escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). The Western Escarpment straddles strong north-south and east-west precipitation gradients, which we exploit in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. The Western Escarpment reflects an ongoing transient response to major tectonic perturbation in the Late Miocene when surface uplift created a steep ramp separating drainage systems above the growing escarpment from those below. Upstream of this ramp, and now forming the uplifted Meseta/Altiplano (~3000 m a.s.l.), streams remain graded to the Late Miocene base level constituted by a series of Tertiary volcanic-volcanoclastic rocks, Streams below the ramp have responded to the Late Miocene surface uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean.
We find that the Tertiary units function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. Upstream of these knickzones, on the Meseta/Altiplano, 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky–1 irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky-1 to 250 mm ky-1 and correlate positively with precipitation rates, but show no significant correlation with hillslope angles or channel steepness. These relationships are explained by the presence of the cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions. In addition, valley width and the extent of dissection both appear to increase with increasing precipitation to the north, consistent with climate-forcing of landscape morphology in the deeply incised terrain beneath the escarpment edge.
Since escarpment retreat and the precipitation pattern were established at least in the Miocene, we speculate that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau.
John Wiley & Sons, 2011. Vol. 36, 1464-1473 p.