LA RISTRA: Lithospheric and Upper Mantle Structure of the Rio Grande Rift Rick Aster, New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, NM Jim Ni, New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM Dave Wilson, Wei Gao and Steve Grand, University of Texas, Austin, Department of Geological Sciences, Austin, TX W. Scott Baldridge, Los Alamos National Laboratory, Earth and Environmental Sciences Division, MS D462, Los Alamos, NM Steve Semken, Arizona State University, Department of Geological Sciences, Tempe, AZ Michael West, Alaska Volcano Observatory, Geophysical Institute, University of Alaska, Fairbanks, AK. LA RISTRA Colorado Plateau, Rio Grande rift, Great Plains, Seismic Transect Experiment 57 seismic station, 950 km-long, broadband passive PASSCAL experiment (54 stations on main transect) 3.6 km station spacing. Currently being extended across the Colorado Plateau and into the Great Basin (RISTRA 1.5) meters All stations equipped with IRIS PASSCAL STS-2 (120 s) seismometers. Hand dug vaults successfully coordinated with diverse landowners and agencies. Continuous recording at 20 samples/second. Data openly available from the IRIS Data Management Center. Abstract Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment show a pure shear extension mechanism for the Rio Grande rift (RGR) at the lithosphere scale. Receiver function results show crustal thickness ranging from 45 to 50 km beneath both the Colorado Plateau and the Great Plains, with crustal thinning to a minimum of approximately 37 km centered beneath the RGR axis. The centering of the thinnest crust on the rift axis indicates that the deep crust has undergone primarily pure shear extension. Inversion of LA RISTRA surface wave data and tomographic inversion of teleseismic body-wave delay times for upper-mantle structure show a broad low velocity region, also centered beneath the rift axis. This low-velocity region is interpreted as rift-centered lithospheric extension, indicating that lithospheric deformation, like that of the deep crust, is also primarily pure shear. A pure shear extensional mechanism for the RGR is consistent with geochemical evidence and regional gravity data which both suggest lithospheric thinning, with the greatest thinning centered beneath the rift axis. We find distributed lithospheric extension that is roughly twice the width of the surface expression of the rift. This geometry suggests less concentrated vertical mantle upwelling than would arise from more localized deformation, which may have led to decreased partial melting, magmatism, and less vigorous convection that might have otherwise occurred. While the upper crust along the RGR has undergone brittle deformation expressed as a series of asymmetric grabens, the lower crust and mantle lithosphere of the RGR have undergone distributed ductile deformation, symmetric about the rift axis, with a broad mantle signature that was likely controlled by pre-rift structure and thermal conditions. New data is currently being collected along an 18-station overlapping extension, RISTRA 1.5, that traverses the northwestern edge of the Colorado Plateau and extends into the easternmost Great Basin.RISTRA is oriented approximately along the age gradient of continental accretion. RISTRA encompasses the southern Great Plains, the Rio Grande Rift, the Colorado Plateau, and the easternmost Great Basin. Laramide tectonics immediately prior to rifting caused widespread deformation including the uplift of the Rocky Mountains The RGR has been opening for the last 30 Ma, along with widespread western U.S. Cenozoic extension. Tectonic Setting (Compiled from Woodward, 1984; Baldridge et al., 1995; Reed and others, 1993) Deployed from July, 1999 through May, earthquakes recorded with m b 5.6 and 129 earthquakes with m b 6.0. RISTRA is approximately on-azimuth for Alaskan/South American earthquakes. m b 6.0 RISTRA Data Raypaths for commonly observed receiver function phases. Timing of receiver function phases depends on: P-wave velocity S-wave velocity Depth to the discontinuity Distance to earthquake (incidence angle of the incoming wavefield). This produces a seismic trace Fourier transform that is ideally solely a function of the Earths structure near the receiver, a Receiver Function. Receiver functions isolate near surface (crust and upper mantle) structure from teleseismic body waves. R( ) source * path * horizontal Earth response near receiver V( ) source * path * vertical Earth response near receiver = Receiver Function Deconvolution H( ) = We use traditional receiver functions calculated from teleseismic body waves (assumed to be nearly planar) by deconvolving the vertical seismogram from the radial horizontal seismogram. In the Fourier domain, this can be characterized by a spectral division: Receiver function migration Kirchhoff pre-stack depth migration. Each of the three typically observed phases is migrated onto a separate 2-d output grid. Output grids can then be compared to validate the velocity model, and/or stacked to reduce imaging artifacts. Where the different receiver function phases coincide in space, they will stack constructively, thus suppressing multiple reverberation and other imaging artifacts. West et al. Colorado Plateau RGR Great Plains Low velocity region is symmetric about the rift axis Ray paths Receiver Function Earths Surface Ps Moho PpPs Moho Regularized receiver function deconvolution decreases sensitivity to instabilities in individual receiver function deconvolution operations. Phases which are consistent between adjacent receiver functions are enhanced. Summary: Crustal thinning and upper mantle lithospheric deformation beneath the Rio Grande rift is highly symmetric about the rift axis, indicating a pure shear rifting mechanism at the lithosphere scale. The broad lateral distribution of lithospheric deformation may have led to decreased partial melting, magmatism, and reduced convection that otherwise could have further driven the rifting process. This geometry is most likely the result of a combination of a regionally elevated geotherm at the time of rift initiation and low strain rates. Also, pre-rift tectonism in the RGR region may have weakened the lithosphere allowing more laterally distributed ductile deformation. This may explain why the RGR has only experienced relatively small volumes of rift related volcanism when compared to other rift systems (5-10% of the Kenya rift). S-wave velocity model from body wave tomography (after Gao et al.) Low velocity region is over twice the width of the surface expression of the rift. Upper mantle S- wave velocity model from surface wave inversion (after West et al.) Migrated cross section with imaging artifacts suppressed via multimode stacking (Wilson et al., 2005) Delaware Basin Moho Receiver Function Amplitude Station # Colorado Plateau RGR Great Plains RGR Crustal thinning is centered on, and is roughly symmetric about the rift axis, favoring a pure shear deformation model. The Moho shows complex impedance structure and topography on the several km scale. Pure Shear Simple Shear End member modes of lithospheric extensional deformation Symmetric lithospheric cross-section Greatest lithospheric thinning offset laterally from greatest crustal thinning (after Buck, 1988) Other evidence supporting symmetric thinning of the lithosphere: Isotopic geochemical data ( Nd values) showing an evolution from magmatism with an enriched source region (lithospheric source) to a depleted source region (asthenospheric source) (Perry et al, 1987; 1988). This transition occurred between 8 and 4 Myr ago in the central RGR, and was relatively symmetric about the rift axis (Baldridge et al., 1991). The decompensative gravity anomaly (Bouger gravity corrected for changes in crustal thickness and composition) shows approximately symmetric thinning of the lithosphere which tracks the axis of the rift (Cordell et al., 1991). This, along with RISTRA seismic imaging results, favors a pure shear rifting mechanism for the Rio Grande rift. The observation that RGR deformation in the mantle lithosphere is over twice the width of the surface expression of the rift may have played an important role in rifting. A narrow zone of concentrated lithospheric stretching leads to increased vertical transport of asthenospheric material. Increased partial melting Increased convection to further drive rifting A laterally- distributed zone of lithospheric stretching leads to lesser vertical transport of asthenospheric material. Less heat produced, reduced partial melting, and less volcanism. Decreased small-scale convection (from Mutter et al., 1988) Acknowledgements: Essential field and data handling assistance was provided by the IRIS PASSCAL Instrument Center at the New Mexico Institute of Mining and Technology (NMT). This research supported by NSF grants EAR and EAR , Los Alamos National Laboratory Institute for Geophysics and Planetary Physics, the New Mexico State University Arts and Sciences Research Center, and the NMT Geophysical Research Center. Persons wishing to conduct geological investigations on the Navajo Nation must first apply for and receive a permit from the Navajo Nation Minerals Department, PO Box 1910, Window Rock, Arizona, 86515, (4x vert. Exagg.) References: Baldridge, W.S., F.V. Perry, D.T. Vaniman, L.D. Nealey, B.D. Leavy, A.W. Laughlin, P. Kyle, Y. Bartov, G. Steinitz, and E.S. Gladney, Middle to Late Cenozoic Magmatism of the Southeastern Colorado Plateau and Central Rio-Grande Rift (New-Mexico and Arizona, USA) - A Model for Continental Rifting, Tectonophysics, v. 197, no. 2-4, pp , Buck, R.W., F. Martinez, M. S. Steckler, J.R. Cochran, Thermal Consequences of Lithospheric Extension: Pure and Simple, Tectonics, v. 7, no. 2, pp , Cordell, L., Y.A. Zorin, and G.R. Keller, The Decompensative Gravity Anomaly and Deep Structure of the Region of the Rio-Grande Rift, J. Geophys. Res., v. 96, n. B4, pp , Gao, W., S. Grand, W.S. Baldridge, D. Wilson, M. West, J.F. Ni, and R. Aster, Upper Mantle Convection Beneath the Central Rio Grande Rift Imaged by P and S Wave Tomography, J. Geophys. Res., 109, B03305, doi: /2003JB002743, Mutter, J., W.R. Buck, and C. Zehnder, Convective Partial Melting, 1, A Model for the Formation of Thick Basaltic Sequences During the Initiation of Spreading, J. Geophys. Res., v. 93, no. B2, pp , Perry, F.V., W.S. Baldridge, and D.J. DePaolo, Chemical and Isotopic Evidence for Lithospheric Thinning Beneath the Rio-Grande Rift, Nature, v. 332, no. 6163, pp , West, M., J. Ni, W.S. Baldridge, D. Wilson, W. Gao, R. Aster, and S. Grand, Comparison of the uppermost mantle across the Colorado Plateau, Rio Grande Rift and Great Plains from surface wave dispersion, J. Geophys. Res., 109, B03309, doi: /2003JB002575, Wilson, D. and R. Aster, Seismic imaging of the Crust and Upper Mantle Using Regularized Joint Receiver Functions, Frequency- Wavenumber Filtering, and Multimode Kirchhoff Migration, J. Geophys. Res., 110, B05305, doi: /2004JB003430, Wilson, D., Aster, R., Ni, J., Grand, S., West, M., Gao, W., Baldridge, W.S., Semken, S., Imaging the Seismic Structure of the Great Plains, Rio Grande Rift, and Colorado Plateau with Receiver Functions, J. Geophys. Res., 110, B05306, doi: /2004JB003492, A subtlety in calculating receiver functions is proper regularization of the commonly ill-posed deconvolution problem. We employ a receiver function estimation technique which takes advantage of characteristic moveout to enhance receiver function arrivals, while filtering out features attributable to noise and deconvolution instability. We do this by filtering receiver function gathers in the frequency-pseudowavenumber domain to remove spurious apparent high--moveout receiver function features while preserving true receiver function phases (Wilson and Aster, 2005). pseudowavenumber RISTRA 1.5 stations ( ) extending from the central Colorado Plateau into the Great Basin.