96 



Structure 



STATUTE MILES 



VERTICAL X 50 



SEA LEVEL 



Figure 82. Plot of the varia- 

 tions of the height or depth of 

 mountain tops (T), basin floors 

 (B), and basin sills (S) with dis- 

 tance south of latitude 34°30', 

 the northern border of Chart 

 I. Data are restricted to areas 

 shown by Chart I. From 

 Emery (1954 J, Fig. 4). 



high Transverse Ranges at the north, and 

 the steepest portion of the curve for the bot- 

 toms of basins is, at least in part, the result 

 of the rapid decrease in rate of deposition 

 of basin sediments in an offshore direction. 

 On land the sills are lower than the basin 

 floors because these basins are filled to over- 

 flowing. Far to the south the sills are not 

 as high above the basin floors as at inter- 

 mediate distances, possibly because the orig- 

 inal basin-forming deformation was less in- 

 tense there. Aside from the local steepen- 

 ing, all three curves show an average south- 

 ward decrease in altitude of about 3000 feet 

 per hundred miles, or a slope of about y^° . 

 Three hundred miles southeast of the south- 

 ern border of Chart I the sea floor again 

 rises to form the spur of Baja California 

 near Cedros Island. If the basin and island 

 topography of the whole continental border- 

 land was blocked out at about the same time 

 and at a more or less uniform depth through- 

 out the region, there must have occurred a 

 subsequent broad downwarping between 

 Point Conception and Cedros Island. This 

 warping has continued into post-Pleistocene 

 times as shown by the similar but smaller 



warp of the depth of the shelf break. The 

 latter amounts to about 160 feet per 100 

 miles, about one-twentieth the warp of the 

 mountain tops and the sills and botoms of 

 basins. This ratio is in agreement with the 

 age of the continental shelf, which is much 

 younger than the basin topography in south- 

 ern California. 



A major question concerned with the 

 downwarping is what happened to the rocks 

 that were displaced. The continental border- 

 land contains about 30,000 cubic miles of 

 post- Miocene sediment and about 50,000 

 cubic miles of overlying water between Point 

 Conception and Cedros Island. Perhaps 

 the downwarp was accompanied by a plastic 

 flow at depth, somewhat like that discussed 

 by Lawson (1950). Conceivably, this sub- 

 flow added to the uplift of the Transverse 

 Ranges and to the Peninsular Ranges; how- 

 ever, the volumes of these ranges are too 

 small (2400 cubic miles for the Transverse 

 Ranges) to have absorbed the entire flow. 

 Moreover, no adequate bulge of the abyssal 

 sea floor is present. We might conclude that 

 any lateral flow which occurred must have 

 continued far beyond the region. 



