Erosion rates and mechanisms of knickzone retreat inferred from ¹⁰Be measured across strong climate gradients on the northern and central Andes Western Escarpment

Abstract

A steep escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between~5°S and~18°S (northern Peru to northern Chile). Strong north-south and east-west precipitation gradients are exploited in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. Late Miocene tectonics uplifted the Meseta/Altiplano plateau (~3000 m a.s.l.), which is underlain by a series of Tertiary volcanic-volcanoclastic rocks. Streams on this plateau remain graded to the Late Miocene base level. Below the rim of the Meseta, streams have responded to this ramp uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean.It is found that the Tertiary units on the plateau function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average~10 mm ky −1 on the Meseta/Altiplano, irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky −1 to 250 mm ky −1 and correlate positively with precipitation rates, but show no strong correlation with hillslope angles or channel steepness. These relationships are explained by the presence of a cap-rock and climatedriven fluvial incision that steepens hillslopes to near-threshold conditions.Since escarpment retreat and the precipitation pattern were established at least in the Miocene, it is speculated that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau.

Slow Long‐Term Exhumation of the West Central Andean Plate Boundary, Chile

“…While these rates may appear fast compared to other rates from cosmogenic isotopes reported in the west central Andes (e.g., Abbühl et al, 2010, 2011; Carretier et al, 2015; Kober et al, 2007, 2009; Placzek et al, 2010; Starke et al, 2017), we note that the rates reported here are averages over long timescales of 10 7  years and could contain within them punctuated periods of activity with long periods of little exhumation. Also, though 3 χ 2 acceptable maximum values rise to ~0.8 km/Myr in the PC (Figure 10), these changes occur on shorter timescales and earlier in the mineral cooling histories where apparent exhumation is a less trustworthy indicator for mean regional exhumation or erosion.…”

“…More recent, shorter timescale denudation rates (<~10 6  years) based on catchment‐averaged 10 Be, 26 Al, and 21 Ne cosmogenic nuclide concentrations and exposure ages have been reported, as low as <1 m/Myr, up to several orders of magnitude lower than bedrock thermochronometric exhumation rates in the southern Peruvian and northern Chilean fore arc but range up to 300 m/Myr (~0.3 km/Myr) at higher elevations and slopes, particularly in the modern‐day Precordillera (Abbühl et al, 2010, 2011; Carretier, Regard, Vassallo, Aguilar, et al, 2015; Carretier, Regard, Vassallo, Martinod, et al, 2015; Kober et al, 2007, 2009; McPhillips et al, 2013; Placzek et al, 2010; Starke et al, 2017). Generally, higher rates are reported in southern Peru than in northern Chile but correlations with precipitation or hillslope angles are inconsistent (Abbühl et al, 2011; Kober et al, 2009; Reber et al, 2017; Starke et al, 2017) and extreme climatic events (e.g., El Niño) may be important (Abbühl et al, 2010).…”

“…Generally, higher rates are reported in southern Peru than in northern Chile but correlations with precipitation or hillslope angles are inconsistent (Abbühl et al, 2011; Kober et al, 2009; Reber et al, 2017; Starke et al, 2017) and extreme climatic events (e.g., El Niño) may be important (Abbühl et al, 2010). While local erosion rates on hyperarid surfaces in the Central Depression of northern Chile may be negligible on millennial and even million‐year timescales (Dunai et al, 2005; Kober et al, 2007), intermittent hyperaridity (e.g., Jordan et al, 2014) and the wide range of local and catchment‐averaged erosion rates in the fore arc indicated by these methods is consistent with long‐term semiaridity to aridity suggested by mean exhumation rates on low‐temperature thermochronometry timescales.…”

Possible threshold controls on sediment grain properties of Peruvian coastal river basins

“…(g) Mean annual water discharge (Reber et al, 2017). ter reaches on the Altiplano, where the flat landscape experiences nearly zero erosion, as 10 Be-based denudation rate estimates (Abbühl et al, 2011) and provenance tracing have shown .…”

“…These inferences have been made based on concentrations of in situ cosmogenic 10 Be measured in river-borne quartz (Abbühl et al, 2011;Carretier et al, 2015;Reber et al, 2017) and on morphometric analyses of the western Andean landscape . Accordingly, erosion along the western Peruvian Andes has been related to either the occurrence of earthquakes and thus tectonic processes (McPhillips et al, 2014) or rainfall rates (Abbühl et al, 2011;Carretier et al, 2015) and to the stream's mean annual run-off and thus climatic processes (Reber et al, 2017). Therefore, we hypothesize that hillslope erosion paired with the stream runoff is likely to have a measurable impact on the grain size pattern in the Peruvian streams.…”