YOUNG MOUNTAIN CHAINS, PRECIPITATION, AND REGIONAL CRUSTAL THICKNESS

New and recycled continental crust is formed in orogens chiefly along convergent plate junctions. Due to surficial erosion, the crustal thickness in an active mountain belt is also a function of climatic patterns (oceanic + atmospheric circulation). The volcanogenic Chilean Andes belt borders the outboard convergent plate junction and E-dipping Nazca plate. The highest mountains and thickest continental crust (~70 km) occur in the north-central Andes at ~25° S. Westerly winds warm as they pass over the Atacama Desert, and precipitation is quite modest in the High Andes. Humidity decreases beyond the mountains as air currents descend eastward over the Altiplano. Great aridity, hence low erosion rates help to account for the elevated calc-alkaline volcanic-plutonic contractional arc and internally draining plateau in its rain shadow. Incipient degradation along the north-central Andean margin provides insignificant volumes of sediment to the Chile-Peru Trench, starving the subduction channel. Thus the thickness of the gravitationally compensated sialic crust cannot reflect underplating and/or partial fusion of sediments, but must be due to contraction and calc-alkaline volcanism-plutonism. A very different climate typifies the terrane at ~45° S, where moisture-laden westerly winds pass over a cool continental margin, resulting in abundant precipitation. The alpine landscape is of lower average elevation compared with the north-central Andes, and is supported by continental crust of only moderate thickness (35 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° S, suggesting that erosion, rather than sediment offloading or anatexis is at least partly responsible for the thickness of the mountain belt. The Himalayas + Tibetan Plateau, the northern versus southern Sierra Nevada + Colorado Plateau appear to exhibit similar relationships between crustal thickness and precipitation-linked erosion.