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

Abstract

The record of Tertiary landscape evolution preserved in Arizona's transition zone presents an independent opportunity to constrain the timing of Colo rado Plateau uplift and incision. We study this record of landscape evolution by mapping Tertiary sediments, volcanic deposits, and the erosional uncon formity at their base, 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 EarlyMiddle Miocene, which is close to average modern relief of ~800-1000 m. Although MiddleLate 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 plausi ble and consistent with displacement histories of the Grand Wash fault to the NW and the Diamond Rim fault to the SE. Interestingly, the coin cidence 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 cliff retreat and the morphological expression of the Mogollon Rim in the Verde Valley region. Some modern can yons cut into the retreating escarpment reflect reexcavation, 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 baselevel 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.

Birth and evolution of the Virgin River fluvial system: ∼1 km of post–5 Ma uplift of the western Colorado Plateau

“…Howard et al (2015) made the case that the CR was graded to near sea level by 4.5 Ma, with major aggradation of Bullhead gravels from 4.5 to 3.5 Ma (Howard et al, 2015;Crow et al, 2016). This reinforces the concept that differential incision along the CR system and major tributaries such as the VR can be used to infer uplift of the CP (Lucchitta, 1979;Karlstrom et al, 2007;Karlstrom et al, 2008) instead of downstream subsidence (Pederson et al, 2002;Ott et al, 2018).…”

Evaluating the Shinumo-Sespe drainage connection: Arguments against the “old” (70–17 Ma) Grand Canyon models for Colorado Plateau drainage evolution

“…16B; Karlstrom et al, 2014Karlstrom et al, , 2017Winn, 2019). The Mogollon Rim and Mogollon Slope defined the south margin (Holm, 2001;Ott et al, 2018), while north-flowing paleorivers such as the paleo-Salt River were disrupted by normal faulting to form internal drainage (Potochnik, 2001).…”

Basin record of a Miocene lithosphere drip beneath the Colorado Plateau

“…However, our observations and models do not resolve the absolute magnitude of such uplift. Existing paleorelief data from the Arizona transition zone 55 and thermometry-based paleoelevation evidence from the Bidahochi Basin 56 do not require (nor do they rule out) significant post-Miocene uplift. With improvements in the past decade in carbonate clumped isotope thermometry instrumentation and calibrations, as well as the development of model-mediated lapse rates and evaporative fractionation corrections using triple oxygen isotope analysis, higher-resolution paleoaltimetry data 57 paired with the evidence presented here could possibly allow the quantification of uplift attributable to individual lithospheric drips.…”