Abstract

The Pinaleño Mountains and adjacent Safford Basin are a landscape defined by the extensional tectonics of the Basin and Range physiographic province. However, over the last ∼4 m.y., this coupled basin and range have been actively degrading in the absence of widespread regional extension. While rates of relief generation and upland erosion during active subsidence ca. 12–5 Ma are reflected in the geometry of the basin’s structure and the stratigraphy it contains, rates of post-tectonic landscape evolution from the Pliocene to the present have been heretofore unknown. We combined topographic analyses of the Pinaleño Mountains with cosmogenic nuclide–derived catchment-averaged erosion rates and burial dates of axial and piedmont deposits to quantify rates of post-tectonic landscape evolution and define a chronology for the last stages of deposition and subsequent incision in Safford Basin. In addition to constraining the timing of a deposit’s formation, cosmogenic nuclide burial dates provide paleo-upland erosion rates at the time of deposition. Erosion rates in the Pinaleño Mountains have been generally moderate over the past 4 m.y., ranging between ∼30 and 60 m/m.y. with no strong relationship to the drainage basins’ modern topography. A potential acceleration of erosion rates to 100–250 m/m.y. between 3.5 and 2 Ma correlates with an inferred period of enhanced precipitation as well as the arrival from upstream of the Gila River in Safford Basin sometime shortly before 2.8 Ma. Widespread incision of Safford Basin was under way by ca. 2 Ma, as recorded by the dissection of piedmont basin highstand deposits (Frye Mesa) and two intermediate Gila River terraces on the northeast margin of Safford Basin (dated to 1.8 Ma and 0.64 Ma). Gila River incision rates have ranged from 30 to 60 m/m.y. over the past 3 m.y. Paleo-upland erosion rates and modern millennial-scale upland erosion rates fall within the same range as incision rates of the Gila River in Safford Basin, suggesting that upland erosion rates are predominantly a function of base-level fall driven by axial incision. However, based on similarities between catchment-averaged erosion rates and topography from basins draining into the integrated Safford Basin and the still internally drained Sulphur Springs Basin to the south, it appears that upland erosion rates during the Quaternary are not being driven exclusively by regional incision rates.

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