The zone of active eruptive fissuring (the spreading axis) is investigated at an accretionary segment of the Mid-Atlantic Ridge. High-resolution side-scan images are used to produce a detailed geologic map of the median valley floor of Segment 18, located immediately south of the Atlantis Transform (30 ¿N). The spreading axis is defined by distinguishing between primary eruptive vents (seamounts and hummocky ridges fed by underlying dikes) and secondary vents (seamounts and terraces fed from lava tubes or channels) and is found to be about 2 km wide or less along the entire segment. It 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, the spreading axis is generally equated to the width of the axial summit trough, typically, 50--100 m wide. Within the Brunhes anomaly, the high-resolution structure of the central anomaly magnetic high (CAMH) obtained from several 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 of the spreading axis. In some places the peaks are located several kilometers away from the spreading axis, possibly indicating that large volumes of lava have been deposited there. Our interpretation of many volcanic structures as secondary vents at Segment 18 leads to the conclusion that lava channels and tubes commonly develop to transport lava to these off-axis deposition sites. ¿ 1999 American Geophysical Union