Tectonic Corridors in the South-Atlantic - Evidence for Long-Lived Midocean Ridge Segmentation

The South Atlantic is investigated for evidence of long-lived (tens of millions of years), tectonic segmentation of the mid-ocean ridge by examining a variety of geophysical parameters. These parameters include: subsidence rate of the oceanic crust (east flank, west flank, average, asymmetry), zero-age depth, geoid height decrease with age (geoid Tate), and the residual geoid field. The variability in these parameters along-strike of the mid-ocean ridge is typically systematic. The ridge can be subdivided into segments (flowline corridors in plan view) within which characteristics are uniform. Eight tectonic corridors have been determined in the area between the Ascension Fracture Zone and the Falkland-Agulhas Fracture Zone, which serve to define a large scale ridge segmentation on the order of hundreds of kilometers. This segmentation is in addition to the well-known small-scale, near-axis segmentation. Near-axis geochemical data show boundaries that are generally consistent with the tectonic segmentation. Many of the primary geophysical observations are found to be internally inconsistent with simple thermal conduction models for oceanic crust. Such observations include asymmetric crustal subsidence, large along-strike variations in subsidence rate and geoid rate, the lack of a systematic correlation between subsidence rate and geoid rate, and the abrupt nature of some of the inferred tectonic boundaries. Thus, the data indicate that in addition to uniform lithospheric cooling, other factors such as variable asthenospheric temperatures are important contributors to the creation and subsequent modification of the oceanic crust and possibly the oceanic lithosphere.