The results from a 53-km-long, wide-angle seismic profile across the rift valley of the Mid-Atlantic Ridge south of the Kane transform (near 23¿20'N, MARK area) provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid-ocean ridges. Anomalously low middle and lower-crustal P wave velocities beneath the neovolcanic Snake Pit ridge are consistent with elevated axial temperatures and with the presence of 4¿1% partial melt evenly distributed within the lower crust in preferentially oriented, elongated thin films. If the melt inclusions have larger aspect ratios, melt fractions can be up to 17¿3%. This and other geological observations suggest that the study area is presently in a magmatically active period. The igneous crust is anomalously thin beneath both flanks of the median valley (≤2.3--2.5 km). Thus the mantle rocks observed along the western rift valley wall at Pink Hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting. The crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous, with igneous crustal thickness variations of ≥2.2 km over horizontal distances of ~5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last ~2 m.y. Thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data. ¿ 2000 American Geophysical Union