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Tectonic control on the persistence of glacially sculpted topography - PubMed

  • ️Thu Jan 01 2015

Tectonic control on the persistence of glacially sculpted topography

Günther Prasicek et al. Nat Commun. 2015.

Abstract

One of the most fundamental insights for understanding how landscapes evolve is based on determining the extent to which topography was shaped by glaciers or by rivers. More than 10(4) years after the last major glaciation the topography of mountain ranges worldwide remains dominated by characteristic glacial landforms such as U-shaped valleys, but an understanding of the persistence of such landforms is lacking. Here we use digital topographic data to analyse valley shapes at sites worldwide to demonstrate that the persistence of U-shaped valleys is controlled by the erosional response to tectonic forcing. Our findings indicate that glacial topography in Earth's most rapidly uplifting mountain ranges is rapidly replaced by fluvial topography and hence valley forms do not reflect the cumulative action of multiple glacial periods, implying that the classic physiographic signature of glaciated landscapes is best expressed in, and indeed limited by, the extent of relatively low-uplift terrain.

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Figures

Figure 1
Figure 1. Fitting power laws to valley cross-sections.

Power laws of the form y=axb fit to valley flanks with the exponent b depicting their shape. b=1 represents the straight valley flanks of a V-shaped valley and greater exponents are indicative of progressively more U-shaped valleys.

Figure 2
Figure 2. Study area characteristics.

(a) Location of the main study area in Westland and the two reference study areas, Marlborough (fluvial) and Fiordland (glacial) on New Zealand's South Island. (b) Extent of Westland study area (dashed rectangular outline), LGM extent (dotted outline), modern coastline (grey shading), rock uplift rates (solid contour lines), geological map showing metamorphic grade of Alpine schist south of the Alpine Fault, Garnet-Oligoclase zone (1), Biotite zone (2), and Chlorite zone (3) (ref. 24), plotted on a digital terrain model. Bold black rectangles outline the extents of subfigures c and e. Scale bar, 10-km wide. (c) Shaded relief map and an example cross-section (dashed line; X–X′) for strongly glacial, low-uplift terrain in the Westland study area. Scale bar, 1-km wide. (d) Example cross-section (X–X′; solid line) and fitted power law (dashed line, b=1.95). (e) Shaded relief map and example cross-section (dashed line; Y–Y′) of strongly fluvial, high-uplift terrain in the Westland study area. Black polygons delimit current ice cover and are not included in analysis. Scale bar, 1-km wide. (f) Example cross-section (Y–Y'; solid line) and fitted power law (dashed line, b=0.97). Contour spacing in c and e is 500 m, cyan lines show flow path cells for valley cross-section extraction.

Figure 3
Figure 3. Cross-sectional valley shape, turnover time, rock uplift and erosion rates.

(a) Mean power-law exponents for 20 rock uplift bins from Westland (bin size=0.45 mm per year). Regression lines fitted separately to data with a rock uplift rate <6 mm per year (dashed line, y=−0.043 (±0.002)x+1.569(±0.011), R2=0.54, root-mean-squared error (r.m.s.e.)=0.06, P<0.01) and >6 mm per year (solid line, y=0.007 (±0.002)x+1.236(±0.014), R2=0.08, r.m.s.e.=0.04, P=0.4). Reference exponents: fluvial from Marlborough (1.28; solid grey line) and glacial from Fiordland (1.54; dashed grey line). Error bars indicate±1 s.e. See Supplementary Discussion and Supplementary Fig. 1 for further information on data distribution and statistics. (b) Mean power-law exponents of 20 rock uplift bins from Westland plotted against relief turnover time. Turnover time is defined as relief/erosion rate and we assume rock uplift rates equal erosion rates. Power-law exponent increases with turnover time as y=0.001(±<0.001)x+1.251(±<0.001), R2=0.67, r.m.s.e.=0.05, P<0.01. (c) Power-law exponents plotted against erosion rates for Earth's most rapidly uplifting and eroding but previously glaciated landscapes and one landscape with little glacial modification (Taiwan). A gridded erosion rate data set derived from thermochronology that is common to all four regions is used as a rock uplift rate proxy. For study area maps, see Supplementary Figs 3–5.

Figure 4
Figure 4. A global perspective on tectonics and valley shape.

(a) Rock uplift plotted against average relief of some of Earth's major mountain ranges: Washington Cascades, WA, USA (1); Coast Mountains, BC, Canada (2); Fiordland, New Zealand (3); Greater Caucasus, Georgia/Russia (4); Northern Patagonian Andes, Chile (5); European Alps, Austria/Italy/Switzerland (6); Central Range, Taiwan (7); Eastern Himalaya, China (8); Western Himalaya, Pakistan (9); and High-uplift Westland, New Zealand (10). Contours indicate time (kyr) necessary to renew 20% of the relief, the average fraction of relief turnover time required to transform a U-shaped into a V-shaped valley in Westland. The minimum, mean and maximum durations of the last four interglacial periods are 20, 22 and 26 kyr, respectively, indicating that transformation from glacial to fluvial landscape morphology can occur during a single interglacial period for comparable or lower contour values. Power-law exponents indicate the degree of glacial imprint on topography. See Supplementary Fig. 6 for study area maps.

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