Abstract Title: Small-Scale Convection Along the Edges of the Great Plains and 
Colorado Plateau

Abstract Author(s): Ni, James (NMSU) - West, Michael (NMSU) - Aster, Richard 
(NMTech) - Grand, Stephen (UT Austin) - Gao, Wei (UT Austin) - David Wilson 
(NMTech) - Baldridge, W. Scott (LANL) - Sandvol, Eric (U. of Missouri, 
Columbia)   

Abstract: From LA RISTRA experiment we have learned that the Rio Grande rift is 
a result of highly symmetric stretching of the lower crust and upper mantle. 
These results suggest a new picture of rift dynamics associated with a nearly 
pure-shear rifting mechanism, low strain rates, and high temperature. 
Initially, ~30 Ma, rift-associated mantle upwelling beneath the Rio Grande 
rift, likely a consequences of foundering of the Farallon plate, caused strong 
lateral temperature and viscosity contrasts at the edges of the Great Plains 
and Colorado Plateau. This in turn started small-scale flows in the mantle 
beneath the craton margins. LA RISTRA imaged the products of the convection 
system including a hot and partially melted uppermost mantle, a down-welling of 
cold lithosphere beneath the eastern edge of the Great Plains and a less 
pronounced down-welling beneath the western edge of the Colorado Plateau. These 
findings constitute the most detailed evidence to date of ongoing small-scale 
convection beneath the southwestern U.S. Small-scale convection is an important 
part of mantle dynamics and is the engine that drives continental deformation 
in the southwestern U.S. Such convection interacts mechanically, thermally, and 
magmatically with the overlying lithosphere. Here we suggest that small-scale 
convection controls extensional deformation and volcanism along the rim of the 
plateau and may even be associated with the initiation of low-angle 
detachments. An additional 18 broadband temporary seismographs will be deployed 
(during the summer of 2004) northwest of LA RISTRA linear array, from Lake 
Powell to Clear Lake, Utah. This extended line has a station spacing of 25-30 
km and is about 400-450 km long. The new data will provide the first complete 
cross-section of the Colorado Plateau and its western margin.