HR: 10:50h
AN: T12A-03    [Abstracts]
TI: Magmatic Segmentation at the Northern and Southern East Pacific Rise: Magma Supply Versus Magma Plumbing
AU: * Toomey, D R
EM: drt@newberry.uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403
AU: Hooft, E E
EM: emilie@newberry.uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403
AB: Competing models for what controls the intensity of ridge crest processes are often at odds on the scale of magmatic segmentation of the mantle and crust. One popular and long held view is the magma supply hypothesis proposed by Macdonald and others, wherein magma is centrally injected at mantle depths into a ridge segment bounded by long-lived tectonic discontinuities and ridge parallel flow of magma over distances of 10's of kilometers gives rise to along-axis variations in ridge crest processes. In this view, the scale of mantle melting at depths of 30 to 60 km is thought to govern the scale of magmatic segmentation of the mid-ocean ridge. Axial variations in volcanic, tectonic and hydrothermal activity are attributed to waxing and waning episodes of melting events. Where magma supply is high, greater amounts of volcanic and hydrothermal activity are predicted. Where the ridge is `starved' of magma, tectonic activity predominates. Recent studies at the East Pacific Rise (EPR) Integrated Study Site (ISS), however, bring up several challenges to the magma supply hypothesis and an alternative model which we refer to as magma plumbing. The two essential features of the magma plumbing hypothesis are the scale of along-axis segmentation of shallow mantle upwelling and the vertical (mis)alignment of the mantle and crustal magmatic systems. In this view, magma injection from the mantle to the crust occurs at more-or-less equally spaced intervals within a ridge segment bounded by tectonic discontinuities and the cross-axis offset between a center of mantle upwelling and an axial volcano regulates the intensity of ridge crest processes. When the mantle and crustal plumbing systems are aligned, the ridge crest is more volcanically and hydrothermally active. Conversely, when the mantle and crustal plumbing systems are misaligned by many kilometers, the intensity of extrusive volcanic and hydrothermal activity is less, while tectonic activity is increased. As a consequence, even at constant magma supply (as determined by crustal thickness) the intensity of seafloor processes can be fundamentally different because of magma plumbing. We will discuss these two competing models for magmatic segmentation in the context of results from geological, geophysical and petrological studies along the northern and southern EPR.
DE: 8434 Magma migration
DE: 7220 Oceanic crust
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting