HR: 11:50h
AN: V31F-06    [PDF]
TI: Mantle Dynamics beneath the western United States
AU: * Dueker, K G
EM: dueker@uwyo.edu
AF: Univ. of Wyoming, Dept. of Geology and Geophysics, 16 and Gibbon, Laramie, WY 82071 United States
AU: Yuan, H
EM: yuan@uwyo.edu
AF: Univ. of Wyoming, Dept. of Geology and Geophysics, 16 and Gibbon, Laramie, WY 82071 United States
AU: Schutt, D
EM: schutt@uwyo.edu
AF: Univ. of Wyoming, Dept. of Geology and Geophysics, 16 and Gibbon, Laramie, WY 82071 United States
AU: Fee, D
EM: fee@uwyo.edu
AF: Univ. of Wyoming, Dept. of Geology and Geophysics, 16 and Gibbon, Laramie, WY 82071 United States
AB: Much uncertainty still exists with respect to the state of the mantle beneath the western United States and the dynamical activity that created this current state. Three end-member dynamical agents may be at work. First, input of heat from below is often used to explain: the Yellowstone Hotspot track (a plume), the high standing elevation of the northern Basin and Range (a plume head), the petrology and geochemistry of southern Nevada young basalts (a plume), and the late-Cenozoic uplift and rifting of the southern Rockies (unspecified source of heat). Second, many publications note the correspondence between the volcano-tectonic activity of the western US and pre-existing lithospheric structures (sutures). For example, the low velocity body imaged beneath the Jemez volcanic lineament (from the CD-ROM project) shows clear mantle velocity layering within the low velocity body suggesting that this velocity anomaly is lithospheric, not asthenospheric in origin. Third, both lithospheric and asthenospheric modes of small-scale convection may be ongoing today. If our understanding of the dynamical parameter space for small-scale convection is correct, then the low upper mantle viscosity ($<$10$^{19}$ Pa-s) and thin Basin and Range lithosphere demands that small-scale convection is occurring. Yet, the abundance of seismic imaging beneath this region cannot presently constrain the planform and modes of small-scale convection. In particular, the magnitude of the thermal anomalies are poorly constrained due to uncertainties in the Q operator and hence the magnitude of the anelastic velocity-temperature derivative. Also, the numerous short term ($<$1-2 year) PASSCAL deployments in the western US, while providing unparallel spatial coverage, generally lacks a sufficient distribution of SKS events to constrain dipping anisotropy. This undersampling problem is unfortunate because dipping anisotropy is probably our best proxy for small-scale convective flow beneath this region Comments on the seismic imaging challenges, that will face the deployment of the USarray component of Earthscope, will conclude this talk.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting