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Abstract: To explore the nature of how the structure and physical properties of the crust vary from extended to relatively unextended domains we present teleseismic receiver functions which measure crustal thickness, shear wavespeed structure and the Vp/Vs ratio at 12 seismic stations in eastern Arizona. The crustal thickness is ∼28 km, increases slightly eastward, and remains nearly uniform beneath the varying elevations in the southern Basin and Range. The observed Vp/Vs ratio in the Basin and Range (∼1.78) exceeds the global average. The southern Colorado Plateau exhibits thicker crust (∼40 km) and a slightly greater observed Vp/Vs (∼1.81). A discrete region in the Colorado Plateau generates an unusually high Vp/Vs ratio (1.90) and contains low wavespeed zones which serve as evidence of partial melt related to Quaternary volcanism. The metamorphic core complexes in the southern Basin and Range likewise exhibit anomalously high Vp/Vs values (1.79–1.87) and lack locally compensating crustal roots. Density models show that ∼85 kg/m3 lighter crust or ∼35 kg/m3 lighter mantle than that of the surrounding Basin and Range helps these metamorphic core complexes maintain their high elevation. Compositional modeling of intrusive bodies exposed throughout the Catalina–Rincon metamorphic core complex indicates that the observed high Vp/Vs ratio and modeled low density could result from substantial amounts of a plagioclase-rich, quartz-poor rock. These Vp/Vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant Tertiary extension may have been preconditioned for orogenic collapse. [Copyright &y& Elsevier]