HR: 08:15h
AN: T11G-02    [Abstracts]
TI: The Influence of Ridge Geometry at Ultraslow Spreading Rates
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, McLean Laboratory, Woods Hole, MA 02543-1539 United States
AU: * Standish, J
EM: jstandish@whoi.edu
AF: Woods Hole Oceanographic Institution, McLean Laboratory, Woods Hole, MA 02543-1539 United States
AB: Ridges spreading at ultraslow rate less than 20 mm/yr have been identified as a unique class of ocean ridge as different from slow spreading as slow spreading are from fast 1. Ridge characteristics, such as the presence or absence of amagmatic accretionary segments, transform faults, axial valleys or axial rises, however, are not a simple function of spreading rate, and it is therefore difficult to define precisely ridge classes simply on this criterion. Ridge morphology, tectonics, and geochemistry are also largely a function of mantle thermal structure, upwelling rate, fertility, and ridge geometry. However, examination of ridge crustal structure with spreading rate clearly shows a sharp break, with seismic measurements of crustal thickness indicating highly variable, generally thin crust associated with spreading rates below 20 mm/yr. In contrast, crust formed at spreading rates greater than 20 mm/yr is generally thicker and less variable thickness, averaging between 6 and 7 km, without a clear relationship to spreading rate. The generally accepted explanation is the influence of conductive heat loss and the formation of a thick axial lithosphere due to slow mantle upwelling rates, thereby limiting melt production at ultraslow spreading rates 2. Comparatively, the influence of conductive heat loss at spreading rates greater than 20 mm/yr is likely negligible except near major large offset transforms. The latter effect is predicted by modeling to increase sharply with decreasing spreading rate below 20 mm/yr. Thus perturbations in ridge geometry that would otherwise have a negligible effect, can dramatically influence melt production and ridge tectonics at ultraslow spreading rates. Investigation of the SW Indian Ridge and along the Gakkel Ridge, for example, shows that where the effective spreading rate for mantle upwelling, which ridge obliquity, falls below ~12 mm/yr, long amagmatic accretionary ridge segments form and replace both magmatic accretionary ridge segments and transform faults. These amagmatic accretionary segments then link with magmatic accretionary segments to form stable plate boundaries on even the most oblique trending ridges. Magma geochemistry is strongly affected, with highly variable, incompatible and isotopically enriched MORB's and alkaline basalts appearing along ultraslow spreading ridges far from mantle hotspots, where the mantle upwelling rate is suppressed and the extent of mantle melting is low. The precise composition of such basalts varies considerably with location, indicating a heterogeneous upper mantle composition that varies both locally and on a global scale. References 1. Dick, H. J. B., Lin, J. & Schouten, H. An ultraslow spreading class of ocean ridge. Nature 426, 405-412 (2003). 2. Reid, I. & Jackson, H. R. Oceanic spreading rate and crustal thickness. Marine Geophysical Researches 5, 165-172 (1981).
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting