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.
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