NEW_Accomplishments.indd - IRIS

2006 IRIS 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE
Seismic Investigation of Upper and Lower Mantle Boundary Layering
Edward J. Garnero, Nicholas Schmerr, Michael S. Thorne • Arizona State University
Both the upper and lower mantle contain important chemical and dynamical processes that likely govern the evolution
and structure of the whole planet. Our work is focused on seismic methods that help to reveal the present day structure in
the upper and lower mantle. The upper mantle transition zone (TZ) is predicted to change thickness as a function of temperature
and chemistry, thus mapping TZ thickness holds promise as a mantle thermometer (Schmerr and Garnero, 2005). In
the deepest mantle, there is evidence for fine-scale layering with ultra-lowered shear and compressional velocities (Thorne
and Garnero, 2004; Rost et al., 2005). These velocity reductions are restricted to the lowest 5-50 km of the mantle, and are
compatible with a partial melt origin; the ultra-low velocities may relate to deepest mantle plume genesis. At slightly larger
vertical scales, the lowermost 200-300 km show evidence for horizontal layering (Thomas et al., 2004) and seismic anisotropy
(Garnero et al., 2004). The IRIS the Data Management System has played a fundamental role in our analyses, as these
studies would not have been possible without it.
Upper mantle discontinuity structure from stacking of SS precursor data to image structure beneath the mid-Pacific, centered on the Hawaiian hotspot.
A) Location of SS precursor stacking bins, each bin is 101⁄4 in radius, and records with Fresnel zones falling within the bin are included in the stack for that
bin. B&C) Topography on the 410 and 660 km discontinuities of the mantle, color scaled to indicate a hot anomaly (yellows) or cool anomaly (reds). The
410 and 660 are generally anticorrelated in this plot. D) Transition zone thickness beneath the region, scaled in the same manner as the discontinuities.
Garnero, E.J., Maupin, V., Lay, T., and M.J. Fouch, Variable azimuthal anisotropy in Earthʼs lowermost mantle, Science, 306, 5694, 2004.
Rost, S., E.J. Garnero, Q. Williams, and M. Manga, Seismological constraints on a possible plume root at the core–mantle boundary, Nature, 435, 2005.
Schmerr, N., and Edward J. Garnero, Probing the nature of 410- and 660-km discontinuities beneath the central Pacific Ocean using SS-precursors, J. Geophys.
Res., in review, 2005.
Thomas, C., Garnero, E.J., and Lay, T., High-resolution imaging of lowermost mantle structure under the Cocos Plate, J. Geophys. Res, 109, doi:10.1029/
2004JB003013, 2004.
Thorne, M.S., and E.J. Garnero, Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves, J. Geophys. Res., 109, B08301,
doi:10.1029/2004JB003010, 2004.
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