Miocene development of alpine glacial relief in the Patagonian Andes, as revealed by low-temperature thermochronometry

Abstract

Apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the Andes. Central Patagonia offers a unique location to study the feedbacks between long-term climate, topography, and erosion due to the high relief and well-resolved mid-latitude glacial history. New apatite (U-Th)/He ages from two vertical transects and two 4 He/ 3 He release spectra in the fjord network around 47ºS reveal fast cooling (15-30 ºC/Ma) from ~10 to 5 Ma. Samples currently at the surface cooled below ~35 ºC by ~5 Ma, indicating slow cooling and little erosion in those regions since 5 Ma. We show that these very low-temperature thermochronometric data are useful indicators of changes in topography, and insensitive to deep thermal processes, such as migration of the Chile triple junction. Map-based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change. Comparisons to predicted cooling ages and rates indicate that our new apatite He data are most consistent with a pulse of early glacial incision, with much of the observed valley relief in Patagonia carved between 10 and 5 Ma. Early onset of glaciation in Patagonia is supported by glacial till with bracketing ages of 7.4 and 5 Ma. We therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 Ma is likely due to an increase in valley relief coinciding with these early glaciations in the Andes. In other glaciated areas at lower latitudes, studies have found a dramatic increase in valley relief at ~1 Ma. This timing has generated the idea that incision of glacial valleys may be related to the mid-Pleistocene transition, when the global glacial cycle changed from 40 to 100 ka periods. Our results from a higher latitude indicate an alternative, that glacial valleys incised rapidly after the onset of alpine glaciation.

The Northwestern Greenland Ice Sheet During The Early Pleistocene Was Similar To Today

“…The lack of AHe ages <150 Ma suggests that the diamict was sourced from areas that underwent little Pliocene or Early Pleistocene incision, although a large‐ n detrital analysis is necessary to resolve potential source areas. Sediment derived from deep erosion of fjords would yield AHe ages younger than known thermal events, similar to northeast Greenland (Bernard et al, ) or the Patagonian Ice Sheet incision of the Andes (Christeleit et al, ). Such sediment sources may include the higher topographic levels of present day fjords and intervening ridges, and/or the low‐relief plateau areas within the Greenland continental margin (Figure b).…”

Tectonic controls of Holocene erosion in a glaciated orogen

“…The balance between post-glacial erosion rates and longer-lived rock uplift rates depends on whether post-glacial climate conditions (e.g., increase or decrease in precipitation) or topographic perturbations (e.g., hillslope steeping or channel shallowing) have changed the activity of extant surface processes. There are many examples of ranges where there have been significant changes to topography during glaciation (Montgomery, 2001;Hassan, 2006, 2007;Brocklehurst and Whipple, 2007;Robl et al, 2008;Hobley et al, 2010;Glotzbach et al, 2013), and erosion during and after glaciation (Reiners et al, 2003;Moon et al, 2011;Christeleit et al, 2017), and others where such changes are not clearly observed (Thomson et al, 2010). More generally, these changes were explored in a coupled ice dynamiclandscape evolution model testing the modification of topography and erosion rates due to alpine glaciation (Yanites and Ehlers, 2012).…”

“…These glaciers possessed variable capacity to erode at the same rate as the rivers that existed before them and regional rock uplift rates. In many mountain ranges, glaciers appear to have accelerated erosion (Hallet et al, 1996;Shuster et al, 2005;Ehlers et al, 2006;Valla et al, 2011;Herman et al, 2013;Christeleit et al, 2017;Michel et al, 2018), while in other areas, glaciers do not appear to have changed erosion rates over the past few million years (Koppes and Montgomery, 2009;Thomson et al, 2010;Willenbring and von Blanckenburg, 2010).…”

U–Th and &lt;sup&gt;10&lt;/sup&gt;Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

“…The results of the numerical experiments of Kaplan et al (2009), regarding the extent of ice related to the elevation of the accumulation area and the depth of the overdeepening, suggest that this overdeepening was most probably already carved before 1 Ma. Recently, Christeleit et al (2017) argued using thermochronometric data that the relief of Patagonia at the latitudes of our study was probably achieved between 10 and 5 Ma. On that basis, one can assume that the recycling time probably remained constant over the period covered by the samples.…”