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Abstract The northwest-trending transition zone (TZ) in Arizona (southwestern United States) is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyperextended Basin and Range province to the southwest. The TZ is widely depicted to have been a Late Cretaceous–Paleogene northeast-dipping erosional slope along which Proterozoic rocks were denuded but not significantly deformed. Our multi-method thermochronological study (biotite 40Ar/39Ar, zircon and apatite [U-Th-Sm]/He, and apatite fission track) of Proterozoic rocks in the Bradshaw Mountains of the west-central Arizona TZ reveals relatively rapid cooling (~10 °C/m.y.) from temperatures of >180 °C to <60 °C between ca. 70 and ca. 50 Ma. Given minimal ca. 70–50 Ma upper-crustal shortening in the TZ, we attribute cooling to exhumation driven by northeastward bulldozing of continental lower crust and mantle lithosphere beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (ca. 70–50 Ma) underplating and initial exhumation of Orocopia Schist to the southwest in western Arizona and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ca. 25 Ma TZ volcanic rocks. 

Abstract The sinking of gravitationally unstable lithosphere beneath high-elevation plateaus is proposed to be a key driver of their uplift. Numerical geodynamic models predict that lithosphere removal can lead to transient, dynamic topographic changes that could be preserved in the surface record, particularly in sedimentary deposits of lakes or playas that are subsequently inverted. However, few such examples have been documented. Here we show that the Miocene Bidahochi Basin, which was partially and intermittently filled by the Hopi Paleolake, preserves a record of the quasi-elliptical surface response to a viscous drip of lithosphere >100 km beneath the Colorado Plateau. New detrital zircon U-Pb, Lu-Hf, and trace-element data reveal systematic isotopic, geochemical, temperature andfO₂transitions in magmatism proximal to the basin. Integration of geophysical, geochemical, and geological evidence supports a spatially and temporally varying record of subsidence and uplift that is consistent with models of progressive dripping beneath plateaus with thick lithosphere. We demonstrate that dynamic topography at the scale of individual lithosphere drips can be recognized on the Colorado Plateau, despite the strength of its lithosphere.