Abstract

A deep-towed instrument survey was made of the floor of the rift valley in the Mid-Atlantic Ridge near lat 37 °N (FAMOUS project). Near-bottom bathymetry, side-looking sonar (SLS), and wide-angle photography are among the data brought to bear on the definition of the American-African plate boundary and the intrusion-extrusion zone between these plates. The valley floor is 1 to 4 km wide and contains eight elongate shield volcanoes that occupy 40% of the axis. Where the volcanoes — called central highs — are absent, there are sometimes shallow depressions — called central lows. Photographic data show three components to the near-bottom environment: massive pillow lavas, well-sorted rock fragments, and sediment cover. Sediment is ubiquitous in all the photos but is scarcest along the axis of the valley floor. Pillows appear the freshest on the central highs except for one locality found at the extreme east side of the valley floor. The rock fragments are evidently pillow joint blocks and are associated with spalling off steep flow fronts and fracturing from faulting. Less well sorted fragments are associated with the large-throw step faults at the edges of the valley floor. Pillow elongation approaches 10:1 and possibly indicates flow directions both across and along contour. Numerous fissures and small-throw step faults were also seen. The SLS records show that the smoothest areas are the well-sedimented regions off the central highs that are yet to be broken by faulting at the floor edges. More than 700 faults were mapped in the valley-floor region. They trend parallel to valley strike (N17 °E) and are generally absent within 500 m of the axis. Fault density is highest at the east inner floor edge, reaching 35/km2. Fissures are present in photos but are not readily recognized with SLS data. Flow edges or ridges also align near N17 °E, but some trend across strike. SLS point targets 25 m high and about 50 by 50 m represent volcanic conelets (haystacks). These align in groups and often are associated with faults. They are not detected away from the axis.

The floor is intensely fractured with fissures and small-throw vertical faults as are parts of the inner walls; this is evidence that the region of tensional faulting is at least 5 km wide. The absence of faults near the axis due to volcanic burial suggests that the extrusion zone is as much as 1 km wide. The construction process in the floor is virtually entirely volcanic, but much of this relief is obscured by the intensive shear (normal) faulting found beyond the inner walls. Many geomorphic features of the floor have direct analogy with those in the Icelandic rift valley and the East Pacific Rise, although there are differences in scale.

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