84 



Structure 



Figure 71. Profiles of magnetic anomaly from Figure 70. Note general absence of relationship to topography. 



alies become progressively more positive 

 until near the edge of the continental slope 

 the values are about + 100 and rise sharply 

 farther seaward to about 250 milligals. 

 (Fig. 72). These Bouguer computations are 

 based on the assumed replacement of sea 

 water by material of density sufficient to rule 

 out topographic effects — for the continental 

 borderland 2.67 (Brisbin, 1957). 



Data computed by Duerksen (1949) allow 

 the construction of an isostatic anomaly 

 map based on the Hayford (Pratt) theory of 

 lower density of crystalline rocks for areas 

 of higher elevation than of rocks for lower 

 areas. Results using depths of compensa- 

 tion of 56.9, 96.0, and 1 13.7 km, as well as 

 the Airy or Heiskanen theories of mass dis- 

 tribution (Daly, 1940, p. 189), show that the 

 region of southern California is negative, 

 averaging —20 to —30 miUigals. This, too, 

 is characteristic of the entire Pacific coast 

 and is difficult to interpret in terms of iso- 

 static theory. 



Prominent minor features of the general 

 pattern are the strong negative Bouguer 

 anomalies in the area of thick sediments of 

 the Los Angeles Basin (McCulloh, 1957). 

 They are the result of the low density of the 

 thick basin sediments, as Ewing and Worzel 



(1954) have shown for the Puerto Rico 

 Trench. Measurements above the sub- 

 merged basins of the continental borderland 

 also show lower Bouguer anomalies than the 

 nearby islands as a result of a low density 

 of sediment fill (Brisbin, 1957). Larger 

 deficiencies in gravity were found for Santa 

 Catalina Basin and San Diego Trough than 

 for San Nicolas Basin, indicating greater 

 thickness of sediments in nearshore than 

 offshore basins. 



Near the continental slope off the Atlantic 

 coast of the United States (Worzel and 

 Shurbet, 1955) a sharp +10- to +70-milli- 

 gal peak in Free Air anomaly may be 

 caused by sediments deposited on the slope 

 with incomplete isostatic downbowing. No 

 such sharp peak exists at the continental 

 slope of southern California because the 

 basins of the continental borderland trap 

 most of the sediment before it can reach the 

 continental slope and produce an apron of 

 sediment. 



Seismic Surveys 



Information about the earth's crust ob- 

 tained by artificial earthquakes supplements 



