Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area):: Implications for crustal accretion processes at slow spreading ridges

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 degrees 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 (less than or equal to2.3-2.5 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 greater than or equal to2.2 km over horizontal distances of similar to5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last similar to2 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.