LONG-TERM INCISION HISTORY OF THE GRAND CANYON INFERRED FROM THE PATTERN OF SHORT-TERM EROSION AND INCISION RATES

Variable erodibility of rocks in and around the Grand Canyon complicates interpretation of its incision history. While a canyon set into a plateau suggests base-level fall rate acceleration, numerical models show similar landforms can evolve under steady base-level fall as the result of rock strength contrast with the canyon formed in stronger rock. The Grand Canyon sequence has erodible post-Permian rocks above the canyon and much less erodible Permian and older rocks exposed below the canyon rim. We ask whether this contrast in rock strength can explain the form of Grand Canyon, or whether an acceleration of incision is required. The pattern of erosion rate differs if incision has accelerated. In acceleration, erosion rate is higher within the canyon while the older, slower erosion rate is maintained in headwater basins that persist above a low erosion rate bench that forms at the contact. For the case where incision has not accelerated, erosion rates in the canyon and in headwaters are the same. Incision rates measured (from terraces 10⁵-10⁶ years old) within the canyon (~150 m/Ma) and in the nearby Grand Staircase (75 m/Ma) suggest that long term base level fall rate has increased. Erosion rates calculated from millennial-scale cosmogenic ¹⁰Be within the Grand Canyon are similar to local incision rate measurements. However, the cosmogenic erosion rates in the Grand Staircase show high erosion rates (~360 m/Ma), above the base level fall rate estimate (75 m/Ma) for the area. This mismatch between cosmogenic erosion rates and long-term base level fall is explained by the undermining of strong rock as weaker underlying rocks erode. During undermining, cliffs form and retreat, inducing locally rapid erosion and producing cliff-and-bench topography. We show that the ratio of cliff erosion rate to base level fall rate is dependent on the ratio of rock strengths and dip angle. Thus, the local high erosion rate can be predicted from rock strength proxies and can be significantly higher than base level fall rate. In the Grand Staircase, quartz is mostly sourced from cliffs and thus cosmogenic erosion rates reflect the high erosion rates along cliffs and not the base-level fall rate, explaining the high erosion rates computed from ¹⁰Be in quartz. Evidence points to an important role of accelerated incision in the Grand Canyon.