The effects of lithology on glacial landscape evolution, paced using terrestrial cosmogenic nuclides: Examples from the Colorado Rocky Mountains and the Kichatna Mountains, Alaska Range, Alaska.

摘要

In Chapter 2, we use cosmogenic 10Be exposure ages from polished, striated bedrock to determine deglaciation histories of the Middle Boulder Creek Valley (MBCV), Colorado Front Range, and the Animas River Valley (ARV), San Juan Mountains, Colorado. In both valleys, the cosmogenic ages suggest that deglaciation occurred from 20 ka through 13 ka. We use a 2D numerical glacier simulation to find climate histories that reproduce the spatial 10Be pattern in each valley. In the ARV, exposure ages are well-explained by a continuous ELA rise throughout deglaciation. Ages in MBCV suggest a hiatus in deglaciation between 16 and 14 ka, followed by rapid retreat.;In Chapter 3, we use cosmogenic 10Be to measure erosion rates of rock walls that tower over deeply-incised valley glaciers. In the granitic Kichatna Mountains, Alaska Range, Alaska, we sampled rockfall debris in medial moraines on each of three glaciers, sourced in areas with identical rock and similar relief of ~1 km. Two of the glaciers yielded similar 10Be concentrations and sidewall erosion rates (~0.5-0.7 mm/yr). The largest glacier of the three indicated the highest sidewall erosion rates (1.3 mm/yr). These rates are reasonable in an alpine glacial setting, and are much faster than long-term exhumation rates of the western Alaska Range.;In Chapter 4, we exploit the Denali (Mt. McKinley) massif and the Kichatna Mountains to illustrate how exhumation of Tertiary granite plutons affects glacial and periglacial erosion in the Alaska Range. Field observations, remote sensing, and digital elevation data indicate that glacial incision is less efficient on the granite. Steep valley walls, maintained by sheeting joints in granite, act as Teflon, shedding snow and enhancing the modern glaciers below. During glaciations, mass is removed efficiently from the surrounding sedimentary landscape, isostatically raising the granitic massifs. These effects can generate very high relief by enhancing glacial erosion in the valleys while preserving the peaks.