DONALD J. EASTERBROOK DISTINGUISHED SCIENTIST AWARD LECTURE: TECTONIC GEOMORPHOLOGY OF COLLISIONAL OROGENS

Quantitative insights on surface processes at the scale of channels, hillslopes, and soils have multiplied over the past few decades. The far greater spatial and temporal scales of active collisional orogens like the Himalaya, Southern Alps, or Central Range of Taiwan typically thwart straightforward applications of geomorphic process rules and require broader, more holistic analytical approaches. The proliferation of digital topographic data has permitted characterization of topographic attributes of whole orogens and has encouraged estimates of spatial variations in erosion based on stream power, relief, channel steepness, and rainfall. Increasing numbers of studies have linked climate to erosion rates and then to orogenic deformation. This talk will focus on several of the unresolved issues at large topographic scales and millennial to million-year time scales, including the importance of orographic precipition, temporal and spatial changes in rates of erosion, the relative influence of tectonic versus climatic forcing on erosion rates, the magnitude and effect of glacial erosion and ice loading/unloading, and the likelihood of topographic steady state. In large orogens, orographic precipitation typically displays profound spatial gradients in rainfall that compromise analyses that use catchment area as a proxy for discharge. Rainfall-based estimates of specific stream power, however, show strong correlations with millennial-scale hillslope and fluvial erosion rates. The retracted and still-shrinking glaciers of the current interglaciation inhibit reliable estimates of relevant erosion rates during full glacial expansions. Although correlations of modern rainfall with long-term erosion rates suggest an appealing causal linkage, some side-by-side orogens with similar climate, rock strength, and topography display order-of-magnitude differences in erosion rates that suggest tectonic forcing is the dominant control. Despite our inability to scale up to orogens from meso-scale observations of channels and hillslopes, broad syntheses of landscape characteristics with both short-and long-term erosion rates and with climate changes are yielding new insights on the controls and evolution of the Earth’s most striking landforms.