Erosion rates driven by channel network incision in the Bolivian Andes

Abstract

The Bolivian Andes flank one of Earth's major topographic features and dominate sediment input into the Amazon Basin. Millennial-scale erosion rates and dominant controls on erosion patterns in this range are poorly known. To define these patterns, we present 48 erosion rate estimates, derived from analysis of in situ 10 Be in quartz-bearing alluvium collected from the Upper Beni River basin.Erosion rates, corrected for the non-uniform distribution of quartz in the sample basins, range from 0·04 mm a ). Hence, our data do not record any significant variation in erosion rate over the last several million years. Mean and modal short-term erosion rates for the Andes are an order of magnitude lower than rates in the Ganges River headwaters in the High Himalaya and an order of magnitude greater than rates typical of the European Alps.In the Upper Beni River region of the Bolivian Andes, short-term, basin-averaged erosion rates correlate with normalized channel steepness index, a metric of relative channel gradient corrected for drainage area. Neither normalized channel steepness index nor basin-averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the Upper Beni River region are at threshold values of slope and local relief. Patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks. Climate and lithology do not appear to exert first-order controls on patterns of basin-averaged erosion rates in the Bolivian Andes.

Drainage reorganization during mountain building in the river system of the Eastern Cordillera of the Colombian Andes

“…Causes and implications of drainage reorganization Second, erodibility differences associated with lithological changes can explain spatial 528 variations in erosion rates (e.g., Safran et al, 2005) and can produce local changes in the 529 organization of drainage. In the Eastern Cordillera of Colombia, the depressions along the 530 main divides are not located on less-resistant lithologies, and they occur indistinctly over 531 shale or sandstone formations, from which we can conclude that the reorganization of the 532 drainage is not controlled by lithology.…”

Migrating deformation in the Central Andes from enhanced orographic rainfall

“…In summary, regardless of the role of glaciers in the headwaters and the mechanisms of uplift , the strongly concave high precipitation reach 24 paired with high channel steepnesses 25 requires a positive crustal feedback response to erosion by crustal advection, isostatic compensation or most likely by a combination of both.…”

“…However, signifi cant correlations exist with rainfall gradients 24 , as predicted by theory 17,20 . Tectonic uplift of rocks can also be inferred in the high rainfall reach based on positive correlations between 10 Be-derived denudation rates and steepnesses of channels 25 . It is not possible to discriminate between geomorphic transience due to fl exural accommodation or steady-state crustal advection based on the stream profi les alone.…”

“…Th e former is unlikely as orthophotos of the Eastern Cordillera show that end moraines are weakly expressed below ~ 3500 metres and are, in some cases, situated well above or within the incised valleys 29 , suggesting that channel incision has not been inhibited 24 . We do not exclude glacial erosion, but high 10 Be-derived denudation rates 25 coupled with the highest frequency of landslides in the orographic rainfall-aff ected reach 30 indicate that fl uvial dissection is the dominant process at least over Holocene timescales.…”

“…Apatite (U-Th ) / He exhumation rates in southern Peru from 60-9 Ma 43 are somewhat higher, averaging ~ 0.01 mm per year. Th e situation on the Eastern Escarpment differs, with exhumation of 0.2 -0.6 mm per year derived from 25-5 Ma apatite fi ssion track ages in the Upper Beni River Basin 44 , and cosmogenic nuclide-derived erosion rates of 0.6 -1.3 mm per year during the Holocene 25 . Apatite fi ssion track lengths also imply an acceleration of cooling at ~ 10 Ma 26 .…”