SPATIAL AND TEMPORAL VARIATIONS OF SUBSIDENCE OF THE EAST PACIFIC RISE (0-23-DEGREES-S)

The distributions of crustal depths as a function of age have been ana lysed for the southeast Pacific region, along the East Pacific Rise, b etween the Equator and the Easter microplate (23 degrees S), Using age data and a new compilation of bathymetric data, subsidence rates (for both eastern and western flanks), asymmetry of subsidence and zero-ag e depths, are computed within flow-line corridors on the Nazca and Pac ific plates. Variations of subsidence rates, axial depths and subsiden ce asymmetry are examined both in space (within corridors) and time (w ithin several age intervals). The variability in these parameters alon g the strike of the East Pacific Rise is systematic and serves to defi ne several orders of ridge segmentation. The largest variations of the se parameters are correlated with the large-scale segmentation of the ridge axis (i.e. transform faults and very large overlapping spreading centres) and are interpreted as related to variations in mantle heter ogeneities mainly dependent upon temperature. Smaller variations of su bsidence parameters are correlated with second (and sometimes third-) order segmentation of the ridge axis, which could be related to variat ions in axial magmatic supply. Across-strike variations of subsidence suggest the existence of small lateral temperature and density variati ons in the mantle. When analysing the slope of the distribution of dep th versus square root of age within corridors, we have observed the ex istence of changes in the slope which occur at specific age limits. We have estimated the subsidence over different age ranges in order to d etermine the temporal evolution of subsidence parameters (rates and as ymmetry). Such an analysis may inform on the past axial segmentation a nd on the persistence of axial discontinuities in time. A Linear relat ionship between subsidence rates and axial depths is determined for ea ch age range and suggests that shallower segments subside faster than deeper segments. Although a similar, statistically defined Linear rela tionship exists for any mid-ocean spreading ridge (both for intermedia te or fast-ultrafast spreading), the resultant slopes of this relation ship vary from ocean to ocean and show that this relationship is not u niversal over all oceans. (C) 1998 Elsevier Science B.V. All rights re served.