PROVINCES OF THE MID-ATLANTIC RIDGE

The Mid-Atlantic Ridge was subdivided by Tolstoy and Ewing (1949) and Tolstoy (1951) into (a) "the central backbone or main range which is shallower than 1600 fathoms," and (b) "the flanks" or "the terraced zone" "between the 1600- and 2500-fathom isobaths." In this paper we use a similar but somewhat differently defined system by dividing the provinces of the Mid-Atlantic Ridge into two categories: (1) the crest provinces, and (2) the flank provinces.

Crest provinces.—The provinces of the crest of the Mid-Atlantic Ridge consist of (1) the Rift Valley (or Valleys); (2) Rift Mountains; and (3) High Fractured Plateau (Fig. 43). The Azores Plateau, which forms part of the crest, presents additional problems and is discussed separately.

RIFT VALLEY: The most striking feature on an average profile across the Mid-Atlantic Ridge is a deep notch or cleft in the crest of the ridge. In a small percentage of the sounding profiles two or three such valleys are present, and on a few profiles no notable depressions are observed. On an average profile the floor of the valley lies at about 2000 fathoms, while the adjacent peaks average about 1000 fathoms below the sea surface. The width of the valley between the crests of the adjacent peaks ranges between 15 and 30 miles, and the depth of the valley floor beneath the highest adjacent peak ranges from 700 to 2100 fathoms. The width of the valley 500 fathoms above its floor ranges from 5 to 22 miles. The range in observed depths of the valley is 1150 to 2850 fathoms in the area of the physiographic diagram. The adjacent peaks range from 500 to 1300 fathoms within the same area (excluding the area near the Azores) (Fig. 47).

Twenty-six crossings of the Rift Valley are shown in Figure 45. The profiles can be divided into three groups: (1) single well-developed rift valley; (2) several well-developed deep valleys; (3) no particularly deep central valley. Most of the profiles (20) fall into the first class; the second class is represented by 5, and only 1 falls in the third class.

The topography of the floor of the rift is rough. In no instance has a flat floor been observed. Where the valley is widest mountains a few hundred fathoms high protrude from its floor.

RIFT MOUNTAINS: The steep walls flanking the rift each form one side of a large rough-sided block. They might be considered as tilted blocks whose facing slopes form the Rift Valley. The back or outer slope of the Rift-Mountains Province is generally broken into mountains as much as 500 fathoms high and 10 miles wide (Fig. 44). The lateral limit of the Rift-Mountains Province is reached when the average slope of the sea floor flattens markedly. Because of the high local relief it is sometimes difficult to pick the boundary of the Rift Mountains, but in almost all recorded profiles the approximate position of the boundary is unmistakable.

HIGH FRACTURED PLATEAU: The High Fractured Plateau is adjacent to the Rift Mountains on either side of the ridge (Fig. 42). The local relief is about 400 fathoms from peak to adjacent valley, and the distance from peak to peak ranges from 8 to 20 miles. In contrast to the adjacent flank provinces there are no filled intermontane valleys, and the valleys are deeper and narrower than in the adjacent Upper Step Province. Within the limits of the physiographic diagram, the average depth of the High Fractured Plateau ranges from 1500 to 1900 fathoms.

Flank provinces.—Between the outer margin of the High Fractured Plateau provinces and the level of the ocean-basin floor lies a succession of parallel provinces, known as the Upper Step, the Middle Step, and the Lower Step. The limits of these provinces are the least well defined of all the provinces described in this paper. The flanks of the ridge are characterized by rough topography (Pl. 18). Peaks of more than 200 fathoms high occur at a frequency of about 15 per 100 miles. Some of the valleys between peaks are smooth, particularly in the provinces south of the Azores Plateau. The flanks of the Mid-Atlantic Ridge are broken by scarps which seem to persist for relatively long distances parallel to the trend of the crest (Pl. 20). These scarps or scarp zones break the ridge into a succession of units here called steps (Fig. 46). An alternative solution also seriously considered by the writers is that the steps might more correctly be considered as a series of tilted blocks which could be referred to as ramps. The difference between the two solutions can be appreciated by inspecting Figures 42 and 46.

The writers must emphasize that the term "terraces" of Tolstoy is in no sense the same as the term "step" used in this paper. Tolstoy defined his terraces as "a succession of smooth shelves, each from 1 to 50 miles in width," which occupy a zone "200-300 miles" wide. Features fitting this definition are called "intermontane basins" in this paper, following a suggestion made by Heezen et al. (1951).

The location of smooth-floored intermontane basins is shown on Plate 20. They are found only in the area extending about 8° southwest of the Azores. Small arrows indicate the slope of the smooth floors. In general all the valleys slope away from the crest of the ridge. Steps, on the contrary, are a succession of average levels separated by scarps or scarp zones and in general are not smooth except that a few basins may be filled. However, this filling is limited to a small area south of the Azores.

The flanks are divided into three steps: upper, middle, and lower. The division of anything into three parts is suspicious, whether it be a geologic period or a physiographic region. Such divisions usually are later replaced as more information is obtained. This is probably true of the three steps. We are more confident of the significance of the boundaries shown on the province chart than of the uniqueness of the enclosed areas, because each boundary marks a major scarp or scarp zone. The steps are defined in part in any limited area on the basis of their mean depth. In general the Upper Step ranges from 1650 to 2300, the Middle Step from 2250 to 2500, and the Lower Step from 2350 to 2800 fathoms. Just as the maximum depth of the ridge and the axis of maximum depth of the basin vary with distance along the ridge (or with latitude), so the steps vary in depth and width (Fig. 47).

Azores Plateau.—The Azores Plateau is an area of 52,000 square miles of sea floor, surrounding the Azores Islands, where the depth is less than 1000 fathoms. The Azores Islands are oriented south-southeast-north-northwest along a topographic trend which strikes off toward the Straits of Gibraltar. This topographic connection between the Azores Plateau and the southern Iberian Peninsula has been called the Azores-Gibraltar Ridge, which the present writers consider as a poorly developed mid-oceanic ridge of the same general class as the Mid-Atlantic Ridge. The Azores Plateau itself merges with the Rift Mountains of the Mid-Atlantic Ridge. The sea-floor topographic trends of the eastern part of the plateau are parallel to the known tectonic and volcanic trends of the Azores Islands (Agostinho, 1937). In the western part of the Azores Plateau trends are north-south parallel to the main trends of the Mid-Atlantic Ridge. Although both Wüst (1940a) and Tolstoy (1951) have published contour charts of the Azores Plateau, it remains largely a mystery whether the trends of the eastern Azores Plateau cross or join the axial trends of the Mid-Atlantic Ridge. The Azores Plateau or bulge is generally considered as a highly fractured tectonic uplift in which vulcanism has played a comparatively small part (Cloos, 1939).

Azores-Gibraltar Ridge.—An ill-defined irregular ridge runs from the eastern end of the Azores Plateau to the Straits of Gibraltar. Largely on the basis of its seismicity we infer that this ridge is structurally and topographically similar to the Mid-Atlantic Ridge. The few existing topographic profiles across this feature suggest that the earthquake belt is associated with a rift valley of the same general type as the central Rift Valley of the Mid-Atlantic Ridge. Depths in this rift appear to reach 2300-2800 fathoms, and the depth of the tops of the adjoining mountains range from 1600 to 2000 fathoms. The flank provinces are even less well developed.

Atlantis-Plato-Cruiser-Great Meteor Seamount Chain.—South of the Azores a chain of great seamounts branches off from the High Fractured Plateau and crosses the Upper and Middle steps in a nearly north-south direction. These seamounts—Atlantis, Plato, Cruiser, and Great Meteor—in general have broad, nearly flat summits at depths of 100-250 fathoms. The largest one, Great Meteor, was discovered by workers on the Meteor in 1937. This seamount, 60 miles across at its base, rises majestically more than 2600 fathoms above the floor of the ocean. Sands and calcareous rocks have been dredged from the summits, and Tertiary sediments have been obtained from the flanks of the seamounts. Photographs of the tops and of the flanks to a depth of 1600 fathoms show ripple marks. This group is described in a paper by Heezen, Ewing, Ericson, and Bentley (in press).