Abstract

Provenance analysis of Pleistocene terrace deposits, together with modern sediments from the same streams, from four catchments draining the western margin of the Andes in Peru is used to infer changes in erosion patterns between the past and the present period by matching detrital zircon ages with crystallization ages of source rocks. Age populations suggest major changes in sediment provenance through the past 100 k.y. At present, sediment sources are mainly located along the steep middle reaches of the rivers, whereas during the Pleistocene, sources were additionally located in the low-relief headwaters of these catchments. These shifts in the loci of erosion are interpreted to reflect changes in precipitation patterns, where periods of stronger precipitation on the Altiplano allowed the entrainment of material from the low-relief plateau in the past. In contrast, modern precipitation patterns result in negligible erosion rates on the Altiplano, and the site of material entrainment shifts to the knickzone reaches where steeper slopes and higher stream power promote erosion. In that sense, this work illustrates that terrace aggradation is associated with major shifts in provenance sources.

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