The composition of the lower crust is a key factor in understanding
tectonic activity and deformation within the Earth. In particular, the
presence or absence of melt or fluids has strong control on tectonic
evolution. Multi-physics inversion results from the western United
States demonstrate that tectonic inheritance plays a much stronger role
in determining the location of melt in the lower crust than previously
thought. Even in a currently active area such as the Yellowstone
Hotspot, fluid dominated structures and fluid free regions are
juxtaposed. This is incompatible with the commonly used model of recent
tectonic activity as a main controlling factor for the presence of
fluids or melt. These results have global implications for how
geophysical models are interpreted and how they can be related to
geodynamic simulations.