated sediments from 

 the 6.2 km/sec basement rock. Redrawn from Shor and 

 Raitt(1956). 



Construction of geological maps of the con- 

 tinental shelf from bottom sampling opera- 

 tions served to locate a linear anticline or 

 fault (Fig. 77) extending southeasterly 

 through San Pedro Bay (Moore, 1954fl), an 

 anticline possibly penetrated by schist base- 

 ment in Santa Monica Bay (Fig. 78) (Terry, 

 Keesling, and Uchupi, 1956, Figs. 32, 37), 

 an anticline extending southward from 

 Point Loma at San Diego, and another 

 probably underlying Coronado Bank also 

 off San Diego (Emery, Butcher, Gould, and 

 Shepard, 1952). Structural contours de- 

 veloped by drilling of oil fields on shore 

 clearly show an extension of anticlines be- 

 yond the shoreline at Huntington Beach, 

 Wilmington, Playa del Rey, Rincon, Monte- 

 cito, Summerland, Elwood, and Capitan(see 

 structural contour maps in California Divi- 

 sion of Mines Bulletin 118, pp. 281-424). 

 Many of these extensions have subsequently 

 been verified and supplemented by marine 

 seismic surveys of the continental shelf 

 (Johnson and Galeski, 1949; Jakosky and 

 Jakosky, 1956). Still other minor structures 

 on the shelf have been detected and mapped 



through use of a powerful echo sounder 

 (Fig. 79) capable of obtaining reflections of 

 sound from strata as deep as 200 feet below 

 the surface of the shelf (Moore, 1957). 



In the Los Angeles Basin four northwest- 

 southeast structural trends are present 

 (Troxel, 1954; Poland, Piper, and others, 

 1945). One of these in particular, the New- 

 port-Inglewood structure, has distinct top- 

 ographic expression as a series of en 

 echelon domal hills. Because the structural 

 relief of these domes increases with depth, 

 folding must have begun early and continued 

 throughout the whole period of filling of the 

 basin with sediments, so that the domes had 

 the form of low hills during all or most of 

 the history of the basin. Since the present 

 sea floor basins are similar in many respects 

 to early stages of the Los Angeles Basin, we 

 might also expect to find structural domes 

 or anticlines in these basins, revealed by 

 hills or ridges rising above the general basin 

 floors. One of the shallowest of these pos- 

 sible secondary structures is the long nose 

 that extends westerly into the Santa Barbara 

 Basin (Fig. 80); (see also 600-foot contour 

 of Chart I). This may be an extension of 

 the Oak Ridge fault (Bailey and Jahns, 

 1954). The broad ridge that rises above the 

 northern end of the San Diego Trough 

 about 15 miles southwest of Dana Point 

 may be similar. A much deeper one may 

 be indicated by the long ridge that extends 

 diagonally up the northeast side of San 

 Nicolas Basin, another by the ridge on the 

 northeast side of Tanner Basin, and lastly a 

 very deep one by the ridge at the north end 

 of Velero Basin. All the basins of the con- 

 tinental borderland contain less pronounced 

 irregularities which might prove to be topo- 

 graphically similar to the Newport-Ingle- 

 wood structure if they were to be very care- 

 fully surveyed. 



General Structural History 



Data presented in the sections on physiog- 

 raphy, lithology, and structure provide a 

 vague outhne of the structural history of the 

 region. Vaguest of all is the pre-Cretaceous 



