526 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



bordering the area, but they trend parallel to the coast and may be 

 genetically unrelated to the deep-sea floor. (H. W. Menard, 1955). 



Baja California Seamount Province 



South of the Murray fracture zone a mountainous area, studded with 

 volcanoes, forms the Baja California Seamount province. 



The continental slope drops off abruptly to a depth of about 2 miles. It is 

 irregular but does not appear deeply dissected by canyons. A smooth apron a 

 few tens of miles wide lies at the base of the slope in some places. Off the 

 southern half of Baja California the continental slope drops abruptly for 2-2/2 

 miles into a series of long thin troughs a few hundred fathoms below the 

 general level of the deep-sea floor to the west. The troughs are flat-bottomed 

 indicating a fill of sediment. 



Widespread vulcanism, particularly recent vulcanism, characterizes the 

 province. Guadalupe Island comprises a group of eroded Late Tertiary or 

 Quaternary volcanoes. Alijos Rocks are three steep-sided remnants of a large 

 volcanic cone. Volcanic islands are so rare in the northeastern Pacific basin 

 that these deserve special consideration, but the evidence supporting unusual 

 vulcanism comes chiefly from submarine volcanoes. Of 51 seamounts, 15 are 

 more than 1 mile high, and every expedition crossing the province finds new 

 seamounts. Seven seamounts have been surveyed, and Jasper and Henderson 

 have been dredged. The volcanoes are typical isolated cones with steep sides 

 and pointed tops. None are guyots with wide flat tops. Henderson Seamount 

 appears to have a flat top at 220 fathoms, but the area is only half a square 

 mile, and this is too small to demonstrate that a sharp peak has been planed 

 off. However, hundreds of pounds of coarse, basaltic gravel were dredged from 

 the top of this seamount, and a large fraction of subrounded and subangular 

 pebbles and cobbles suggests wear in the surf zone. 



Contrasting strongly with the smooth floor of the Deep Plain to the north, 

 the Baja California Seamount province is irregular. Recorded echo soundings 

 show thousands of miles of jagged bottom in which the irregularities have a 

 relief of 100-200 fathoms. The relief must be tectonic, but it is uncertain 

 whether it is caused by vulcanism or faulting. The lack of a smooth blanket 

 of sediment suggests either that the topography was formed relatively recently 

 or that the rate of sedimentation is unusually slow. No large rivers carry sedi- 

 ment from southern California and Baja California into the ocean, and even the 

 limited amount introduced by intermittent small rivers is trapped in the basins 

 of the continental borderland or in the troughs off Baja California (Menard, 

 1955). 



Constitution of Deep-Sea Crust 



A seismic refraction survey by Raitt (1956) indicated that at a position 

 in the Baja California Deep-Sea Province due east of Sebastian Vizcaino 



Bay (Lat. 27°24'N, Long. 121°35'W) in a depth of 4176 meters of water, 

 the crust had the following velocity layers: 



Thickness, km 

 0.26 

 0.93 

 6.24 



Velocity, km/sec 



2.15 (Sediments) 



5.88 ± 0.23 (Volcanics?) 



6.96 ± 0.68 (Crust, gabbroic?) 



■'- 



8.41 ± 0.43 (Mantle) 



Mason uses similar figures in his analysis of magnetic profiles of the 

 Deep-Sea Plain. See subsequent pages and Fig. 32.17. 



Magnetic Intensity Surveys 



Magnetic intensity surveys and contour maps have now been made of 

 a large region off the western United States including a portion of the 

 Deep Plain province and the Murray and Mendocino fracture zones 

 (personal communication, H. W. Menard). The results are striking and 

 tectonically significant. 



Figure 32.14 is a sample of the magnetic intensity map and shows an 

 area 350-400 miles out from the shore along the Murray fracture zone. 

 The lines of equal magnetic intensity have been so adjusted that they 

 do not reflect the increase of the earth's magnetic field across the area. 

 The intensity highs and lows are in sharp zones about 15-25 miles wide 

 and extend conspicuously and rather regularly in a north-south direc- 

 tion. This pattern is dominant west of a less intense and more irregular 

 near-shore zone with a fabric to the north-northeast. Some of the strong 

 north-south magnetic features have been contoured for a length of 370 

 miles on the Deep Plain (Menard and Vacquier, 1958; Mason, 1958). 



Figure 32.15 shows the topography of the ocean bottom of the same 

 area as Fig. 32.14. It will be seen that the Murray fracture zone is fairly 

 narrow here and is reflected clearly in the magnetic intensity contours. It 

 may also be detected that the zone is one of horizontal offset of the 

 intensity pattern. This is brought out forcefully if an east-west profile 

 curve of the anomalies field is plotted both north of the fracture zone 

 and south of it. If the two profiles are then moved east or west they 

 match well but in only one position. This is taken to mean that the 





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