NEW VIEWS OF SEAFLOOR SPREADING FROM THE GEOLOGY OF OPHIOLITES AND OCEANIC CRUST: CONTINUALLY COLLAPSING CALDERAS TO CORE COMPLEXES

The current view of oceanic crust and upper mantle derives from an amalgamation of observations and ideas from marine geology and geophysics and field investigations of ophiolite complexes over the past 30 years. This view is illustrated in nearly all modern earth science textbooks and many research publications with a now-familiar layered sequence of rock units: residual upper mantle peridotites, mafic and ultramafic plutonic rocks, and upper crustal sheeted dike complex and basaltic lavas. Despite widespread acceptance of this interpretation, significant uncertainties remain because of ambiguities in geochemical correlations with spreading environment, correlations between seismic data and rock units, reconstructions of ophiolite geology, and different scales of observation. Recently, geological investigations of the oceanic crust demonstrate very important variations on this highly generalized structure and by inference variations on spreading processes. Integrated studies with submersibles, remotely operated vehicles, and side-scan sonar take advantage of natural cross sections of the oceanic crust ("tectonic windows") at major tectonic escarpments. Crust generated at fast- to intermediate-spreading ridges (>30 m/yr half-rate) appears to have the expected layered sequence but with internal structures that were probably generated by continual caldera collapse above a more or less continuous magma chamber. Important elements include inward-dipping lava flows and outward-dipping dikes in the upper crustal units. Underlying gabbroic rocks that formed in subaxial magma chambers are poorly exposed. Crust from slow-spreading ridges is highly variable and ranges from the expected layered sequence to highly dismembered assemblages. Spreading much like that of fast-spreading ridges occurs where the "magma budget" is relatively high. At low magma budgets faulting dominates and near-axial crust and upper mantle are tectonically stretched and thinned in the context of a sputtering magma supply. Oceanic core complexes with low-angle detachment faults occur over a range of spreading rates and magma supplies. Understanding the evolution of these diverse oceanic crustal structures will require continued investigations of key features in both ophiolites and in contemporary oceanic crust.