Thinnest correspond to two topographic lows in Great Basin Topographically high, avg.
Ranges mostly bounded by high angle normal faults which shallow at depth, thus are listric faults.
Conclusion Regardless of the cause the Basin and Range is a region of greatly attenuated continental crust. The upper crust has extended by brittle normal faulting and the deeper crust has thinned by ductile deformation. In the underlying mantle the extension has been accomodated by a shallow convective inflow of material, allowing for the high heat flow and high elevation of the stretched crustal region.