HR: 10:50h
AN: U41C-03    [PDF]
TI: Seismic evidence for a diffuse ultra-low velocity zone in the CMB region beneath North America.
AU: * Rondenay, S
EM: rondenay@mit.edu
AF: MIT Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Fischer, K M
EM: Karen_Fischer@brown.edu
AF: Brown University Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AB: In recent years, a wide range of geophysical results has offered evidence that Earth's lowermost mantle is characterized by strong lateral variations in material properties. Among the structures of particular interest are intermittent ultra-low velocity zones (ULVZ's), located directly above the CMB, which were inferred originally from the distortion of teleseismic {\it SPdKS} phases. ULVZ's have been modelled as layers with sharp boundaries and seismic velocity reductions $\ge$10%, and interpreted as regions of partial melt. In this study, we further constrain local ULVZ structure beneath North America by signal processing and waveform modelling of the {\it SKS}-coda recorded at broadband seismic arrays. Secondary phases in the {\it SKS}-coda are effectively isolated by eigenimage processing. Residual (i.e., {\it SKS}-less) data sections from various western Pacific events display clear {\it SPdKS} arrivals, followed by a secondary phase whose timing and slowness are consistent with CMB origins. One-dimensional modelling of these phases by reflectivity and generalized ray synthetics favours an asymmetric model, with ULVZ present at only one of the CMB-intercepts. The preferred ULVZ is characterized by reductions in P and S velocities of 18% and 50%, respectively, and a diffuse upper boundary. These characteristics are interpreted in the context of local production and gravitational sinking of dense (i.e., iron rich) partial melt above the CMB. We postulate that a gradational ULVZ beneath North America may mark a lateral transition domain between regions of mantle upwelling, where more uniform ULVZ's exist, and regions of downwelling, where ULVZ's are either nonexistent or imperceptibly thin. The broader existence of such diffuse ULVZ's beyond the portion of the CMB investigated in this study may explain why, in some regions, ULVZ's are detected by {\it SPdKS} phases but not by {\it PcP}/{\it ScP} precursors.
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
DE: 8124 Earth's interior--composition and state (old 8105)
SC: U
MN: 2003 Fall Meeting