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; precipitation 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 continental 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 erosion, 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 continental 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 lithosphere 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 dean 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 precipitation is only Ϸ0.2–0.3 m in the High Andes (36). Air parcels continuing eastward beyond the continental divide sink over the Altiplano (3.6-km average elevation); low humidity characterizes this region, 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 surficial 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 topographic gradients promote more rapid degradation for a given precipitation rate. But as rain and snowfall decrease, contraction and uplift may block external drainage systems and produce elevated, internally draining plateaus (32–34). The regional climate itself is a multivariant function of the Earth’s dynamic atmosphere plus hydrosphere structure, including wind and near-surface ocean circulation patterns (functions of latitude, positions of obstructing land masses, Co- riolis effect, etc.). Annual global precipitation zonations illustrated in Fig. 1 clearly transect some mountain chains at large angles, resulting in longitudinal variations in erosional efficacy.

Do Climatic Processes Impact Crustal Scale Features of an Active Mountain Belt? The present synthesis suggests that the answer is yes. As an example, consider an active continental margin belt such as the contractional, volcanogenic Chilean Andes, which parallels the outboard Chile–Peru Fig. 2. Topography, average annual precipitation in meters, and relative plate velocities along the west convergent plate junction and surmounts coast of South America (simplified from figure 1 in ref. 36). (Left) Topography and plate convergence. the eastward descending Nazca oceanic- (Right) Average annual precipitation. The mean elevation of the Andean mountain belt at 25° south crust capped plate. Fig. 2 presents general exceeds 3 km, reflecting thick continental crust, whereas Ϸ20° latitude farther south in Bernard O’Higgins topographic and climatic relationships. Land, the regional elevation is somewhat less than 1 km, indicating thin continental crust.

Ernst PNAS ͉ October 19, 2004 ͉ vol. 101 ͉ no. 42 ͉ 14999 Downloaded by guest on September 26, 2021 channel (24, 44). Sediment-starved subduction zones might be anticipated to constitute sites of active subcrustal erosion, as is characteristic of some segments of the Chile–Peru continental margin (45). Accordingly, the great regional thickness of the continental crust in this sector must be mainly a consequence of calcalkaline volcanism-plutonism com- bined with major tectonic shortening and feeble erosion, and cannot be caused by accretionary offloading or partial fusion of subducted materials. The climatic situation is markedly dif- ferent around 45° south. Here, the tem- Fig. 3. Average annual precipitation in mm, major peaks, drainage network, and gauging stations in the perature contrast between the Humboldt Himalayas and adjacent Tibetan Plateau (after ﬁgure 1 in ref. 53). Outlined study area of Findlayson et al. Current and the land is reversed, with the (53) includes both eastern and western syntaxes (maximum uplift areas). Note the pronounced rain continent being slightly cooler than the shadow north of the Himalayas. nearby ocean. Low-pressure Ferrell cells in the atmosphere rise at this latitude, raphy for the Himalayas and Tibetan Pla- the Himalayan rain shadow and thus may supplying abundant H2O-laden winds to the cool terrestrial surface. Consequently, teau. Because precipitation zonation pat- owe part of its great elevation and crustal terns coincide with the east-west thickness (54, 55) to aridity; yearly precip- the rugged, fjord-rich but much lower ele- Ͻ vation Bernard O’Higgins Land is richly orientation of the range, the Himalayas itation is 0.5 m, accounting for the at- endowed with Ϸ2–4 m of annual rain and lack a strong axial gradient in erosion tendant low erosion rates. snowfall, producing snowfields and glaci- rates. The remarkable mean elevation of The Sierra Nevada Range in eastern Ͼ ated valleys (36). The alpine landscape is this mountain chain ( 5 km) is chiefly a California lies east of the southeast- supported by a continental crust Ϸ35 km reflection of compressional tectonism. flowing California Current. The latter, thick (46, 47). Reflecting moderate pre- The structurally induced relief on the coupled with westerly winds, brings abun- cipitation downwind, eastward stream southern side of the belt is somewhat dant moisture to the cool Pacific North- drainage is external and a high plateau is modified by extremely rapid erosion (4, 5, west and northern California. Farther lacking as sediments escape to the Atlan- 53). H2O-saturated air derived from the south, the land is considerably warmer tic margin. On the western slopes, heavy Indian Equatorial Gyre is drawn north- than the ocean, resulting in progressively precipitation, vigorous stream flow, and ward in response to low-pressure systems greater aridity. Eastward from central- active erosion transport voluminous clastic created by parcels of hot air ascending southern California, the Sierra achieves its debris westward to the offshore Chile– over central Asia. Juicy monsoonal winds maximum regional elevation in the Mount Peru Trench, and vast amounts of quart- cool as they pass over India and encoun- Whitney area. The crustal thickness of zofeldspathic material are subducted (24). ter the Himalayan Range, resulting in an- the southern Sierra Nevada (42–55? km) Nevertheless, the southern Andean crust nual precipitation rates approaching 3–10 appears to exceed that of the northern is only about half as thick as that at 25° m (43, 53). To the north, the adjacent, Sierra Nevada (Ϸ35 km), according to the south, demonstrating that intense ero- internally draining Tibetan Plateau lies in limited amount of geological and geo- sional decapitation (a reflection of climatic patterns), rather than sediment underplating plus͞minus anatexis is at least partly responsible for the modest crustal thickness. Although divided into segments of dif- fering subduction inclinations, extents of recent glaciation, and amounts of tectonic shortening, the physicochemical nature of the Nazca plate is roughly similar in both north-central and southern Andes; thus, primary calcalkaline igneous contributions to crustal thickness should be roughly comparable in both portions of the mountain belt. A substantial sediment cushion in the subduction zone at 45° south might be expected to inhibit subcrustal erosion and instead favor accretionary offloading and growth, but if such processes are op- erating (45), they have not generated a massively thick arc in Bernard O’Higgins Land. Ongoing continental collision and intra- crustal thrusting accounts for the great Fig. 4. Average annual precipitation throughout the American West in inches (modiﬁed from regional thickness (65–80 km) of the Himalayan map, Oregon Climate Service at www.ocs.oregonstate.edu͞prism). The Colorado Plateau lies far down- continental crust (48–52). Fig. 3 shows wind from the high-elevation southern Sierra Nevada but is separated from it by the Neogene extensional annual precipitation as a function of geog- crust of the Basin and Range Province.

15000 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0406557101 Ernst Downloaded by guest on September 26, 2021 physical data available (56–59). Contrasts environment of the Japanese arc is quite the landward belt where new continental in Sierran crustal thickness are not as complex, with the old Pacific lithosphere crust is added. Alpine-type chains form marked as in the Andes and Himalayas, descending beneath northern Honshu and during underflow of an oceanic plate that presumably because the constructional the young Philippine Sea plate sinking transports continental crust into the sub- phase of mountain building largely ceased under western Chugoku (60). The Japan duction zone. Growth of the continental in the Late Cretaceous. And what about Trench is receiving only modest volumes margin crust is largely a regional function the Colorado Plateau (2.4-km average of sedimentary debris, compared to the of the operation of petrotectonic pro- elevation), located downwind from the clastic load entering the Nankai Trough. cesses, such as calcalkaline magmatism, southern Sierra? It lies well to the leeward Nevertheless, northeast Japan appears to terrane accretion, and tectonic contrac- of the high Sierra, but the intervening have a continental crustal thickness of tion. Mountain building results in regional Basin and Range Province may represent Ϸ40 km (61–63), in contrast to southwest- crustal thicknesses that are comparable to, Neogene crustal extension of an initially ern Japan where the crust is on the order or exceed, those of old, cold, stable cra- somewhat larger plateau. As illustrated in of 30–35 km thick (64–66, †). tons (67). Fig. 4, the average annual precipitation But reflecting the intensity of erosional for the American Southwest appears to be Summary removal, the thickness of the continental more or less inversely correlated with ele- Silicic͞intermediate crust is generated by crust in a geologically youthful contractional vation, and thus the thickness of the crust. calcalkaline magmatism in island arcs and mountain belt is also to some extent a func- Episodes of extensional tectonics clearly continental margins situated over conver- tion of climatic patterns. For a given conver- have severely stretched the continental gent plate junctions; some of it represents gent plate tectonic environment, the mean crust in the intervening Basin and Range net additions to the continental crust. elevation of a mountain chain and therefore Province, but it seems possible that, in Paired Pacific-type mountain belts de- the regional thickness of the isostatically addition to volcanic-plutonic construc- velop above zones of long-lived subduc- compensated continental crust are inversely tional processes, erosion linked to present- tion of oceanic crust-capped lithosphere. correlated with precipitation and consequent day climate patterns may have influenced They consist of an outboard, low-heat- erosion. In addition, tectonic shortening the current regional crustal thickness in flow accretionary prism deposited in and (crustal thickening) and attendant rapid up- both the Sierra Nevada and Colorado landward from the trench, and an in- lift in rain-shadow realms can defeat rapid Plateau. board, high-heat-flow volcanic-plutonic stream erosion and promote the formation Another well studied mountain belt, the arc. The trench assemblage consists domi- of internally draining, high-elevation plateaus Japanese island arc, exhibits strongly nantly of clastic debris derived from the typified by only modest sedimentary deposi- zoned rainfall patterns (Fig. 1). Annual nearby, contemporaneous arc, but in- tion and͞or erosion. precipitation in western Chugoku, cludes tectonic fragments of the oceanic Shikoku, and Kyushu exceeds that in plate. A massive magmatic arc dominates I thank Kelin Whipple, George Hilley, Simon northern Honshu, reflecting moist winds Klemperer, and Norm Sleep for constructive that blow off the warm, northward flow- feedback and reviews and these researchers †Murakoshi, T., Takenaka, H., Suzuki, S., Shimizu, H. & ing Kuroshio Current and over southwest- Uehira, K. (2003) Trans. Am. Geophys. Union 84, Suppl., and Stanford University for their help. This ern Japan. However, the plate tectonic S32A–0841 (abstr.). study was supported by Stanford University.

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