NEW_Accomplishments.indd - IRIS

SURFACE OF THE EARTH: NORTH AMERICA
2006 IRIS 5-YEAR PROPOSAL
Lithospheric Structure of the Rio Grande Rift
The RISTRA Group
The seismic structure of the crust and upper mantle of the southwestern United States has been examined using teleseismic
arrivals recorded by IRIS PASSCAL Instruments in LA RISTRA (Colorado PLAteau Rio Grande Rift-Great Plains Seismic
TRAnsect). Receiver function estimation and filtering methods developed by Wilson and Aster (2005) produce receiver
functions with decreased sensitivity to noise. Crustal thickness and V P /V S are estimated using both direct and reverberated
P-to-S receiver function modes. Regularized receiver function migration methods produce a multiple- suppressed image of
the velocity discontinuity structure of the subsurface. Crustal thickness averages 44.1 +/- 2.3 km beneath the Great Plains
(GP) and 45.6 +/- 1.1 km beneath the Colorado plateau (CP). Crustal thinning beneath the RGR is broadly symmetric about
the
beneath the RGR (figure 2). We observe a prominent northwest-dipping discontinuity, ranging from 65-85 km deep beneath
the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are
consistent with preserved ancient lithospheric structures such as
relict suture zones associated with Proterozoic subduction. An
upper mantle discontinuity at 220-300 km depth may correlate
with similar structure observed beneath eastern North America.
Flat discontinuities at 410 and 660 km depth suggest that there
is not a large-scale thermal anomaly beneath the RGR at these
depths (Wilson et al., 2005b). Research supported by the NSF
Geophysics program and Los Alamos IGPP; instruments and field
support provided by IRIS PASSCAL.
Figure 2. Moho modeling for specific lower-crust:upper-crust extension ratios,
with the best fit being LC:UC=2.0:1. Note extensive widening of the rift structure
at depth. Background colors depict S-wave variations from joint body/surface
wave inversion (3.6-4.7 km/s; West et al., 2004). Both after Wilson et al. (2005a).
Figure 1. The 960-km RISTRA transect
across the Rio Grande rift (RGR) region.
Areas of Cenozoic extension are hatched.
West, M., J. Ni, W.S. Baldridge, D. Wilson, R. Aster, W. Gao, and S. Grand, crust and upper mantle shear wave structure of the southwest of the southwest
United States: Implications for rifting and support for high elevation. J. Geophys. Res. 109, doi: 101029/2003JB002575, 2004.
Wilson, D., Aster, R.,lter -
ing, and multimode Kirchhoff migration, J. Geophys. Res., in press, 2005.
Wilson, D., Aster, R., West, M., Ni, J., Grand, S., Gao, W., Baldridge, W.S., Semken, S.,Lithospheric Structure of the Rio Grande Rift, Nature, 433, doi:
10.1038/nature03297, 2005a.
Wilson, D., Aster, R., Ni, J., Grand, S., West, M., Gao, W., Baldridge, W.S., Semken, S., Imaging the seismic structure of the crust and upper mantle beneath
the Great Plains, Rio Grande Rift, and Colorado Plateau, J. Geophys. Res., in press, 2005b.
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