HR: 09:25h
AN: T51B-06    [PDF]
TI: Upper Mantle Structure Beneath the East Pacific Rise
AU: * Gu, Y J
EM: gu@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Webb, S
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Lerner-Lam, A
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Gaherty, J B
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Dziewonski, A M
AF: Harvard University, 20 Oxford St, Cambridge, MA 02138 United States
AB: Depth extent and characteristics of low velocity zones under spreading centers have long been debated. Evidence from recent global and regional surface wave studies suggests considerable shear velocity variations at depths possibly as large as 300 km. The upper mantle beneath midocean ridges are also known to exhibit highly anisotropic behaviors, especially in the top 200 km. In this study we combine regional observations with global constraints to investigate shear velocity structure (both isotropic and anisotropic) and mantle discontinuities under the East Pacific Rise (EPR). The data set consists of intermediate-period (15-35 sec) records from ocean-bottom seismometers (OBS) deployed by the TOES (1995) and MELT (1996) experiments, as well as long-period surface ($>$45 sec) and body waves from on-land, permanent stations worldwide. Our preliminary surface wave (mostly Rayleigh wave) analysis of the OBS recordings shows considerable differences in 1-D isotropic velocities between cross-ridge and along-ridge paths. The presence of a distinct low-velocity zone within the top 100 km is required to fit the observed waveforms. The average velocity for several source-receiver paths in the distance range 8-25 degrees appears to correlate with the path-integrated age of the oceanic lithosphere. A joint inversion for isotropic/anisotropic shear velocity structures reveals a radially anisotropic anomaly (possibly as strong as 2.5%) $\sim$230 km beneath various parts of fast spreading ocean ridges, most notably north EPR. A positive difference between the vertical and horizontal wavespeeds implies near-vertical alignments of olivine fast crystallographic {\it a} axis at asthenospheric depths. The anomalous nature of the EPR is further evidenced by the presence of a deep ($\sim$270 km) reflector in north EPR. This feature could potentially represent a deep Lehmann discontinuity, though the latter is generally associated with major continents. Waveform comparisons between observed S multiples and synthetic calculations suggest either a positive jump that is nearly as strong as the 220-km discontinuity in PREM or, alternatively, a gradient in the $V_{SV}$ wavespeed exists below 200 km. The nature of this anomaly, especially its potential association with anisotropy under ocean ridges, is being investigated.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting