Numerical modelling of glacial landscape response to tectonic and climatic forcing

Glacial erosion represents a potentially crucial coupling between tectonic processes and climate change. However the role of glaciers in this context remains little explored. Previous digital topographic analyses have suggested that the response of glaciers to tectonic forcing is strongly dependent on glacier size; large glaciers behave as an effective glacial "buzzsaw", while small glaciers behave much like rivers and steepen in response to rock uplift. A second striking feature is the development of tall cirque headwalls in association with rapid rock uplift. Numerical modelling of glacial longitudinal profile evolution allows us to explore landscape response to tectonics under a variety of different climate scenarios. We explore different rates of uniform rock uplift, and tilting scenarios with rock uplift rate either increasing or decreasing linearly along the profile. Climatic forcing is based on temperature, either uniform temperature or simplified 100kyr glacial cycles. The initial condition is a concave profile, typical of a fluvial landscape, with a short plateau reach at its head that acts as a source of windblown snow. In all circumstances a uniform climate results in a steady decrease in glacier size, as found in previous studies. Both uniform and variable temperatures result in headwall lengthening, but the valley floor response is markedly different. Uniform temperature efficiently creates a cirque form, with a shallow downvalley gradient. Under warmer conditions, the cirque glacier only lowers the upper part of the profile, whereas the valley glacier that forms under cooler conditions flattens most of the valley floor. Variable temperature causes erosion along a greater proportion of the valley floor for a given mean temperature, but less dramatic decreases in downvalley gradient. Changes in the imposed tectonic regime are weakly reflected in the valley profile, but strongly influence headwall relief. These results, and their consistency with field observations, emphasise the potential for substantial landscape modifications by glaciers that are strongly dependent on the details of both tectonic and climatic forcing.