ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN 



151 



easterly, probably forms the south boundary of the Outer Ridge, described 

 above by Hersey et al., but if so, it does not show in section H-H'. The 

 Blake Plateau south of the fault, at any rate, stands 15,000^30,000 feet 

 above the block on the north, in reference to the base of the interpreted 

 sedimentaries. The deeply filled block extends northward at least to 

 32° N. Lat. 



Regarding the origin of the Outer Ridge, Hersey et al. point out that 

 two velocity layers appear there that are unusual, namely the 5.20-5.67- 

 km/sec layer and the 7.21-7.73-km/sec layer. The 5.2-km/sec layer is 

 interposed between the sedimentary layers and the basaltic "oceanic 

 layer" (6.5 =*= km/sec), and the 7.5-km/sec layer interposed between the 

 oceanic layer and the mantle. Since profile D-D' shows a 5.22-5.52- 

 km/sec layer under the Blake Plateau the rock represented by this 

 velocity range is probably not unique to the Outer Ridge. The 7.5-km/sec 

 layer, however, seems more restricted to the Ridge, but it, nevertheless, is 

 known to extend as far north as the northern end of profile H-H'. 



The 5.2-km/sec layer is regarded as a mass of extruded volcanic mate- 

 rial, lighter and more porous than the basaltic "oceanic crust" layer, and 

 the 7.5-km/sec layer is taken to be a mixture of mantle rock with the 

 oceanic crust, probably by intrusion of peridotitic magma into basalt, in 

 the manner postulated for the Mid-Atlantic Ridge (Fig. 10.13). 



Hersey et al. (1959) speculate that the ultrabasic intrusions fed the 

 volcanic extrusions, then at the surface, and that the two are comple- 

 mentary. Another theory might be one in which basalt is formed by 

 partial melting of the mantle, with the basalt rising to concentrate in mesh 

 fashion in the upper part of the mantle. This basalt could then rise in 

 fissures and vents through the oceanic crust to eruption at the surface. 

 See Chapter 33 on igneous rock provinces. 



Mid-Atlantic Ridge 



Topography. The Mid-Atlantic Ridge is a broad arch or swell that 

 occupies approximately the center third of the ocean (Figs. 10.11 and 

 10.12). The higher and central part is less than 1600 fathoms below sea 

 level, and the flanks fall between 1600 and 2500 fathoms. The Ridge is 

 very rough as the profiles indicate, and the most striking feature is a 

 deep notch or cleft in the crest of the arch, called the Rift Valley. On an 



average profile the floor of the valley lies at about 20CK) fathoms below sea 

 level, whereas the adjacent peaks average about 1000 fathoms. The re- 

 lief from floor to adjacent peaks ranges from 700 to 2100 fathoms. The 

 width of the valley between crests of the adjacent peaks ranges between 

 15 and 30 miles; at an elevation of 500 fathoms above its floor the width 

 is from 5 to 22 miles (Heezen et al., 1959). 



On either side of the Rift Valley are terranes of sharp and strong relief 

 called the Rift Mountains. Immediately adjacent to the central Rift 

 Valley are the High Fractured Plateaus with local relief of 400 fathoms 

 and ranges 8 to 20 miles apart. Flanking the High Fractured Plateaus 

 is a succession of provinces known as the Upper Step, Middle Step, and 

 the Lower Step. The topography here likewise is rough with local relief 

 of 200 fathoms. Peaks over 200 fathoms high occur at about the fre- 

 quency of 7 per each 100 miles. The steps appear to be separated from 

 each other by scarps of considerable length. 



Seismicity. The High Fractured Plateaus and Rift Valley make up a 

 zone of considerable seismicity. See Fig. 10.16. Another zone extends from 

 the Rift Valley through the Azores eastward to Gibraltar. 



Sediments. Photos taken on the sides of seamounts in the Rift Moun- 

 tains show scour and ripple marks indicating deep-ocean currents. Cores 

 taken in intermontane basins show interlayering as turbidity current 

 deposits. 



Rocks. The lithology of the Mid-Atlanic Ridge is known from three 

 sources: (1) rocks dredged from the sea floor, (2) detrital rock frag- 

 ments found in sediment cores, and (3) rocks exposed on the islands of 

 the Ridge. These all point to olivine gabbro, serpentine, basalt, and dia- 

 base as the predominating rock types. One limestone sample probably 

 of Tertiary age was collected from the Rift Valley at about 30° N. Lat. 

 (Heezen et al, 1959). 



Crustal Structure. Seismic refraction records have been obtained in 

 about twenty places on the Mid-Atlantic Ridge, and the following layer- 

 ing is reported (Heezen et al, 1959). See Fig. 10.13. 



. . . the average crustal structure of the crest provinces and Upper Step 

 consists of 0.4 km of low-velocity sediment and 2.8 km of rock with a velocity 

 of 5.1 km/sec. overlying a substratum in which the velocity is 7.3 km sec. 

 The thickness of the layer of low-velocity sediment varies considerably From 



