Synthetic seismograms for the Samail ophiolite, computed using the full reflectivity technique, are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust. If so, then this ophiolite complex can be compared with the older (45 m.y.) Bay of Islands (BOI) ophiolite to develop an aging model for oceanic crust. Synthetics for this young emplacement age (5-8 m.y.) ophiolite are compared with ocean bottom hydrophone data from 0.5-and 4.5-m.y.-age crust obtained during the Rivera Ocean Seismic Experiment. Second-arrival phases from reflected refractions from one ophiolite velocity-depth model and data from 4.5-m.y.-age crust are in excellent agreement; this agreement suggests that the Samail ophiolite is a good working model for young oceanic crust. Earlier work has shown that the BOI ophiolite is a good prototype for mature oceanic crust. The BOI ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited. We use the petrologic and geochemical properties of the Samail and BOI ophiolites to develop a two-stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials. refractions.' The crustal models used are based on velocitydepth functions from both Manghnani et al. [1981] and Christensen and Sinewing [1981, and personal communication, 1982] for the Ibra and northern section of the ophiolite complex, respectively. The seismograms are compared with ocean bottom seismometer (OBS) refraction data from phase I Of the Rivera Ocean Seismic Experiment (ROSE) that samples oceanic crust of similar (4.5 m.y.) and younger (0.5 m.y.) age [see Ewing, 1979].Our results sugget that within the limits of the seismic velocity and density samples for the Samail ophiolite, synthetic seismograms can be produced which offer good approximations to seismic refraction observations from young oceanic crust. Some variation between the synthetics and the observations occurs in the second arrivals. Our results suggest that this problem arises because reflected refractions are particularly sensitive to the P velocity gradient in the uppermost crust. Unfortunately, this portion of the crust and the ophiolite suites is poorly constrained. However, the agreement between synthetics for the Samail ophiolite and seismic refraction observations from young crust is impressive; the differences noted above are well within the expected variability of oceanic crustal structure.We have investigated the BOI ophiolite complex of Newfoundland in a similar manner [Kempnet and Gettrust, this issue] and show that synthetic seismograms for the BOI ophiolite fit marine crustal refraction data from older (60 m.y.) ocean basin crust. That investigation is based on a comparison of seismic refraction observations from the northeast Pacific with the BOI ophiolite complex. These similarities between seismic observations sampling young and mature oceanic crust with synthetic seismograms for ophiolite suites of similar ages suggest that it is rea...

A series of synthetic seismograms based on the Bay of Islands ophiolite data are compared with ocean bottom seismometer seismic refraction observations which sample 60-m.y. crust in the northeastern Pacific. The general agreement between the synthetics for the ophiolite and the observations support the idea that some ophiolites are representative of normal ocean crust. This support extends to similarities in complex second arrivals in the synthetics and observations which often are not used in the interpretation of marine seismic data. The use of the ophiolite model also allows us to show how the interpretation of seismic data is affected by experimental design and interpretation procedures; as the sampling density increases and as more phases in the observations are accounted for, the velocity-depth model more closely approaches that found for the ophiolite. The results of this modeling support sharper P velocity transitions within the crust than are found in young (<12-m.y. age) oceanic crust. The need for a thin (approximately 1 km thick) P low-velocity zone at the base of the crust is interpreted as support for serpentinization at the crust-mantle boundary; serpentinizat, ion of the ultramafic component in the Bay of Islands ophiolite, therefore, may include a small preemplacement component. ß ß ß ß ß ß

Analyses of slant stacks of synthetic record sections generated from velocity models for ophiolites by the reflectivity method show that even 90‐km‐long refraction lines having a 10‐Hz source and a 1‐km intertrace spacing barely distinguish between fine‐scale crustal structures which are of current interest. Such data are sufficient, however, to distinguish between young and old oceanic crust. Tripling the spatial sampling period degrades the quality of the synthetic slant stacks for this temporal frequency content. A major implication of the study is that most currently available field data, being sparser and poorer in quality (due to both noise and instrument responses), offer little hope of resolving thin low‐velocity layers or small changes in velocity gradients by using newly proposed inversion techniques based on slant stacking. Last, a simple, first‐order correction to a downward continuation algorithm, namely, the inclusion of the phase shift caused by postcritical reflection at each ray parameter, is proposed.

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