PACIFIC SUBMARINE PROVINCES 



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on a cross trend. The maximum relief of the Clipperton Ridge is 18,000 feet 

 from Clipperton Island to the deepest spot in the trough at 2,960 fathoms. 



DEEP SEA PROVINCES 



Gulf of Alaska Seamount Province 



The northernmost division of the northeastern Pacific basin is the Gulf 

 of Alaska Seamount Province (Menard and Dietz, 1951). Its northwestern 

 boundary is the Aleutian trench and its western the continental shelf 

 slope, which here is only about 8000 feet high. A rather steep apron 

 flattens seaward and appears to be a graded profile. The apron and 

 smooth deep-sea floor are interrupted by thirty-six majestic submarine 

 volcanoes. Eleven of these are guyots, and their flat tops indicate they 

 were once truncated by erosion. Most of them are now about 2500 feet 

 below sea level and some are much deeper, so it is concluded that a like 

 amount of subsidence has occurred since the truncation. 



The region is seismically inactive, and the topography is old with a 

 thick apron of sediment evidently across the entire province. Major 

 subsidence of the region is postulated but some time in the geologic 

 past, possibly Cretaceous. 



Ridge and Trough Province 



The continental slope of the Ridge and Trough Province is about 132 

 miles high and is dissected by several well-known submarine canyons. 

 An apron of sediment spreads from the base of the slope off Queen 

 Charlotte Island in the northern part of the province, but a long, narrow, 

 seismically active trough lies between the apron and the base of the 

 slope. Evidently the top of the apron has been faulted down so recently 

 that sediment moving out from the continent has not yet filled the trough 

 to re-establish an even gradient seaward (H. W. Menard, 1955). 



The sea floor presumably was block-faulted into long thin ridges which trend 

 northeast or north. From the ridges rise a few submarine volcanoes some of 

 which are only a few fathoms below the surface, but most crossings of the 

 ridges indicate steep-sided, low blocks, unlike volcanoes. 



The long ridges roughly parallel the continental slope and guide the flow of 

 turbidity currents moving sediment out from the continent. One of several 



leveed channels on the otherwise smooth plain at the base of the continental 

 slope off Oregon was traced southward for almost 200 miles. Apparcnth the 

 turbidity currents cannot surmount the ridges to flow west (direction of the 

 regional slope) but are diverted southward to a divide through which they again 

 flow westward or fan out to fill low spots on the downstream side of the ridges. 

 A few basins appear entirely ringed by high ridges so that turbidity flows 

 moving along the bottom cannot fill them with sediment. These basins are 

 thousands of feet below the level of the surrounding alluvial plains formed 

 by deposition from turbidity currents; their bottoms are irregular, which sug- 

 gests that deposition from suspension in the main mass of the ocean may 

 be much slower than deposition from turbidity currents moving in concentrated 

 clouds along the bottom. 



Deep Plain 



South of the Mendocino escarpment the sea floor is about half a mile 

 deeper than it is to the north, and it is called the Deep Plain. It is bounded 

 on the south by the Murray escarpment, and south of the Murray escarp- 

 ment the sea floor is roughly a quarter of a mile higher than it is to the 

 north. 



The continental slope off central California forms the eastern boundary 

 of the Deep Plain. It drops off abruptly to a depth of more than 2 miles, 

 and three great deep-sea fans form an apron which grades imperceptibly 

 into the gently sloping Deep Plain at a depth of about 2% miles. Crossing 

 the fans are leveed and unleveed channels. 



The Deep Plain is unique in that it appears to contain few seamounts. 

 Five seamounts, probably volcanoes, rise from the continental slope 



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Fig. 32.13. View to southwest toward Clipperton Island and the Clipperton Ridge. Reproduced 

 from Menard and Fisher, 1958. 



