Crustal thickness and structure along three contrasting spreading segmentsof the Mid-Atlantic Ridge, 33.5 degrees-35 degrees N

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrast ing morphologies and gravity signatures are determined from a seismic refra ction study. These segments lie between the Oceanographer and Hayes transfo rms and from north to south have progressively deeper axial valleys with le ss along-axis relief and smaller mantle Bouguer gravity lows. Major variati ons in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the se gment 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 i s similar for each segment (5.6+/-0.4, 5.7+/-0.4 and 5.1+/-0.3 km). Near th e 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 e nds, the thickness of the upper crust is relatively uniform along axis (sim ilar to 3 km), although its internal structure is laterally heterogeneous ( velocity anomalies of +/-0.6 km s(-1) over distances of 5 km), possibly rel ated to the presence of discrete volcanic centers. The along-axis crustal t hickness variations are primarily accommodated in the lower crust. The cent er of the northern segment (OH-1) has an unusually thick crustal root (exce ss thickness of 2-4 km and along-axis extent of 12 km). Our results are con sistent with an enhanced supply of melt from the mantle to the segment cent ers and redistribution of magma along axis at shallow crustal levels by lat eral dike injection. Along this portion of the Mid-Atlantic Ridge, our resu lts 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 segmen t length is similar for all three segments despite their different morpholo gies and gravity signatures. This argues against excess melting of the mant le beneath segment OH-1. Instead, we suggest that the thickened crust at th e segment center is a result of focusing of melt, possibly due to the influ ence of the thermal structure of the Oceanographer fracture zone on melt mi gration in the mantle.