IGPP Seminar Series

Transient Landscape Evolution and Development of High Topography in Convergent Orogens

by Dr. Michael E. Oskin
University of North Carolina at Chapel Hill

Abstract

Development of high topography in crystalline basement rocks is fundamental to understanding dynamic equilibrium between shortening and denudation of convergent orogens. Exposure of basement during mountain range growth suppresses erosion rates and initiates a phase of transient landscape evolution as topography rises. Dynamic equilibrium may be reestablished at a new, higher mean elevation via recovery of erosion rates or by migration of deformation elsewhere in the orogen. The Kyrgyz Range, located on the northern margin of the western Tien Shan, illustrates long-term 106- to 107-year transient landscape evolution in response to exposure of erosionally resistant Paleozoic basement rocks in the hanging wall of an active reverse fault. Eastward propagation of the reverse fault tip progressively initiated exposure of basement capped by a tilted, planar, pre-Cenozoic unconformity. The resulting range-scale topography is examined in the context of a space for time substitution where systematic along-strike spatial topographic patterns inform understanding of the temporal development of geomorphology at a site within the range. The easternmost, least topographically developed portion of the Kyrgyz Range is dominated by surface uplift that rapidly builds elevation. An analytic model based on the stream power-law predicts transient river profile incision into progressively exposed basement rock and quantifies preservation of the pre-Cenozoic unconformity as a marker surface in the landscape. Initial channel slope and transient channel form are found to be sensitive to the degree of non-linearity of the stream power-law and permit field calibration of model parameters. Glacial erosion overprints the landscape as the range crest crosses the Pleistocene glacial equilibrium line altitude. The pattern of glacial erosion with respect to the reconstructed unconformity surface reveals that cirque retreat dominates and begins to limit peak elevations. Mean elevation of the Kyrgyz Range stabilizes as erosion by glaciers and glacially-charged fluvial systems balances rock uplift rate. The final stage of transient landscape evolution occurs through amalgamation of adjacent catchments via competitive fluvial incision and stream piracy. This process results in higher peak elevations and deeper canyons but does not substantially change mean range elevation. By limiting peak elevations and modifying fluvial systems, alpine glacial erosion emerges as a critical process for the attainment of dynamic equilibrium between shortening and denudation in the Tien Shan. Higher equilibrium line altitude and higher convergence rates are predicted to promote growth of higher topography and/or propagation of shortening into the adjacent foreland.
Tuesday, 02 May 2006
3845 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM