SECT. 2] THE MID -OCEANIC RIDGE 391 



floor. On either side the peaks of the rift mountains drop abruptly to depths 

 of 1600-1800 fm of the high-fractured plateau. These features are illustrated 

 in Fig. 2. 



B. Flank Provinces 



The flank provinces of the Mid- Atlantic Ridge have been arbitrarily divided 

 into several steps parallel to the axis (Fig. 3). Outward-facing scarps of varying 

 height and intermittent length form boundaries of these steps. Except for 

 the local smooth-floored intermontane basins, peaks of 200-400 fm form the 

 moderately rugged relief of the flank provinces. The smooth floors of the 

 intermontane basins result from sediment filling, and, in general, a series of 

 adjacent basin floors deepen progressively from the crest to the flank. 



The continuity along the ridge axis of the step provinces has not been firmly 

 established, perhaps because more detailed surveys are needed to evaluate 

 the eff"ects of fracture zones. 



Fracture zones were originally discovered in the floor of the northeast 

 Pacific. Studies of magnetic-anomaly patterns have disclosed large slip dis- 

 placements along these zones. In the Atlantic similar zones offset the Mid- 

 Atlantic Ridge. The magnitude of these displacements is judged by topo- 

 graphic discontinuities in the position of the axis of the ridge. These fracture 

 zones are predominantly east-west in the Atlantic as they are in the Pacific, 

 a fact of certain but unknown significance. Prominent fracture zones displace 

 the Mid-Atlantic Ridge at 30°N, 15°N, 11°N, 1°N, TS and 2^S. There are 

 indications of several additional fracture zones but data at present are in- 

 sufficient for complete confirmation. 



C. Seismicity 



Even before the establishment of seismograph stations, a study of seaquakes 

 (Rudolph, 1887) had indicated that the Mid- Atlantic Ridge is seismically 

 active. As the world net of seismograph stations became more complete, the 

 repeated observations of mid-oceanic earthquakes firmly established the 

 seismicity of the Mid-Atlantic and Mid-Indian Ocean Ridges (Tarns, 1931; 

 Rothe, 1954). However, recent morphological investigations cited above reveal 

 that although the epicenter belt in the North Atlantic is no more than 100 miles 

 in width, the Mid-Atlantic Ridge is 1000-1200 miles in width. When we con- 

 sider the probable inaccuracy of epicenter determination, it is clear that the 

 actual width of the epicenter belt is probably even narrower (Fig. 1). The 

 present accuracy of epicenter determinations in this area is about + 1° in the 

 most favorable areas, but the earlier epicenters are probably never located 

 better than + 1°. A careful study of each profile across the crest of the Mid- 

 Atlantic Ridge reveals that the axis of the epicenter belt coincides with 

 the axis of the rift valley. The plotted positions of most epicenters define 

 a belt which includes the rift valley and whose width represents the probable 

 error in epicenter determination. It therefore seems most probable that the 



