HR: 1340h
AN: T23A-0530    [Abstracts]
TI: Direct Observation of Upper Mantle Shear Flow Since 14.9 Ma in the Central Basin and Range
AU: * Biasi, G P
EM: glenn@seismo.unr.edu
AF: University of Nevada Reno, Seismological Laboratory MS-174, Reno, NV 89557 United States
AB: Asthenospheric flow in the Basin and Range province of the western United States has been hypothesized in dynamic models of the region. Models have included southwest flow in response to a hotspot-like upwelling in northeast Nevada, rapid northeast flow in response to extension and Pacific-North American margin shear, and slow, top-to-the-west simple shear. Detailed P-wave tomographic imaging in southern Nevada provides a direct measure of mantle flow to >200 km since ~14.9 Ma. A nearly vertical, 1-2.5% high velocity column presently exists immediately beneath the Timber Mountain/Silent Canyon caldera complex (37.2° N, 116.2° W) that is interpreted as the root of the complex. Volcanism reached 500 km³ in eruptive volumes (comparable to Long Valley Caldera) by 14.9 Ma, and a total of over 7,000 km³ by 11.7 Ma, whereupon the system declined drastically in activity. The root is interpreted from tomographic, geochemical, and petrological considerations to correspond to a de-watering feature that conveyed a pulse of volatile rich volcanism and left a dry, depleted, and slightly cooled column behind. As a melt ascension residual of some sort, we infer that it was originally nearly vertical. With that assumption, the root structure comprises a shear flow marker for activity since it formed. The edifice is 0-15 km west of its center at 200 km depth, indicating that there has been little or no top-to-the-west shear and no channelized flow to the east or west since ~14.9 Ma. Approximately 40 km of top-to-the-south flow is observed, consistent with some geological models. Below 200 km the root appears to plunge NE at 45 degrees to the bottom of the model at 400 km. Synthetic modeling indicates that the deep delay structure is not an artifact of the root, but array aperture is too small to eliminate association with even deeper structures. Hydrous, subsolidus conditions to ~150 km are inferred on a NE linear trend southeast of the root structure. In Nevada this trend does not correlate with volcanism but does underlie a zone of crustal weakness based on fault density and seismicity. This trend may continue NE and be associated with the St. George volcanics of southern Utah. In sum, to 200 km, there is little flow of any sort in the asthenosphere in the Basin and Range at 37°N, and water appears to be the main agent for velocity variations.
DE: 7270 Tomography (6982, 8180)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005