HR: 0800h
AN: T51C-0464    [Abstracts]
TI: Crust and upper mantle structure of the Southern Basin and Range and Colorado Plateau of Arizona: Project COARSE
AU: * Frassetto, A
EM: andyf@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St, Tucson, AZ 85721 United States
AU: Gilbert, H
EM: hgilbert@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St, Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St, Tucson, AZ 85721 United States
AU: Owens, T J
EM: owens@seis.sc.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Department of Geological Sciences University of South Carolina, 701 Sumter Street, Columbia, SC 29208 United States
AU: Beck, S L
EM: beck@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building #77 1040 E 4th St, Tucson, AZ 85721 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Department of Geological Sciences University of South Carolina, 701 Sumter Street, Columbia, SC 29208 United States
AB: Project COARSE (COnsortium for Arizona Reconnaissance Seismic Experiment) has operated a temporary broadband seismic network across Eastern Arizona for the past 18 months. COARSE stations sample lithosphere across the Southern Basin and Range and onto the Colorado Plateau. Here we present observations aimed at improving our understanding of the creation and maintenance of the stable Colorado Plateau and highly deformed Basin and Range, which has undergone substantial compression and extension. Forces related to subhorizontal subduction of the Farallon plate beneath the North American plate followed by a slab steepening or detachment event at ~35 Ma are common to geologic and tectonic models of the deformation and volcanism within the western United States. However, how these tectonic events have left their signature in the lithospheric structure remains unknown. Receiver functions from COARSE stations display distinct structural differences between the Colorado Plateau-Transitional Zone and Basin and Range stations including variations in crustal structure, Moho signature, low velocity zones within the crust and upper mantle, and crustal anisotropy. A clear high amplitude Moho arrival corresponding to crustal thicknesses near 30 km can be seen in receiver functions from the southern Basin and Range. Differently, receiver functions from Colorado Plateau stations exhibit a high amplitude, negatively polarized arrival at ~1-2 seconds which could result from a layer of upper crust containing anomalously slow shear wave speeds. Additionally, the Ps converted phase from the Moho has considerably lower amplitude than those from the Basin and Range possibly indicative of reduced impedance contrast across the Moho, or that reverberating energy from shallow structures obscures the primary Moho arrival. To investigate these possibilities, we have begun to construct a suite of models that can produce the observed receiver functions from the Colorado Plateau and Basin and Range. Utilizing waveform modeling to create synthetic receiver functions from models of Vp, Vs, density, and crustal thickness, we attempt to determine what combination of characteristics best match our observations at each station. Preliminary modeling results indicate that a 30 percent drop in Vs centered about a 3.25 km thick low velocity body at 9 km depth matches observed receiver functions for the USNSN station WUAZ on the plateau. We speculate that the low velocity feature may relate to the presence of partial melt beneath the Plateau that is associated with Pleistocene age volcanism in the vicinity.
DE: 8109 Continental tectonics--extensional (0905)
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting