EROSION AND TECTONICS AT THE MARGINS OF CONTINENTAL PLATEAUS

The topography across the eastern margin of the central Andean plateau north of 18-degrees-S (Beni region) bears a strong resemblance to the topography of the southern margin of the Tibetan plateau (Nepal Himal aya), with both regions featuring a steep frontal slope and high peaks at the plateau edge. In contrast, the topography of the eastern margi n of the central Andean plateau south of 18-degrees-S (Pilcomayo regio n) tapers toward the foreland more gently and has no line of high peak s at the margin. Both the Himalayan and the Beni regions have been the sites for large amounts of denudation, and in both regions, geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward die plateau inte rior during the Neogene. We hypothesize that the steep frontal slope a nd high peaks of the Beni region and Himalayan front largely reflect t he high orographic precipitation and high erosion rates occurring in t hese regions and that the more gentle topography of the semiarid Pilco mayo region reflects a tectonic landform only slightly modified by ero sion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long-wavelen gth slope. A numerical model coupling orographic precipitation, erosio n, and tectonic uplift demonstrates the plausibility of this hypothesi s. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya su ggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent p rior domination of tectonic influx and may reflect a decrease in the r ate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.