Geologic Structure of the Uppermost Oceanic Crust Created at Fast- to Intermediate-Rate Spreading Centers

Geological investigations of major fault scarps (“tectonic windows”) and DSDP/ODP Drill Holes provide direct views of the uppermost oceanic crust generated at fast- to intermediate-rate spreading centers. These areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing (“inward”) dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of “outward” dip. Widespread fracturing, faulting, and hydrothermal metamorphism accompanied magmatic construction. These geological relationships can be interpreted in terms of dramatic, asymmetrical, subaxial subsidence of upper crustal rock units that diminishes across the very narrow (few kilometers wide) zone of lava accumulation and dike intrusion at the ridge axis. This type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological complexity.