Seismic reflection surveys shot over oceanic crust are contaminated by source-generated coherent noise that limits the identification and interpretation of subbasement reflections. Coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface; both types of arrival may be enhanced by conventional common midpoint (CMP) stacking. Some turning arrivals also exhibit negative moveout in a CMP gather due to the increase of crustal velocity with depth. In unsedimented areas, coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout, with or without a subsequent zero-offset migration prestack, and suppressed, but turning arrivals will usually remain. Turning arrivals generated by linear features in the seafloor, such as fault scarps, appear on a stack section as nearly linear coherent arrivals, the apparent dip of which is related to the features' angle to the seismic line. After migration, these turning wave arrivals can extend close to the topography that produced them and continue to late times, leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut, but do not offset, the Moho. Careful identification of crustal reflections on a seismic profile along the MARK area rift valley indicates that the Snake Pit hydrothermal field marks the northern limit of identifiable crustal reflections, at least some of which appear to be related to magma injection. The recent extensive volcanism of the northern MARK area may well be a result of faulting caused by the strong northward growth of the southern spreading center, permitting melt to rise more easily to shallow crustal levels.¿ 1997 American Geophysical Union