How does the base of a plate migrate? Inferences from the Colorado Plateau.

The patterns of surface heat flow on the Earth evolve in space and time, due to plate motion and deformation, reflecting mantle convection patterns. In this talk, I will develop the question of the extent to which surface heat flow changes can reflect changes in plate thickness driven primarily by thermal and chemical disequilibrium between the convecting mantle and the mechanical plate. Geologic observations suggest that depleted mantle lithosphere can be "refertilized" by infiltrating melts. Rock deformation experiments show that stress can cause melt to segregate and that this process is enhanced by chemical disequilibrium. These geologic and experimental observations motivate our interpretation of the structure and dynamics of the western margin of the Colorado Plateau (CP). New seismic velocity models show that the uplifted margins of the CP are underlain by slow velocities (i.e. very thin lithosphere), forming a ledge-like structure, while the core of the CP is a seismically fast cratonic keel. Volcanism is migrating inward towards the keel at a rate of 4 km/myr. On the western margin, surface volcanism is closely associated with faults and high strain rates and seismicity rates. On the southern margin, no such associations exist. We propose that thermal/chemical corrosion of the lithospheric mantle is producing this ledge structure, and melt migration is highly focused up the lithosphere-asthenosophere boundary (LAB) forming the walls of the keel, aiding in the corrosion and stress concentrations. This corrosion front migration could be steady state if the upwards LAB velocity is on the order of the surface volcanic migration rate. Controls on the kinetics of the corrosion rate are an open question, but may be greatly enhanced by deformation. Finally, detailed interpretation of the seismic velocity structure in terms of temperature, melt fraction and compositional properties will place constraints on the thermodynamic driving forces for LAB migration.