Lithospheric structure of the Rio Grande rift.
A high-resolution, regional passive seismic experiment 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 geochemical and other geophysical 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 convection 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 rift.
Wilson
David
D
Department of Earth and Environmental Science and Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. davew@ees.nmt.edu
Aster
Richard
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West
Michael
M
Ni
James
J
Grand
Steve
S
Gao
Wei
W
Baldridge
W Scott
WS
Semken
Steve
S
Patel
Paresh
P