HR: 0800h
AN: S41B-0557    [Abstracts]
TI: Complex Upper Mantle Seismic Structure Across the Southern Colorado Plateau / Basin and Range I: Results from Shear Wave Splitting Analysis
AU: * Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AU: Gilbert, H J
EM: hersh@purdue.edu
AF: Purdue University, Dept. of Earth and Atmospheric Sciences, West Lafayette, IN 47906, United States
AB: The origin of the stark transition from the highly extended and deformed geologic terrane of the southern Basin and Range to the stable, laterally undeformed region beneath the Colorado Plateau has been a long-standing question in understanding the tectonic evolution of the southwestern United States. To provide new constraints for this enigmatic region, we are utilizing the excellent regional coverage afforded by the USArray Transportable Array (TA) and other permanent and temporary broadband seismic stations to image anisotropic crust and mantle structure. This study focuses on our results from shear wave splitting analyses, but is part of a broader study of regional seismic anisotropy that includes receiver function analyses. As of summer 2007, we have performed shear wave splitting analyses on over 7,500 broadband waveforms. Our results provide the first clear evidence for anisotropic complexity laterally and with depth for this region. These variations appear to be correlated with regional geologic terranes. Splitting times range from 0.6 sec to 2.7 sec and are generally larger for Basin and Range stations. For stations located within the southern Basin and Range, fast directions are oriented from ENE-WSW to N-S. While regional similarities exist for some areas, significant variations exist over spatial scales of 70 km or less for events from similar backazimuths. We also observe well-resolved differences in fast direction with event backazimuth. Conversely, fast directions for stations located on the Colorado exhibit a more limited range of fast directions from NNE-SSW to NE-SW. Splitting for stations near the Arizona Transition Zone, between the Colorado Plateau and southern Basin and Range, tend to reflect the range in fast directions exhibited by the southern Basin and Range. The regional variations in shear wave splitting likely result from a combination of crust, mantle lithosphere, and asthenospheric fabric. The large splitting times suggest that sublithospheric anisotropy plays a strong role. Additionally, we note that receiver function imaging of the 660 km mantle discontinuity in this area exhibits backazimuthal dependence, suggesting contributions from deep upper mantle structure to the observed splitting complexity. Future work will include the analysis of additional TA waveforms, the distribution of null splitting results, and understanding the relationship between shear wave splitting results and anisotropy apparent from receiver function analyses at these stations.
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: 2007 Fall Meeting