HR: 08:15h
AN: T31D-02 INVITED     [Abstracts]
TI: Global Upper Mantle Structure from Finite-Frequency Surface-Wave Tomography
AU: * Zhou, Y
EM: yingz@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Nolet, G
EM: nolet@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Dahlen, F
EM: fad@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Laske, G
EM: gabi@mahi.ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225 United States
AB: We report global shear-wave velocity structure and radial anisotropy in the upper mantle obtained by finite-frequency surface-wave tomography, based on complete three-dimensional Born sensitivity kernels developed by Zhou et al (2004). Because wavefront healing effects are properly taken into account, finite-frequency surface-wave tomography improves the resolution of small-scale mantle heterogeneities using long-period surface waves. The resulting S-wave velocity models fit the dispersion data better, and show stronger small-scale mantle anomalies compare to traditional ray-theory-based tomographic models. Separate inversions of Love wave (SH-type) and Rayleigh wave (SV-type) dispersion provide insight into the radial anisotropy in the upper mantle. In our model, the globally averaged radial anisotropy is positive ($V_{\rm SH} > V_{\rm SV}$) (horizontal flow) in the top 220 km, and becomes negative ($V_{\rm SV} > V_{\rm SH}$) (vertical flow) below 220 km depth. In cratons, both SH and SV velocities show strong fast anomalies down to 250 km depth, and the fast anomalies gradually diminish below 250 km. Radial anisotropy beneath cratons is positive, which largely agrees with a recent global model by Gung et.~al (2003). The old Pacific plate is characterized by strong positive anisotropy with its maximum centered west of Hawaii; this supports an earlier observation on Pacific radial anisotropy by Ekstrom & Dziewonski (1998). The depth extent of mid-ocean ridges and the primary force that drives plate tectonics has been a long-standing question. In our model, ridge anomalies are characterized by strong negative radial anisotropy (vertical flow). Ridge anomalies at fast-spreading centers are stronger than those at slow-spreading centers at shallow depth, but the amount of velocity reduction rapidly decreases below 250 km. However, at slow-spreading centers such as the north Mid-Atlantic ridge (MAR) and East Africa (Red Sea), ridge anomalies extend down at least to the top of the transition zones. The different depth extent of the ridge anomalies indicates that the primary driving force of slow-spreading seafloors may be different from that of fast-spreading seafloors; active upwelling beneath slow-spreading ridges may play a major role in the opening of the slow-spreading sea floor. The deep origin of the slow anomaly at MAR at about $20^{\circ}$N may be responsible for the initial opening of the Atlantic ocean between Africa and North America plate 180 million years ago.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting