Data on the geophysics and geology of the central Andes are interpreted in terms of plate theory and a model for Andean evolution is presented. Analysis of upper mantle structure and seismicity shows that the underthrusting Pacific plate is now about 50 km thick and the overriding South American plate 200 to 300 km thick. Underthrusting of the Pacific plate probably began in Triassic time and has continued without substantial change to the present. Prior to underthrusting, the west coast of South America was quiescent, and great thicknesses of Paleozoic continental shelf deposits were laid down in an area east of the present volcanic arc. In Late Triassic or Early Jurassic time, an incipient arc began to form at or west of the present coast of South America. Igneous activity has since migrated eastward, culminating in the Pliocene-Pleistocene volcanic episode. The crust beneath the volcanic cordillera is more than 70 km thick and probably consists largely of rocks compositionally equivalent to those of the volcano-plutonic suites observed at the surface.

Increase in crustal volume of the volcanic arc between Cretaceous time and the present implies either that the mantle above the under-thrust plate has undergone 18 to 36 percent partial melting or that 1 to 2 km of rock has been melted from the underthrusting plate. The intrusion of melt into the crust beneath the volcanic cordillera and the resultant crustal dilatation produced continentward compression of the Paleozoic sedimentary rocks which form an easterly belt of thrust and fold mountains. Here crustal shortening has produced crustal thicknesses of 50 to 55 km. Few deposits of the type normally termed eugeosynclinal, and no ophiolites, are observed between trench and volcanic arc; only in the intermontane foredeep behind the arc has a clastic wedge of geosynclinal proportions formed.

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