Seismic stratigraphy of the oceanic Moho based on ophiolite models

Synthetic seismograms that model vertical-incidence seismic reflections from the oceanic Moho were calculated from detailed laterally varying velocity-depth models of two ophiolite complexes. The synthetic seismograms reproduce the rapid lateral variability of waveforms, amplitudes, and two-way travel times of reflections from the oceanic Moho that are observed in existing MCS data. The large variations observed in the synthetic seismograms were caused solely by horizontal changes in the geologic structure along the crust/mantle boundary because focusing or defocusing of seismic energy by the upper crustal units was not modeled. The petrologic contrast producing the strongest reflector from the oceanic crust/mantle transition is highly variable, so it is difficult to identify what the particular Moho reflections represent geologically, although with broad-band data, velocity gradients can be distinguished from laminated zones. More important, however, is our finding that the seismic-refraction oceanic Moho and the seismic-reflection oceanic Moho usually do not coincide and that either or both may cross major geologic contacts.