EROSION OF AN ACTIVE FAULT SCARP LEADS TO DRAINAGE CAPTURE IN THE AMAZON REGION, BRAZIL

Far from the continental margin, drainage basins in Central Amazonia should be in steady state but are not. Abandoned remnant fluvial valleys up to hundreds of square kilometers in size are observed throughout Amazonia and are evidence of significant landscape reorganization. While major drainage shifts occurred in the Late Miocene due to initiation of the transcontinental Amazon River, local landscape reorganization has remained active until the late Quaternary. Driven either by dynamic topography, tectonism, and/or climatic fluctuations, landscape reorganizations in Amazonia provide a natural experiment for assessing the geomorphic response of low-slope basins to sudden, capture related base-level falls. This paper evaluates the timing of reorganization by examining an across-scarp drainage capture event involving the Cuieiras and Tarumã-Mirim river basins northwest of the city of Manaus in the Amazon State, Brazil. A system of capture related knickpoints were generated by base-level fall following drainage capture. Through numerical modeling of knickpoint initiation and propagation, the capture event is identified to have occurred between the Middle and Late Pleistocene, consistent with other studies of landscape reorganization in surrounding areas. Results herein suggest base-level fall can increase erosion rates in low slope settings like the Amazon by more than an order of magnitude in the active portion of the basin. The analysis further reveals that even in a low relief setting such as the Amazon, 1000 km² basins respond to base-level fall over timescales of 10⁴ – 10⁵ yrs. In addition to numerical modeling, geomorphological and structural findings bear on the geologic elements active in the Neogene, as well as landscape reorganization of the lower Rio Negro, and on the geomorphic response of basins to base-level fall.