82 



Structure 



continuity is about 15 km thick under the 

 continental borderland and that it thickens 

 to about 35 km under the Transverse Ranges 

 and to about 50 km under the Sierra Ne- 

 vada. These estimates accord reasonably 

 well with data provided by the average 

 depth of earthquake foci and of gravity 

 interpretations. 



Geomagnetism 



For several decades the U. S. Coast and 

 Geodetic Survey's Coast Pilot has reported 

 a magnetic compass variation of 3° near 

 San Clemente Island and another one near 

 Sixtymile Bank. In order to investigate 

 these irregularities and to learn something 

 about the general magnetic trends in the 

 region, an aeromagnetic survey was made in 

 1949 as a cooperative project of the U. S. 

 Geological Survey (Bromery, Emery, and 

 Balsley, in press). A pattern having a total 

 flight distance of 1300 miles was flown, 

 mostly at an elevation of 1500 feet. 



After removal of the regional magnetic 

 intensity, the measurements have a range of 

 between plus 1365 and minus 130 gammas. 

 The anomalies are only slightly related to 

 topography, and they reveal only a few of 

 the local sharp discontinuities that might be 

 interpreted as faults (Figs. 70, 71). High 

 magnetic anomalies were found over Santa 

 Cruz Island, Santa Monica Mountains, and 

 San Gabriel Mountains where outcrops of 

 granodiorite were crossed (see also Schoell- 

 hamer and Woodford, 1951). Outcrops of 

 volcanic and metamorphic rocks failed to 

 produce high anomalies except near the 

 basaltic seamounts of the deep sea floor. 

 These anomalies are perhaps similar to ones 

 found (Alldredge and Keller, 1949) around 

 seamounts of the North Pacific Ocean, where 

 the topographic projections were believed to 

 be polarized by the magnetic field of the 

 earth. Sedimentary rocks of the topograph- 

 ically high areas and unconsolidated sedi- 

 ments of the basins produced no definite 

 magnetic eff'ects, so the broad anomalies en- 

 countered in these areas are presumed to re- 



flect variations in the composition of the 

 deeply underlying basement rocks. 



The greatest magnetic high anomaly is ad- 

 jacent to the most conspicuous magnetic low 

 near San Clemente Island. The area may, 

 therfore, be one in which a considerable vol- 

 ume of relatively highly magnetic basement 

 rock has been faulted much nearer to the sur- 

 face than in the adjacent basin and, in fact, 

 than in most of the continental borderland. 

 Because the magnetic intensity here is greater 

 than over the outcrops of granodiorite, the 

 basement below San Clemente Island may 

 be more magnetic than the granodiorite 

 to be found elsewhere. Possibly the high 

 anomaly results from a large covered diorite 

 or gabbro intrusive. Such an interpretation 

 would fit the known pattern of distribution 

 of basement rock types better than would 

 the presence of granodiorite. The belts of 

 high and low magnetic anomaly near San 

 Clemente Island are elongate in a north- 

 westerly direction and are paralleled by four 

 less intense belts of high and low anomaly 

 located farther seaward. The northwesterly 

 extension of these belts parallels the struc- 

 tural trend of the region, but the significance 

 is admittedly unknown at present. 



Gravity 



Published information on gravity measure- 

 ments for southern California was so sparse 

 that up to 1955 the intensity was sufficient 

 to be considered only as preliminary recon- 

 naissance (Woollard, 1955). About 30 pen- 

 dulum stations were established in the region 

 by the U. S. Coast and Geodetic Survey 

 (Duerksen, 1949), and some additional 

 measurements were made by WooUard (1949) 

 as portions of transcontinental or transoce- 

 anic profiles. Subsequently, a very thorough 

 study of gravity in the region was made by 

 Brisbin (1957) who occupied about 300 sta- 

 tions on land plus 17 others on islands. A 

 very detailed study of the western part of 

 the Los Angeles Basin was also made by 

 McCuHoh (1957) on the basis of 840 stations. 

 Other local gravity meter studies have been 



