Plot of the interval of accelerated extension, determined at a particular location, versus the distance to the outer parabola alone a line trending N55°E (from Anders, 1994). Assuming the migration direction can reasonably be determined, the slope of the line reflects the migration rate of the thermomechanical effects of an assumed sub-lithospheric plume or hotspot. From these data the migration rate of the thermal source is determined to be 2.2 ± 0.2 cm/yr over the last 10 m.y. This is significantly less than the 2.9 cm/yr to 4.5 cm/yr values that have been suggested based on the location of volcanic centers.  If the thermal source is indeed fixed relative to the North American plate, as predicted by the standard hotspot model, then the relative North American plate velocity is 2.2 cm/yr in a roughly N55°E direction. This estimate is the same as that calculated for the last 3 m.y. using sources other than the Yellowstone hotspot (see DeMets et al. 1990).