On the mechanisms resulting in the formation of the Quaternary staircase terrace systems in the valleys of the western Andean margin of Peru

Schlunegger, Fritz; do Prado, Ariel H.; Norton, Kevin P.; Delunel, Romain (2023). On the mechanisms resulting in the formation of the Quaternary staircase terrace systems in the valleys of the western Andean margin of Peru. Geomorphology, 442, p. 108923. Elsevier Science 10.1016/j.geomorph.2023.108923

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River terraces are among the most intriguing geomorphic features that have been reported from the Peruvian Andes hosting some of the most classical staircase terrace systems. Here, we explore how the large-scale tectonic architecture of the Andes in combination with cyclic variations of climate has impacted the generation and routing of sediment and the formation of these terrace systems. We invoke a hydroclimatic and geomorphic control where the landscape steepness and precipitation rates need to exceed a threshold for the release of sediment stored on the hillslopes, so that aggradation begins downstream in the valley. Once the sediment reservoirs on the hillslopes are void of material, the trunk streams start to recycle the previously deposited material, thereby forming a terrace level. Several of such out-of-phase cyclicities between water and sediment discharge have occurred during the Quaternary, and they were synchronous with the glacial periods at the global scale. Most of these climate shifts in Peru are explained by orbitally controlled changes in the solar insolation of the southern American continent and the Altiplano, which impacted the strength of the Andean jet. On its way towards the south, the jet becomes diverted towards the SE and thus away from the Andes where it intercepts with the mountainous topography of the Bolivian Orocline on the eastern flank of the Andes. These are also the latitudes where we mapped the thickest terrace deposits and the highest frequency of terrace occurrence along the Peruvian coast. Accordingly, the large-scale tectonic architecture of the Andes in combination with orbitally controlled variations in insolation has not only influenced the cyclic change in the strength of the Andean jet, but it has also conditioned the region along the Peruvian margin where the out-of-phase cyclicities between water and sediment discharge was large enough for thick staircase terraces to form.

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