OPHIOLITES AND THE INTERPRETATION OF MARINE GEOPHYSICAL DATA: TOWARDS MORE COMPLICATED MODELS

Ophiolite studies have facilitated the interpretation of geophysical data collected over the world's oceans. The "ophiolite model" is the standard against which models for seismic structure are tested. More recently, there has been a divergence between the models found in geophysical studies and the increasingly complex models that are derived from ophiolites. Ocean crustal structure, as revealed from geophysical studies, is most striking for its uniformity. This is particularly true on intermediate to fast spreading ridges. Structural uniformity is surprising, because variations in the crust forming processes ought to lead to changes in crustal parameters. For example, ridges that exhibit topographic highs would be predicted to produce thicker crust than those with rift valleys or topographic lows. This is because the high ridges are generally associated with robust magmatic upwelling. Such a prediction is consistent with some ophiolite models, but is not consistent with geophysical observations of intermediate to fast spreading ridges. The puzzling uniformity of the geophysical models may result for several reasons. Crustal variations may be too small to be resolved geophysically. Alternatively, perhaps the variations are smeared out by processes that occur as the crust ages. We know that the formation of abyssal hills leads to faulting that may change the porosity, and hence the seismic velocities and density in the crust. Finally, perhaps the morphological configuration of a given ridge axis is ephemeral, and over time the ridge will produce, on average, a typical crustal structure. In any event, caution must be used when applying the simple ophiolite model. We have accumulated data from intermediate to fast spreading ridges with a variety of morphologies. The mature crusts of each are similar in their seismic and density structure. We observe age-related changes, such as changes in upper crustal velocities and thickness that can explain some of the uniformity. However, it appears that the lack of correspondence between seafloor morphology and crustal structure cannot be explained by aging processes alone. Differences in mantle properties may be present, or new models for ridge processes must be invoked. These may require changes in how the ophiolite model is applied to the oceanic crust.