Use of Geophysical Logs for Quantitative Determination of Fracturing, Alteration, and Lithology in the Upper Oceanic Crust

The upper layers of the oceanic crust are characterized by large changes in geophysical properties both sub-areally and with submarine depth which belie their volcanic modes of origin. Feeder dikes connecting surface-flow basalts with the magma chamber below, produce new seafloor with similar volcanic structure worldwide within the spreading systems active on all mid-ocean ridges. All ocean crust consists of pillow basalts overlying sheeted dikes which in turn rest upon gabbroes which are the frozen remains of old magma chambers. Even the chemistry of the oceanic crust is essentially the same from ocean to ocean. It is the rare ocean basalt which differs from MORB, or mid-ocean ridge basalt composition. Yet geophysical properties of Mid-Atlantic Ridge flank crust can be very different from East Pacific Rise crust. For example, the seismic velocity in the very uppermost crust is often slower in the Atlantic than in the Pacific (Houtz and Ewing 1976). The most highly altered metamorphic rocks are invariably found in the Atlantic.