Post-tectonic landscape evolution of a coupled basin and range: Pinaleño Mountains and Safford Basin, southeastern Arizona

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 catchmentaveraged 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 deposi tion 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 baselevel fall driven by axial incision. However, based on similarities between catchmentaveraged 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.

Transverse canyon incision and sedimentary basin excavation driven by drainage integration, Aravaipa Creek, AZ, USA

“…In Safford Basin most paleo‐erosion rates inferred from burial dating of basin fill average ~30 m/Myr. Paleo‐erosion rates from two Pliocene/Pleistocene terraces of the Gila River in Safford Basin are between 3 and 20 m/Myr (Jungers and Heimsath, ), which agrees well with this inferred minimum long‐term average of Aravaipa Creek.…”

“…Our burial dates from samples (AZ56 series) ~10 m below the surface of Rattlesnake Mesa thus represent a maximum age constraint for the final stages of deposition and the onset of basin‐wide incision. Burial dates for AZ56C and AZ56G are 2.92 ± 3.5 Myr and 3.01 ± 3.4 Myr (1σ uncertainty), respectively (Figure ), suggesting the final stages of aggradation in Aravaipa Creek basin was about 3 Myr, roughly correlative to similar regional deposits such as Frye Mesa in the Safford Basin to the northeast and the Martinez surface in Sonoita Creek Basin to the southwest (Menges and McFadden, ; Jungers and Heimsath, ). The imprecision on these two burial dates (AZ56C and AZ56G) is the result of higher than ideal concentrations of native Al in each sample, and very low 26 Al concentrations leading to poor analytical precision during AMS measurements.…”

“…In Safford Basin to the east of Aravaipa Creek Basin (Figure ), the sedimentary basin is as deep as 4600 m near the Pinaleño Mountains’ range front (Houser et al, ). Given tectonic quiescence by 2–3 Ma (Jungers and Heimsath, ), we can infer a maximum subsidence rate of 920 m/Myr if extension of the Safford Basin half graben initiated late – i.e. 8 Ma – during the Basin and Range Disturbance.…”

“…From 2.8 Myr to 1.3 Myr, the Gila River incised 30 m at a rate of 20 m/Myr. Incision accelerated to 60 m/Myr from 1.3 Myr to 0.64 Myr, and then slowed again to 40 m/Myr from the Mid‐Pleistocene to the present (Jungers and Heimsath, ). One structural basin upstream of Safford Basin in the Duncan‐Clifton Basin, the Gila River apparently incised twice as rapidly from 0.64 Myr (Lava Creek B Ash) to the present at a rate of 80 m/Myr (Dethier, ).…”

“…Previous work inferred a rough correlation between all basin high stand remnants and the Martinez Surface of the Sonoita Basin (Figure ) dated to ~ 3 Ma by paleomagnetism (Menges and McFadden, ; Morrison, ). Recent work by Jungers and Heimsath () in Safford Basin, just northeast of Aravaipa Creek Basin, suggests such a correlation is not so simple considering 2 Ma burial dates of latest stage basin fill in Frye Mesa (Figure ).…”