Random sequences of lithologies exposed on the Mid-Atlantic Ridge

Magmatic, tectonic, and sedimentary processes at slow spreading ridges are known to create complex distributions of rock types at the surface that do not reflect the classical model for oceanic crust with horizontally stratif ied lithologic layers comprising extrusives, sheeted dikes, and gabbro over lying mantle rocks. A database of seafloor lithology observed in > 100 subm ersible dives is analyzed in order to characterize this complexity, As a wa y of summarizing the relations between units exposed on rock slopes, the tr ansitions between different lithologies are counted to create transition pr obability tables, which represent the relative likelihood of passing betwee n different rock types. They reveal that extrusives exposed on Atlantic roc k slopes crop out above dikes, serpentinite, and gabbro, in order of decrea sing, though with similar, likelihood. Tables derived from the occurrences of the different rock types in dredge hauls are similar to the tables deriv ed from dive observations, so the dredges record a similar heterogeneity. I t is suggested that slope failure of gabbroic and ultramafic escarpments, i mplied by sedimentary breccias in off-axis drill sites, may contribute to t he heterogeneity, in addition to the magmatic and tectonic processes that h ave been discussed extensively by previous workers. Further analysis of the tables reveals that the net effect of tectonic, magmatic, and sedimentary processes is to produce a distribution of rock types that is indistinguisha ble from a random distribution. This result is derived from the immediate t ransitions between units and therefore applies to the fine-scale structure. If oceanic crust is also heterogeneous on a larger scale, however, it coul d have more general implications for resolving different crustal models. Fo r example, the presence of a sheeted dike layer is critical evidence for co ntinuous seafloor spreading, but its presence or absence in any one locatio n may be an artifact of the heterogeneity or discontinuous exposure. Method s will therefore be needed to characterize large-scale heterogeneity so tha t the significance of these large-scale observations can be assessed.