ELEVATION AND SUBSIDENCE OF OCEANIC CRUST

A moving oceanic crustal plate is elevated where it is created at the traiting and it subsides as it grows older. The subsidence continues far beyond the obvious topographic boundaries of midocean ridges. The rate of subsidence averages 9 cm/103 yr for the first 10 million years; 3.3. cm/103 yr for the next 30 million years; and is estimated at 2 cm/103 yr for the next 30 million years. Even older crust in the Pacific subsided at a slower rate for an additional 25 million years. In the Atlantic, however, the rate was much faster during the same period. Subsidence is related to the interaction of mantle degassing, erosion, sedimentation, mantle counterflow, and sea floor spreading. Midplate rises" exist in all the major ocean basins. Many are broad elevations characterized by thick pelagic sediment overlying faulted and uplifted turbidities. They show no signs of intense vulcanism or sea floor spreading center phenomena. They probably mark the locus of small transient convection cells which act under rather than at the edges of large crustal plates. © 1969."