PALEOZOIC TERRANES IN THE BASEMENT OF THE ANDES

The basement of the Andes is formed by a series of continental blocks accreted to the proto-Pacific margin of South America through different episodes of contraction and extension associated with amalgamations and detachments. Some of these blocks are partially exotic to Western Gondwana, and have been derived from Laurentia and Eastern Gondwana. Most of them share a common Grenvillian-age basement, which clearly indicates their participation in the Rodinia Supercontinent. The other terranes are Gondwanic in their origin, and have been detached and re-accreted to the margin through successive orogenic episodes.

Main periods of amalgamation occurred in the latest Proterozoic-Early Cambrian times during the Brasiliano-Pan African orogeny; in the Middle to Late Ordovician during the Famatinian orogeny, and at Late Carboniferous to Early Permian times. This last docking episode was related in the northern part of the Andes to the Alleghenides orogeny, while in the southern Andes is linked to the Gondwanides orogeny.

These terranes are associated with high pressure metamorphism, ophiolitic assemblages, magmatic arcs, and fold and thrust-belts in the foreland areas. In recent years a great advance in the understanding of these terranes was related to new dating of the age of the magmatic rocks and related metamorphism in these basement rocks. Besides subsurface studies in the foreland areas allowed reconstructing the deformation and basin evolution associated with periods of docking and accretion of these terranes.

A dominant characteristic is the recognition of extensional periods following the amalgamation of the terranes, which have been explained by different processes. However, in the last years there is some consensus in assigning these periods to changes in the absolute motion of the Gondwana Supercontinent. These changes have been fundamental to produce the Terra Australis orogen through most of the Paleozoic along the Pacific margin of South America.