HR: 10:20h
AN: T32A-01    [Abstracts]
TI: Formation of the Rio Grande Rift and Upper Mantle Seismic Wave Velocity Anomaly
AU: * van Wijk, J
EM: jolante@lanl.gov
AF: Los Alamos National Laboratory, MS D443, Los Alamos, NM 87545, United States
AU: van Hunen, J
EM: jeroen.van-hunen@durham.ac.uk
AF: Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, United Kingdom
AU: Goes, S
EM: s.goes@imperial.ac.uk
AF: Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom
AB: The Rio Grande rift extends from central Colorado through New Mexico where it merges with the southern Basin and Range province, to Chihuahua in Mexico. Rifting started in the Oligocene-early Miocene, and strain rates were probably very modest since the Late Miocene. The rift is currently active (strain rates ~1-2 mm/yr) but rifting is not accompanied by seismic activity. The Rio Grande rift was formed close to a step in lithosphere thickness between the Great Plains and Colorado Plateau. The La RISTRA passive seismic experiment reveals a shallow low seismic wave velocity perturbation under the Rio Grande rift, extending to a depth of about 100-180 km. Prior explanations for this observed upper mantle heterogeneity include a broad region of lithosphere thinning and asthenospheric upwelling, compositional variations, presence of partial melt in the upper mantle, thermal anomalies, or a combination of these factors. There is no evidence for deep mantle upwelling beneath the rift and therefore active rifting of the Rio Grande rift is not supported. Here we present a model for formation of this continental rift system and the low seismic wave velocity anomaly in the upper mantle based on results of a lithosphere-upper mantle finite element model. The numerical results suggest that formation of the Rio Grande rift took place west of the Great Plains at the location where the lithosphere was thermally weakened by small scale convection. The rift zone was formed at the time that the lithosphere of the south-western US was no longer under compression. Localized extension at the rift zone further thinned the mantle part of the lithosphere and asthenosphere material welled up under the rift, replacing the colder lithosphere. From the temperature structure predicted by the model we calculated the expected seismic wave velocity perturbation in the upper mantle. Comparison with tomographic results shows a good agreement, and we conclude that the low seismic wave velocity anomaly is the result of upward flow of asthenospheric material below the rift zone.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8100 TECTONOPHYSICS
DE: 8109 Continental tectonics: extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting