Locating the spreading axis along 80 km of the Mid-Atlantic Ridge south ofthe Atlantis Transform

The zone of active eruptive fissuring (the "spreading axis") is investigate d at an accretionary segment of the Mid-Atlantic Ridge. High-resolution sid e-scan images are used to produce a detailed geologic map of the median val ley floor of Segment 18, located immediately south of the Atlantis Transfor m (30 degrees N). The spreading axis is defined by distinguishing between p rimary eruptive vents (seamounts and hummocky ridges fed by underlying dike s) and secondary vents (seamounts and terraces fed from lava tubes or chann els) and is found to be about 2 km wide or less along the entire segment. I t is impossible to narrow the width further with the data we have in hand. For comparison, at faster spreading ridges such as the East Pacific Rise, t he spreading axis is generally equated to the width of the axial summit tro ugh, typically, 50-100 m wide. Within the Brunhes anomaly, the high-resolut ion structure of the central anomaly magnetic high (CAMH) obtained from sev eral cross-axis, deep-tow tracks is used to define regions of recent, major volcanism. The locations of the CAMH peaks generally conform to our pick o f the spreading axis. In some places the peaks are located several kilomete rs away from the spreading axis, possibly indicating that large volumes of lava have been deposited there. Our interpretation of many volcanic structu res as secondary vents at Segment 18 leads to the conclusion that lava chan nels and tubes commonly develop to transport lava to these off-axis deposit ion sites.