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Regional Crustal Thickness and Precipitation in Young Mountain Chains

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Regional Crustal Thickness and Precipitation in Young Mountain Chains Regional crustal thickness and precipitation in young mountain chains W. G. Ernst* Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 Contributed by W. G. Ernst, September 7, 2004 Crustal thickness is related to climate through precipitation-induced erosion. Along the Andes, the highest mountains and thickest crust (Ϸ70 km) occur at 25° south, a region of low precipitation. Westerly winds warm passing over the Atacama Desert; precipita- tion is modest in the High Andes and eastward over the Altiplano. Severe aridity, hence low erosion rates, helps to account for the elevated volcanogenic contractional arc and high, internally draining plateau in its rain shadow. Weak erosion along the north- central arc provides scant amounts of sediment to the Chile–Peru Trench, starving the subduction channel. Subcrustal removal might be expected to reduce the crustal thickness, but is not a factor at 25° south. The thickness of the gravitationally compensated conti- nental crust cannot reflect underplating and͞or partial fusion of sediments, but must be caused chiefly by volcanism-plutonism and contraction. Contrasting climate typifies the terrane at 45° south where moisture-laden westerly winds encounter a cool margin, bringing abundant precipitation. The alpine landscape is of lower average elevation compared with the north-central Andes and is supported by thinner continental crust (Ϸ35 km). Intense erosion supplies voluminous clastic debris to the offshore trench, and vast quantities are subducted. However, the southern Andean crust is only about half as thick as that at 25° south, suggesting that ero- sion, not subcrustal sediment accretion or anatexis, is partly responsible for the thickness of the mountain belt. The Himalayas plus Tibetan Plateau, the Sierra Nevada plus Colorado Plateau, and the Japanese Islands exhibit analogous relationships between crustal thickness and climate. topic of intense geomorphic roughly parallel the inclined subduction Climatic Patterns and Crustal Thickness research involves quantifying the zone, whereas the broad magmatic arc Near Young Convergent Plate Junctions competing influences of contrac- is typified by open folding and a high- General Statement. The thickness of the tional and accretionary tectonic temperature, high-thermal-flux regime continental crust is a dynamic product of Aprocesses plus uplift versus surficial, pre- (6–10). The former is generated exclu- competing constructional and destruc- cipitation-induced erosion in controlling sively on oceanic crust (e.g., Franciscan, tional processes (19–24). The former in- the topography of active mountain belts Aleutian, and Chile–Peru trench systems); volve primary calcalkaline magmatic (1–5). Because of the orographic effect, the latter is constructed on a preexisting additions derived from the mantle, plate climate and surface elevation must be basement consisting of continental margin tectonic convergence (contraction), trans- intimately coupled in terms of cause and or island arc plus͞minus older oceanic form motion (strike-slip), and͞or diver- effect. Reflecting regional attendant gravi- crust (e.g., Sierran, Indonesian, and An- gence (extension), terrane suturing plus͞ tative equilibrium (isostacy) the thickness dean arcs). minus accretionary prism offloading͞ of the mountain belt crust is also partly a Collision-type convergent plate bound- underplating. The latter include surficial function of rain and snowfall. Using the aries form where subduction rollback erosion, subduction-induced subcrustal Andes, Himalayas, Sierra Nevada, and (11) of ocean lithosphere results in the removal, and delamination plus founder- Japanese Islands as examples, the modern insertion of a continental promontory, ing of the lower, mafic crust. Pacific-type thickness of the continental crust is shown microcontinent, or island arc beneath a paired mountain chains consist of an out- to be related in part to present-day annual nonsubducted, continental crust-capped board subduction complex and a sub- precipitation. Consequently, the interplay parallel inboard Andean arc, whereas of igneous activity, sedimentary and tec- plate. A salient of the downgoing conti- nental crust may descend to great depth Alpine-type mountain belts chiefly display tonic accretion, compressional shortening, the effects of continental collision. An- subcrustal removal, exhumation, and cli- imbedded in the sinking oceanic litho- sphere because the overall density of the cient platforms have attained thermal and mate all play important roles in determin- architectural stasis throughout the litho- dominantly oceanic plate exceeds that of ing the regional crustal thickness of active sphere, reflecting tectonic quiescence pro- the underlying asthenosphere. During un- mountain belts. vided by long-term isolation from upper derflow, the thickness of the suprasubduc- mantle circulation, and the consequent Contrasts Between Pacific- and tion-zone continental crust is increased by establishment of an unperturbed mantle- Alpine-Type Mountain Belts underplating, contraction, and amalgam- ͞ crust geothermal gradient. In contrast, Andean-type convergent plate boundaries ation continental collision. Thrust sheets geologically youthful mountain belts may evolve where thousands of kilometers of dipping beneath the stable plate and a possess thinner, or more commonly, oceanic lithosphere are consumed without paucity of calcalkaline igneous activity thicker, crust than do old cratons because, the introduction of substantial amounts of characterize such collisional mountain although regional Airy isostatic equilib- continental crust into the subduction belts. Typical examples include the Urals, rium is closely approximated, the complex zone. This plate-tectonic realm produces Alps, and Himalayas (12–16). Of course, interplay between constructional and de- an outboard, largely metasedimentary ac- continental lithosphere can be carried structional forces is influenced substan- cretionary trench complex, a medial, lon- down beneath young, hot, thus less dense, tially and to varying degrees by tectonism gitudinal forearc basin, and an inboard oceanic lithosphere (e.g., Oman and Su- and local erosion rates. calcalkaline volcanic-plutonic arc. The lawesi; refs. 17 and 18), but such cases are Reflecting regional gravitative equilib- relatively narrow trench depositional uncommon because most oceanic lithos- prism consists of a low-temperature, low- pheric plates are negatively buoyant (and heat-flow belt in which folds are over- roll back) relative to continental plates. *E-mail: [email protected]. turned oceanward and thrust faults © 2004 by The National Academy of Sciences of the USA 14998–15001 ͉ PNAS ͉ October 19, 2004 ͉ vol. 101 ͉ no. 42 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0406557101 Downloaded by guest on September 26, 2021 PERSPECTIVE Andes at 20–25° south (25, 37–41). This is the atmospheric realm of high-pressure descending, heating Hadley cells. Cold upwelling water and the north-flowing Humboldt Current lie directly offshore. Along this continental margin, the weak westerly winds are relatively cold, thus low in H2O content; they warm as they pass over the Atacama Desert, and yearly pre- cipitation is only Ϸ0.2–0.3 m in the High Andes (36). Air parcels continuing east- ward beyond the continental divide sink over the Altiplano (3.6-km average eleva- tion); low humidity characterizes this re- gion, with yearly precipitation rates of Ϸ0.3 m. Low precipitation plus low Fig. 1. Modern average annual precipitation in inches around the globe (modified from figure 3 in ref. erosion rates partially account for the ele- 35). The cool California, Humboldt, and Tasman currents provide moisture-laden westerly winds at higher vated volcanic-plutonic arc and the inter- latitudes and relatively H O-poor winds at lower latitudes. The warm Indian Equatorial Gyre and the 2 nally draining high plateau on the east Kuroshio Current bring oodles of moisture via warm, H2O-laden winds to the south-facing Himalayas and southwestern Japan. Note the marked north-south variation of rain and snowfall along the Sierra Nevada, (34, 42, 43). Only minor amounts of sedi- Chilean Andes, and Himalayas-Tibetan Plateau, as well as the Japanese Islands. ment are transported eastward toward the Amazon Basin and westward toward the Chile–Peru Trench. Because erosion is rium, the mean elevation of a growing The highest mountains and intensely weakly developed along the north-central mountain belt is a time-integrated reflec- shortened, thickest continental crust cordillera, little volcanoclastic debris is tion of the petrotectonically accumulated (55–75 km) occur in the north-central carried down the Chile–Peru subduction volume per unit area, modified by surfi- cial degradation (25–30). As a very rough approximation, erosion rates in modern mountain ranges track with precipitation (31). This is a gross oversimplification of a much more complex set of variables (2), including chemical and physical weather- ing, nature and integrity of the geologic substrate (e.g., resistance of the bedrock complex to erosion), proximity to local and regional erosional base levels, etc. Other factors being equal, greater topo- graphic gradients promote more rapid degradation for a given precipitation rate. But as rain and snowfall decrease, con- traction and uplift may block external drainage systems and produce elevated, internally draining plateaus (32–34). The regional climate itself is a multivariant function
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