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Climate, Tectonics, and the Morphology of the Andes

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Climate, Tectonics, and the Morphology of the Andes Climate, tectonics, and the morphology of the Andes David R. Montgomery Greg Balco Sean D. Willett Department of Geological Sciences, University of Washington, Seattle 98195-1310, USA ABSTRACT Large-scale topographic analyses show that hemisphere-scale climate variations are a ®rst-order control on the morphology of the Andes. Zonal atmospheric circulation in the Southern Hemisphere creates strong latitudinal precipitation gradients that, when incor- porated in a generalized index of erosion intensity, predict strong gradients in erosion rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and maximum elevation re¯ect gradients in both the erosion index and the relative dominance of ¯uvial, glacial, and tectonic processes, and show that major morphologic features cor- relate with climatic regimes. Latitudinal gradients in inferred crustal thickening and struc- tural shortening correspond to variations in predicted erosion potential, indicating that, like tectonics, nonuniform erosion due to large-scale climate patterns is a ®rst-order con- trol on the topographic evolution of the Andes. Keywords: geomorphology, erosion, tectonics, climate, Andes. INTRODUCTION we argue for the ®rst-order importance of earthquake cycle. Some studies have attribut- The presence or absence of mountain rang- large-scale climate zonations and resulting dif- ed local variations in structural, metamorphic, es at the global scale is determined by the lo- ferences in geomorphic processes to the mor- and geomorphic characteristics of the central cation and type of plate boundaries. Other fac- phology of mountain ranges. Andes to erosion (Gephart, 1994; Masek et al., tors become important in the evolution of 1994; Horton, 1999), but none has considered individual mountain systems. In particular, TECTONIC AND CLIMATIC SETTING variations in erosional mass removal at the spatially variable erosion resulting from cli- OF THE ANDES scale of the entire mountain range. mate gradients may localize exhumation and The in¯uences of climate, erosional pro- The highly variable climate of the Andes deformation in orogens and thereby in¯uence cesses, and tectonics on orogen morphology re¯ects its position transverse to hemisphere- the geologic structure and morphology of may be deconvolved in the Andean orogen be- scale, Hadley cell-driven precipitation regimes mountain ranges (Beaumont et al., 1991; Zei- cause it is a hemisphere-scale, north-south± (Fig. 1). In the Intertropical convergence zone tler et al., 1993; Avouac and Burov, 1996). oriented range with large gradients in temper- (108N±38S), both sides of the range receive 21 Earlier studies of climatic geomorphology ature and rainfall across a single convergent annual rainfall exceeding 2 m´yr . In the sub- have limited relevance to this issue because margin. Uplift of the Andes began ca. 25 Ma, equatorial northern Andes (38S±158S), oro- they simply classify Earth into normal (¯uvi- concomitant with accelerated convergence be- graphic interception of the trade winds deliv- 21 al), glacial, and arid zones and generally de- tween the Nazca and South America plates ers .2 m´yr of rainfall to the Amazon side 21 pict an alpine area as a single morphoclimatic (Allmendinger et al., 1997). Early theories of of the range and ,0.2 m´yr to the Paci®c zone that crosscuts multiple low-elevation formation of the Andes emphasized crustal side, and westerly winds produce the opposite morphoclimatic zones (Tricart and Cailleux, growth by magmatic processes, but estimates relationship in the temperate latitudes south of 1972). Even though the large-scale morphol- of structural shortening and evidence for sym- 338S. The central part of the range (158S± ogy of mountain belts must record the com- metric paleomagnetically de®ned rotation on 338S) is in the subtropical belt of deserts, bined effects of climatic and tectonic process- the northern and southern ¯anks of the Alti- where there is little precipitation on either side es, only a few studies explore climatic factors plano gave rise to the hypothesis that the var- of the range, or on the high plateau of the (Willett et al., 1993; Brozovic et al., 1997). iable size and thickness of the range result Altiplano. These major climate boundaries in Here we show that geomorphometric pa- from nonuniform crustal shortening, with the Andes are not dependent upon orographic rameters such as cross-range asymmetry, hyp- maximum shortening and consequent thick- effects, but are robust features of the general sometry, and maximum elevation of the An- ening at the center of the Andean orocline (Is- circulation in the Southern Hemisphere, and des re¯ect the in¯uence of zonal climate acks, 1988; Gregory-Wodzicki, 2000). How- therefore may be considered a priori condi- regimes on the nature and intensity of ero- ever, direct structural shortening estimates are tions under which the mountain range sional processes. In addition, we show that limited to the Eastern Cordillera and Suban- developed. consequent latitudinal gradients in erosion po- dean fold and thrust belt. In the Altiplano and tential are correlated with the crustal mass dis- Western Cordillera, crustal structures are ob- TOPOGRAPHIC ANALYSIS tribution and inferred orogenic shortening of scured by sedimentation or volcanism, and We focus on four aspects of the large-scale the range, suggesting an ambiguity in the cur- global positioning system measurements (Nor- geomorphology of the Andes: (1) a general- rent interpretation of crustal mass distribution abuena et al., 1998; Kendrick et al., 1999) ized index of erosion intensity based on re- as the result of variations in the tectonic en- may be in¯uenced by short-term strain accu- gional slope and ¯uvial discharge, (2) cross- vironment. On the basis of these observations mulation associated with the subduction-zone range asymmetry, (3) regional hypsometry q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; July 2001; v. 29; no. 7; p. 579±582; 3 ®gures. 579 Figure 1. A: Maximum (dark line) and mean (gray area) elevation in 18 latitude bins. Red circles are elevations of modern perennial snowline and blue circles are lowest elevation of Pleistocene glacier extent, both from Schwertfelder (1976). B: Topography and convergence velocity. Vectors headed in open circles denote long-term velocity of Nazca and Antarctic plates relative to South American plate (DeMets et al., 1994); those headed in closed circles denote global positioning system (GPS) velocities at coastal sites, relative to stable South America (Norabuena et al., 1998; Kendrick et al., 1999). C: Mean annual precipitation, overlain on shaded-relief map of western South America. D: False-color image of South America showing areas with steep slope in yellow, high precipitation in blue. Red pixels have calculated IE above 90th percentile relative to all pixels in image. E: Cross-range asymmetry, de®ned to be fraction of range volume above sea level that drains to west: values greater than 0.5 (lighter shade of gray) indicate that bulk of range is west of divide. (the elevation distribution of the topography), determined by summing the annual precipita- IE values shows that the zone of maximum and (4) the relationship between the maximum tion (P) over the matrix of upslope grid cells predicted erosion is on the eastern side of the elevation and the perennial snowline. We used each of drainage area A: range in the northern Andes and on the west- topography from the global 30 s GTOPO30 ern side in the southern Andes. Only small, digital elevation model; topography, slope, IE 5 34O PAii S. (1) localized areas of high IE are predicted in the and ¯ow direction from the 1 km HYDRO1K central Andes (Fig. 1D). DEM; and mean annual precipitation digitized We used this simple approach because (1) it from Hoffmann (1975). For purposes of our is not clear which process formulation is most Cross-Range Asymmetry analysis, we de®ned the eastern boundary of appropriate for modeling landscape-scale ero- We de®ned a cross-range asymmetry index the Andes as the approximate limit of Tertiary sion rates across 1 km grid cells in which net as the ratio of the volume of the topography or older units mapped on continental-scale erosion re¯ects an aggregation of ®ner scale above sea level on the west side of the divide geologic maps (UNESCO, 1978). effects from multiple, interacting processes; to that of the entire range within a given lat- (2) vegetation and land use, which cannot be itude band (Fig. 1E). Between 28S and 428S Erosion Index predicted from digital elevation models, com- most of the range is to the east of the drainage Rates of ¯uvial and hillslope erosion are plicate simple relationships between precipi- divide, whereas south of 428S most of the governed by processes characterized by dif- tation and erosion rate; (3) erosion models at range is west of the drainage divide. North of ferent erosion laws, but the net large-scale this scale inherently require calibration be- 28S, the inclusion of the areas draining to the erosional potential of a landscape increases cause slopes calculated from coarse-resolution Caribbean Sea with areas draining to the Pa- with precipitation, drainage area, and slope. grids are gentler than actual gradients (Zhang ci®c Ocean places most of the range on the Thus, we evaluated large-scale patterns in ero- and Montgomery, 1994); and (4) data on dif- west side of the drainage divide. Cross-range sion potential by using a simple parametric ferences in erosivity due to soil type and par- asymmetry tracks latitudinal variations in measure of erosional intensity (IE) based on ent lithology generally are not available at the moisture delivery due to prevailing wind the product of local slope (S) and discharge scale of interest. In the Andes, the pattern of directions. 580 GEOLOGY, July 2001 1957), here the aggregate pattern is geograph- ically consistent with variations in erosional processes.
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