An assessment of competing factors for fluvial incision: An example of the late Quaternary exorheic Moyobamba basin, Peruvian Subandes

Abstract

Three competing factors for late Quaternary fluvial incision in the Moyobamba piggyback basin, located in the northeastern Peruvian Subandes, were investigated, namely base-level change as a result of basin opening, changes in the ratio sediment load:discharge (Qs: Qw) controlled by orbital and suborbital climate cyclicity, and thrust-related tectonic uplift. To assess the relative importance of these factors, the fluvial terrace staircases of five rivers were studied in terms of their sedimentology, stratigraphy and geomorphology. A new dataset of eight Optically Stimulated Luminescence (OSL) and four 14C ages shows that the final phase of endorheic alluvial fan aggradation lasted until 18.0 ± 1.4 ka, after which basin-wide incision occurred. Incision at the basin outlet as a result of adaptation to the new base level is decoupled from the observed incision in the middle and upper parts of the basin. There, incision is the result of a northwestward increase in uplift related to movements of the Cerro Blanco thrust at the western border of the Moyobamba basin. Incision was continuous from 18 ± 1.4 ka until 10.3 ± 0.7 ka without terrace formation, but between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka up to four terrace levels were formed. Deposition of fluvial terrace sediments may have occurred during short periods characterised by a more intense South American Summer Monsoon (SASM) controlled by suborbital climate cyclicity, but a more precise age control is needed to confirm this theory. Precession-controlled, increased Holocene aridification between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka may have resulted in a decrease in the ratio Qs: Qw leading to a five-fold increase in fluvial incision rates of up to 6.0 mm a−1 and flattening of terrace profile gradients with rates of up to 2.6 mm a−1. After 5.5 ± 0.4 ka, rates dropped again. Over longer timescales, movement of the Cerro Blanco thrust caused basin-wide uplift and fluvial incision at a more constant pace. During the past ~20 ka, incision increases from the southwest basin border towards the northwestern basin border with corresponding uplift rates increasing from 0.2−0.2+0.3 mm a−1 to 2.1−0.2+0.2 mm a−1; and with shortening rates increasing northwestward from 0.2−0.2+0.4 mm a−1 to 2.8−0.2+0.3 mm a−1. The highest, presented uplift rates are in agreement with independently published data for our study area. Our data shows that over longer timescales of ~20 ka, incision rates can be used as proxies for tectonic uplift, but over shorter timescales incision rates may be seriously affected by climate change and cannot be used to infer uplift rates. It is further suggested that continuous uplift and erosion of the Cerro Blanco thrust system, and subsequent sedimentation in the basin, in combination with increased precipitation during Heinrich events 1 and 2 and the Last Glacial Maximum (LGM), led to basin overflow. Simultaneously, river capture through headward erosion, guided by strike-slip faulting, may have led to the definitive opening up of the Moyobamba basin. To our knowledge, this is the first time that late Quaternary basin opening has been demonstrated for the South American Andes and one of the few, known cases worldwide.