The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid‐Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along‐axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along‐axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH‐1) has an unusually thick crustal root (excess thickness of 2–4 km and along‐axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid‐Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH‐1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

Abstract. In this paper we re-examine the relationship between seismically constrained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment-scale variations in axial depth, morphology, basalt geochemistry, and hydrothermal activity that have often been attributed to along-axis differences in the supply of magma to the mid-ocean ridge. Along >800 km of the fast spreading SEPR, good correlations exist between axial depth, ridge cross-sectional area, mantle Bouguer anomaly, and the MgO weight percent of basalts recovered from the rise axis. These correlations indicate along-axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of •-!00,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of seismic layer 2A, and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross-sectional area. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (seismic layer 2A) thickness. We hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (-10-100 years) than the longer-term variations in magma supply reflected in alongaxis changes in the shape and depth of the ridge axis.

A stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic Ž . crustal structure similar to that observed in seismic data from the East Pacific Rise EPR , in ocean drill holes, and in ophiolites. In this model the location of successive dike intrusion events relative to the ridge axis is determined by a Gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a Gaussian-shaped lava pile. We interpret wide-angle seismic reflections from the steep velocity gradient at the base of seismic layer 2A to be the extrusiversheeted dike contact. Seismic data from the northern and southern EPR place constraints on the on-axis Ž . Ž extrusive layer thickness 230 " 50 m , the distance over which the thickening of the extrusive layer occurs width of the . Ž . Ž . accretion zone s 1-3 km and its off-axis thickness 300-800 m . Ophiolites and ocean drill holes DSDP Hole 504B provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from Ž . extrusives to sheeted dikes ; 100-200 m . A simple stochastic emplacement model, where the lavas are described by one mean flow length, fits the thickening of the extrusive layer off-axis inferred from the deepening of seismic layer 2A, but the predicted transition from sheeted dikes to extrusives is too thick. In order to match the dimensions and flat-topped shape of the seismic layer 2A boundary as well as the thickness of the extrusive-sheeted dike transition, a bimodal distribution of Ž . lava flows is used. Short flows, confined within the axial summit caldera ASC , build up approximately half the extrusive volume. Occasional voluminous flows spill out of the ASC, or erupt outside of the ASC, and pond at a considerable distance off-axis to build up the remainder of the extrusive section. The upper part of the final extrusive section will be dominated by the off-axis flows, while the lower portions will be primarily composed of short flows erupted within the ASC. Magnetic Ž . transition widths predicted from the overlap of lavas ; 2 km in this model are similar to those measured in deep-tow studies. Assuming a smoothing function which acts over one seismic wavelength, the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the EPR. The ages of seafloor lavas in this model are younger than the tectonic spreading model ages by ; 30-70 kyr, in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the EPR.

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