Thermochronometry Reveals Headward Propagation of Erosion in an Alpine Landscape

Abstract

Glacial erosion of mountain ranges produces spectacular alpine landscapes and, by linking climate with tectonics, influences a broad array of geophysical phenomena. Although the resultant landforms are easily identified, the timing and spatial pattern of topographic adjustment to Pleistocene glaciations remain poorly known. We investigated topographic evolution in the archetypal glacial landscape of Fiordland, New Zealand, using (U-Th)/He thermochronometry. We find that erosion during the past 2 million years removed the entire pre-Pleistocene landscape and fundamentally reshaped the topography. Erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins, a behavior expected if subglacial erosion rate depends on ice sliding velocity. The Fiordland landscape illustrates complex effects of climate on Earth's surface morphology.

Thermochronology in Orogenic Systems

“…Exemplary studies have been conducted in the North American Cordillera (Batt et al, 2001;Benowitz et al, 2011;Cecil et al, 2006;Colgan et al, 2008;Farley et al, 2001;Fitzgerald et al, 1995;Hacker et al, 2011;Spotila et al, 2001), the Andes (Barnes et al, 2008;Gunnell et al, 2010;Insel et al, 2010;Schildgen et al, 2007Schildgen et al, , 2009Schildgen et al, , 2010Spikings et al, 2010), the Southern Alps of New Zealand (Batt et al, 2000;Shuster et al, 2011;Kamp, 1993, 1995), and the Himalayan-Tibetan orogenic system (Kirby et al, 2002;Ouimet et al, 2010;Wobus et al, 2008;Zeitler et al, 2001).…”

Glacial buzzcutting limits the height of tropical mountains

“…Furthermore, most mountain ranges affected by Pleistocene glaciation have a hypsometric maximum between the upper and lower bounds of ELA fluctuation, much in the way that Brozović et al (1997) first described the glacial buzzsaw's signature in the Karakoram. It should also be noted that thermochronometric data support the 20 coincidence of a rapid shift in exhumation rate and the onset of Pleistocene glaciation in many places that bear the glacial buzzsaw signature (Thomson et al, 2010;Shuster, 2011;Herman et al, 2013;Fox et al, 2015), thus adding confidence that, at least in some places, the topographic signal of buzzcutting is matched by an expected increase in exhumation rate coincident with Pleistocene glaciation. In summary, the glacial buzzsaw hypothesis is presently supported by a topographic signature that can be found on a global scale, 25…”

“…The signature of the glacial buzzsaw is a hypsometric maximum within the bounds of Pleistocene ELA fluctuation, and on the scale of the entire Talamanca and Central Range, there is very little area near the lower limit of ELA fluctuation. However, a hypsometric maximum near the ELA is thought to arise from headward cirque propagation (Brocklehurst & Whipple, 2004;Oskin & Burbank, 2005;Shuster et al, 2011;Gran-20 Mitchell & Humphries, 2015), and since this process acts on the scale of a first-order glacial catchment, a hypsometric maximum near the ELA for a large area (~1000 km 2 ) presumably records the aggregate of many individual valleys with hypsometric maxima near the ELA. Our first goal was to assess whether the glacial buzzsaw signature was recorded in individual glaciated valleys, and to do this we implemented focused hypsometric analysis.…”

Zircon 4He/3He thermochronometry

“…These data constrain permissible thermal paths between temperatures of $30 and 90°C, which corresponds to shallow crustal depths of approximately 1-3 km and, therefore, can be related to the evolution of the subsurface thermal field due to km-scale topographic changes (e.g. Schildgen et al, 2010;Shuster et al, 2011). At greater temperatures and depths, 40 Ar/ 39 Ar thermochronometry of K-feldspar via multi-diffusion domain modeling constrains continuous paths between $200 and 350°C, which can be related to tectonically-driven phenomena (e.g.…”