Crustal and upper mantle seismic structure beneath the rift mountains and across a nontransform offset at the Mid-Atlantic Ridge (35 degrees N)

We present new results on the crustal and upper mantle structure beneath th e rift mountains along two segments of the Mid-Atlantic Ridge and across a nontransform offset (NTO). Our results were obtained from a combination of forward modeling and two-dimensional tomographic inversion of wide-angle se ismic refraction data and gravity modeling. The study area includes two seg ments: OH-1 between the Oceanographer fracture zone and the NTO-1 at 34 deg rees 35'N and OH-2 between NTO-1 and the NTO at: 34 degrees 10'N. The cente r of OH-1 is characterized by anomalously thick crust (similar to 8 km) wit h a thick Moho transition zone with V-p = 7.2-7.6 km/s. This transition zon e, coincident with a gravity low,:is probably composed of gabbro sills alte rnating with dunites, as observed in some ophiolites. OH-1 has larger along -axis crustal thickness variations than OH-2, but average crustal thickness es are similar (6.0 +/- 1.2 km at OH-1, 6.1 +/- 0.7 at OH-2). Thus we do no t find significant differences in magma supply between these segments, in c ontrast to what has been inferred-from morphological and gravity studies. A t both segments the shoaling of the Moho is more rapid at the inside than a t the outside corners, consistent with models in which the inside-corner cr ust:isl tectonically modified. The structural differences between inside- a nd outside-corner crust are more apparent at OH-2, suggesting that the extr usive layer is thinner at the inside corner of OH-2 than at the inside corn er of OH-1, probably due to differences in axial morphology and along-axis magma transport. NTO-1 is characterized by a nearly constant velocity gradi ent within the upper 5 km and low upper mantle velocities (7.4-7.8 km/s). T he anomalous structure beneath NTO-1 is interpreted as fractured mafic crus t. The P wave velocities:and densities required to match the gravity data s uggest that serpentinites are common beneath the NTO-1 and possibly beneath the inside corners. Serpentinization could be as much as 40% at similar to 3.8 km below seafloor and probably does not occur at subseafloor depths gr eater than similar to 6.2 km at the NTO-1. Our results indicate that in a s low spreading environment where magmatism and tectonism are equally importa nt, the seismic Moho cannot be correlated with an unique geological structu re. At the center of a: segment the seismic Moho may represent the lower bo undary of an interlayered grabbro-dunite transition zone, while beneath the inside corner and NTO where the crust is thinner, it may correspond to an alteration front.