EVALUATING THE EFFECTS OF SMALL-SCALE CONVECTION ON LITHOSPHERIC STRUCTURE WITH THE COLORADO PLATEAU TRANSITION ZONE

The Colorado Plateau (CP) is a high (~2 km above sea level), low-relief, orogenic plateau located within the interior of the southwestern United States. Although, commonly identified as a region of tectonic quiescence within the highly-deformed North American Cordillera, recent work has shown that deformation and volcanism are encroaching into its interior from its margins. This may be driven by small-scale convection within the upper mantle beneath the transition zone, a process which is expected to warm the Colorado Plateau lithosphere and produce decompression melting.

In order to better understand transition zone tectonism, we utilize data from the EarthScope Transportable Array network to measure mantle flow, image the CP lithosphere, and inform thermodynamic models of CP lower crustal composition. SKS wave splitting was used to estimate mantle flow directions beneath the region and Rayleigh wave phase velocities were inverted for shear velocity. In order to provide greater context to these results, mantle temperatures were calculated from seismic velocities and the thermodynamic modeling program Perple_X was utilized to forward model crustal densities, seismic velocities, stable mineral assemblages, and water content based on pseudosections calculated using published rock bulk compositions.

Our seismic and modeling results from the transition zone show low velocities in the upper mantle, high lower-crustal and upper mantle temperatures, and increased SKS splitting times, consistent with small-scale convection within this region. Thermodynamic modelling shows that warming and extension associated with this process have likely led to melting and reduced the density of the crust along the plateau margins by as much as 110 kg/m³, providing a possible mechanism for uplift of the plateau. This work provides a model of how mantle processes can uplift and thermally erode stable lithosphere eventually causing it to succumb to deformation and volcanism.