Incision history of the Verde Valley region and implications for uplift of the Colorado Plateau (central Arizona)

The record of Tertiary landscape evolution preserved in Arizona's transition zone presents an independent opportunity to constrain the timing of Colorado Plateau uplift and incision. We study this record of landscape evolution by mapping Tertiary sediments, volcanic deposits, and the erosional unconformity at their base, ⁴⁰Ar/³⁹Ar dating of basaltic lava flows in key locations, and constructing geological cross sections along canyons to restore the paleorelief on the Tertiary erosional unconformity to test whether canyon incision requires young (< 10 Ma) Colorado Plateau uplift. Our cross sections and new 40Ar/39Ar ages document that in the Verde Valley, relief across the ancestral Mogollon Rim that marks the southern edge of the Colorado Plateau was up to 1000 m and averaged ~700 m in the Early-Middle Miocene, which is close to average modern relief of ~800-1000 m. Although Middle-Late Miocene volcanics and sediments in places onlap the ancestral Mogollon Rim, suggesting an erosional origin, northeastward erosional retreat of an earlier tectonic escarpment is both plausible and consistent with displacement histories of the Grand Wash fault to the NW and the Diamond Rim fault to the SE. Interestingly, the coincidence of a rugged, sharply defined Mogollon Rim and a wide bench cut into the Hermit shale below the escarpment today suggests that exposure of the Hermit shale may play an important role in cliffretreat and the morphological expression of the Mogollon Rim in the Verde Valley region. Some modern canyons cut into the retreating escarpment reflect re-excavation, deepening, and headward propagation of Miocene paleochannels largely buried by Middle-Late Miocene basalts. Despite evidence for similar total paleorelief, most canyons show significant (~35%) deepening since 5 Ma with young incision decreasing to the SE. Incision rates during the period of active Miocene volcanism were below 20 m/m.y. but accelerated to 50-80 m/m.y. during the past 5-8 m.y. and were probably 100-160 m/m.y. during the Quaternary. The accelerated incision reflects base-level fall associated with breaching of the Verde Lake basin by ca. 2.5 Ma and integration of the Verde River and thus does not require post-Middle Miocene uplift of the southwestern edge of the Colorado Plateau.