SUBSIDENCE OF THE SEA-FLOOR IN THE ATLANTIC AND PACIFIC OCEANS - REGIONAL AND LARGE-SCALE VARIATIONS

We present a detailed survey of seafloor subsidence over the entire Atlantic Ocean and in the Pacific Ocean. We examine the evolution of depth as age increases within narrow (200 km wide) tectonic corridors elongate in the direction of spreading. Over the part of the young seafloor where the depth-age(1/2) relationship is valid, we estimate the subsidence rate by a least-squares adjustment. In the three oceanic regions (West Atlantic, East Atlantic and Pacific) the subsidence rate shows regional (1000-2000 km) and basin-scale fluctuations of the order of 100-150 m/m.y.(1/2). We also note that in some regions subsidence rate and axial depth are correlated, with shallow ridges showing fast subsidence and deep ridges slow subsidence. On the basis of published models, we infer that regional variations in subsidence rate reflect temperature changes in the subaxial mantle of similar to 100 degrees C. Asymmetrical subsidence is observed in the Atlantic, especially in the South Atlantic. The age of departure from the depth-age(1/2) linear trend is highly variable, with values larger than 100 m.y. reported in several instances but very low values (20 m.y. or less) characterizing the central Pacific. Flattening predicted by the plate model occurs in some areas. In a number of cases however, quite distinct behaviour is observed, with a sudden depth increase where the depth-age(1/2) relationship breaks down, and a slow rise of the seafloor at older ages. Our results suggest that the subsidence behaviour of the seafloor is much more complex than global plate or half-space cooling models predict.