IMAGING ROCK FABRIC DIFFERENCES ACROSS THE 410 KM AND 660 KM SEISMIC DISCONTINUITIES BENEATH THE CHEYENNE BELT

A change in rock fabric and hence velocity anisotropy is expected across the 660 km discontinuity if this phase boundary acts as a barrier to mass exchange between upper and lower mantle. In addition, whether velocity anisotropy, created by dislocation creep in the mantle transition zone exists, remains an open question. To address these questions, we study radial and transverse receiver function stacks from the 410 km and 660 km discontinuity boundaries. These receiver functions were recorded by the Lodore array in NW Colorado which straddles the Cheyenne belt. This array was run from September 1997 to July 1998 and produced 2600 "good" receiver functions.

Previous research reveals that in some regions significant discontinuity topography exists whereas other places show relatively flat topography at the 410 and 660 km discontinuities. In regions of flat topography, analysis of the anisotropic velocity effects upon converted P-to-S waveforms can provide roust estimate of how mantle anisotropy (if any) is distributed across the upper mantle. Previous SKS splitting results from the Lodore array show little upper mantle anisotropy. We expect to see phases on the tangential receiver functions due to the SH-SV partitioning of P-to-S conversion at the discontinuity boundaries. Results from the Lodore array will be presented in the meeting.