32. 



PACIFIC SUBMARINE 

 PROVINCES 



DISCOVERY OF STRONG SUBMARINE RELIEF 



It was current opinion until 1925 that the ocean floors were montonous 

 plains. The continental shelves above the floor and the great deeps below 

 the floor were known, but not their details. The technique of echo sound- 

 ing was successfully introduced in 1925 by the U.S. Coast and Geodetic 

 Survey, and since then remarkable progress in mapping the floor of both 

 the Pacific and Atlantic oceans has been made. Many thousands of miles 

 of traverses have been run, and with progressively more accurate means 

 |af location available the contouring has become more accurate and the 

 topography better known. The Gulf of Alaska was explored before 1940, 



and instead of a featureless floor a number of bold seamounts were dis- 

 covered. The most detailed early survey was off the coast of southern 

 California, where basins, banks, ridges, and escarpments of comparable 

 size to those on the adjacent land were indicated. 



In addition to many seamounts in the northeastern Pacific, various 

 ridges, depressions, and trenches were discovered, and by 1955, the length 

 of sounding lines to show the extent and some of the details of these 

 features had reached about 80,000 miles (Menard, 1956). This work 

 was done chiefly aboard ships of the Navy Electronics Laboratory and 

 the Scripps Institution of Oceanography. Several expeditions each year 

 since 1955 continue to add to an ever amazing picture of the Pacific ocean 

 floor. 



Study of the submarine topography is pertinent to an understanding of 

 the deformation of the oceanic crust, and most interpretations to date have 

 been made from the relief features. Valuable supplementary informa- 

 tion has come from seismic and gravitational surveys, and most recently 

 from extensive magnetic intensity surveys. 



SUBMARINE PROVINCES 



Basins, Banks, and Ridges off California 



The submarine topography for 150 miles off the southern California 

 shore is one of basins, banks, and ridges comparable with that of the 

 adjacent land. Shepard ( Shepard and Emery, 1941 ) calls it the continental 

 borderland. See Figs. 32.1 and 32.2. 



In this borderland are eleven basins which would contain large lakes 

 if the land became emergent. Some of them would cover 1000 square 

 miles and would range up to 2880 feet deep. The basins are roughly oval 

 and elongated northwesterly. Their walls are generally steep, long, and 

 straight, but are gashed by a few valleys. However, abrupt changes in 

 direction exist. The basin floors are very flat, and do not possess the 

 piedmont slopes of their land counterparts in southern California and 

 Nevada. The general elevation of the basins and their overflow sills be- 

 comes greater to the southeast (Shepard and Emery, 1941). 



The elevations on the continental borderland are numerous and diverse. 



515 



