Near-bottom geophysical observations made over the median valley of the Mid-Atlantic Ridge near lat 37°N show that the median valley can be divided into four provinces: the outer walls, terraces, inner walls, and floor. Block faulting along normal faults with as much as 600 m throw is the dominant mechanism for creating median-valley relief, particularly at the inner and outer walls. Most of these faults dip toward the median-valley axis an average of 45°. Many of the block tops slope 3° to 10° away from the valley axis. Some of the major fault scarps are linear for more than 20 km. Large-scale block faulting occurs less than 1 km from the axis of spreading. Inversion of near-bottom magnetic anomalies suggests that the principal zone of intrusion may be no wider than 1 to 2 km and is along the valley axis. This spreading center is marked by either a central ridge or central depression, which suggests either discrete magma sources or discrete taps of a linear source beneath the median-valley axis. The central ridge is a volcanic construction, although faulting and (or) volcano-tectonic uplift may contribute to its relief.

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