POST-TECTONIC LANDSCAPE EVOLUTION OF A COUPLED BASIN AND RANGE: PINALENO MOUNTAINS AND SAFFORD BASIN, SOUTHEASTERN ARIZONA

The Pinaleño Mountains and adjacent Safford basin are a landscape defined by the extensional tectonics of the Basin and Range physiographic province, although currently this coupled basin and range are decaying under post-tectonic conditions. While rates of relief generation and upland erosion during active subsidence ~12-5 Myr ago 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 are heretofore unknown. We combine topographic analyses of the Pinaleño Mountains with ¹⁰Be derived catchment-averaged erosion rates and ²⁶Al/¹⁰Be burial dates (isochron and conventional) of axial and piedmont deposits to quantify rates of post-tectonic landscape evolution and develop 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, our burial dates provide paleo-upland erosion rates at the time of deposition.

Erosion rates in the Pinaleño Mountains have been generally slow over the past 4 Myr, ranging between ~30-60 m/Myr with no strong relationship to drainage basins’ modern topography. A notable acceleration of erosion rates to 100-250 m/Myr between 3.5-2 Myr correlates with an inferred period of enhanced precipitation as well as the arrival of the Gila River in Safford basin sometime shortly before 2.8 Myr. Widespread incision of Safford basin was underway by ~2 Myr as recorded by the dissection of piedmont basin high stand deposits and three Gila River terraces on the northeast margin of Safford basin (dated to 2.8, 1.3, and 0.64 Myr). Gila River incision rates have ranged from 30-60 m/Myr over the past 3 Myr. 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 of the Gila River system.