HR: 16:00h
AN: T34A-01    [Abstracts]
TI: What is the Asthenosphere? What is the Lithosphere?
AU: * Phipps Morgan, J
EM: jp369@cornell.edu
AF: EAS Dept., Snee Hall, Cornell Univ., Ithaca, NY 14853 United States
AB: At the dawn of plate tectonics the concept of a weak asthenosphere layer sandwiched between strong overlying plates and more viscous underlying mantle was widely accepted, albeit vaguely defined. This interpretation was reinforced by both 1-D seismic interpretations of a `low-velocity zone' just beneath the (seismically defined) lithosphere and post-glacial rebound interpretations of a shallow weak asthenosphere layer overlying deeper, more viscous mantle. However, more than a century since it was first suggested, there is still no consensus view on whether there even is a rheologically weak asthenosphere, let alone its spatial bounds. I start by summarizing various lines of geophysical evidence for an asthenosphere region within the earth. This evidence suggests that the asthenosphere is patchy, with a well-developed asthenosphere existing beneath the relatively thin lithosphere found in the ocean basins and younger continental regions, and a ~100km deeper-beginning and substantially more-viscous `asthenosphere' (if we want to use the same term) found beneath continental cratons. Some geodynamic implications of a `patchy' asthenosphere will be assessed. Furthermore, if the sub-oceanic asthenosphere is to a large extent fed by concentrated upwelling from regions hotter than typical mantle (e.g., thermal plumes), then sub-oceanic asthenosphere should typically be hotter than underlying mantle. Recent seismic evidence (depth-dependent attenuation patterns, and even the wavespeed structure of the oceanic low-velocity zone) and other supporting geophysical evidence will be reviewed that to me seem more consistent with this idea than the alternative that the asthenosphere typically has the same potential temperature as underlying mantle. Finally, I will discuss some implications of this concept for oceanic off-axis volcanism, in particular introducing the idea that the heat of the asthenosphere may be conducted downwards to induce melting in more volatile-rich parts of its underlying mesosphere - with these melts inducing the short-lived off-axis volcanic lineaments sometimes speculatively attributed to short-lived microplumes.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: Fall Meeting 2005