A high-resolution, regional passive seismic experiment1,2,3,4,5,6 in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust–mantle topography. Synthesizing these results with geochemical7,8,9 and other geophysical10,11,12,13 evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection14 than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift15,16.
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We thank G. R. Keller and Roger Buck for comments. 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 was supported by NSF grants, the 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. A permit is necessary to conduct geological investigations on the Navajo Nation from the Navajo Nation Minerals Department.
The authors declare that they have no competing financial interests.
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Wilson, D., Aster, R., West, M. et al. Lithospheric structure of the Rio Grande rift. Nature 433, 851–855 (2005). https://doi.org/10.1038/nature03297
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