Anomalously high relief on Denali, Alaska, caused by tectonic, lithologic, and climatic drivers

Abstract

We assess the growth of anomalously high relief on Denali, located in the Alaska Range, Alaska, and the tallest mountain in North America (6190 masl). Denali is 3000 m taller than most surrounding peaks. It lies inside a 19° restraining bend in the active Denali fault system that is moving at about 7 mm/yr, providing a tectonic and structural driver for ongoing rock uplift. High relief around Denali is also due, in part, to its granitic rock type and low fracture density relative to adjacent metasediments. Here we show that unique climatic conditions at high elevations also contribute to the growth of relief. We examine ¹⁰Be concentrations in 1) three new gravel samples between 3500 and 5200 m elevation from sites unaffected by recent glacial erosion, 2) previously published samples from a sidewall of the Kahiltna Glacier from 2400 to 2800 masl, 3) previously published data for samples collected from medial moraines along the length of the Kahiltna Glacier from ∼500 to 1400 masl, and 4) previously published data for alluvial samples collected along the Kahiltna River at an elevation of ∼200 masl. These samples constitute a transect extending >5000 vertical meters, and the data establish that erosion rates decrease with elevation and contribute to the growth of relief. Erosion rates for the three new high-elevation samples are calculated to 4.6 ± 0.6 mm/ka at 5200 masl, 28.6 ± 3.7 mm/ka at 4000 masl, and 38±5 mm/ka at 3500 masl. Erosion rates at intermediate elevations, on the sidewall of the Kahiltna Glacier, range between 160 and 327 mm/ka. Along the medial moraines inferred erosion rates range between 140 and 537 mm/ka, and basin-wide erosion rates calculated from sediments in the river below the glacier range between 450 and 896 mm/ka. These differences in erosion rates can create relief of 3 km within 1–10 Ma, well within the estimated period of increase in rock uplift and exhumation on Denali over the last ∼6 Ma. Meteorological data from 2130 to 5550 masl at 5 sites show temperatures rarely exceed freezing above 4000 masl elevation, indicating that frost weathering currently plays a diminished role in erosion at high elevations. The immediate implication of this temperature and erosional correlation is an increase in relief. This is the first study to directly measure a significant decrease in erosion rates at high elevations in the relative absence of frost weathering. The results highlight the combined influence of rock type, glacial erosion, and permanent sub-zero temperatures on erosion rates. In combination with active faulting, the data explain the resultant increase in relief along the southern side of the Alaska Range over the past 100 ka.