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Abstract

The evolution of mountain belts is a balance between forces that build topography and those that destroy topography. This study focuses on the destructive forces, weathering and mass wasting, fluvial or eolian erosion, glaciers, etc., that are inherently surfacial processes. In order to understand how orogens change through time, we need to identify and quantify the processes responsible for denudation. We apply a combination of novel and proven techniques to address the question of how these forces modify the landsurface. Morphometric analysis based on high resolution LIDAR data is used to identify transient landscapes. These geomorphic data are then combined with cosmogenic nuclides, 10Be from quartz in sediment specifically, in order to quantify process rates. In a case study in the Alpine Foreland, we identified two adjacent drainage basins which have responded differently to the same forcing event, a fall in stream base-level at 16,000±3,000 yrs BP. One of the watersheds has a coupled hillslope-channel system, while the other is decoupled. Denudation rates in the coupled system, 380±50 mm ky-1, are equal to those in the headwaters of the decoupled system, 380±50 mm ky-1. However denudation rates in the decoupled system increase downstream to 540±100 mm ky-1, with denudation in the incised region occurring at extremely high rates, ~1800 mm ky-1. The denudation rates show that within 16,000 yrs, only the channels themselves have responded, and spatially extensive erosion occurs only after a significantly longer lag time. The advantage of this approach is that the geomorphic response of a landscape to external forces can be identified by morphometric analysis, and the process rates that are measured with cosmogenic nuclides can be used to quantify landscape response times on the time scale of climate change or fault movement.