The depths of mantle melting zones can be constrained by forward (in terms of physicochemical thermodynamics) or inverse (in terms of equilibrium thermodynamics) modeling. However, there is discrepancy in this respect between fluid-dynamic models of decompression melting in convecting upper mantle and thermodynamic models of basaltic magma sources beneath mid-ocean ridges. We investigate the causes of the mismatch in melting depth predictions with reference to the magmatic systems of the Basin and Range Province in the western margin of North America. The inverse solutions turn out to represent melts from different substrates (depth facies) in the lithospheric mantle, while modeling decompression melting in convecting fertile upper mantle refers to the depths the faults in spreading zones never reach. The discrepancy between forward and inverse solutions may be due to the fact that the respective depth estimates correspond to different levels of the same mantle–crust magmatic systems.