Facts and theories on the Andes: Twenty-sixth William Smith Lecture

Abstract

Theories on the relation of oceanic plates with continents frequently use the Andes as a model. Simplified assumptions neglect the fact that only the recent morphogenic uplift made the apparently uniform Andes, masking a most complicated geological history. Three main subdivisions are recognized. (I) The Southern Andes that show a virgation leading into the Scotia arc. Oceanic formations characterize the interior ranges and disappear towards the Gulf of Penas, where the Chile fracture zone joins the continent. (2) From here the Central Andes begin with a Coastal Cordillera of metamorphic, highly compressed Paleozoic and probably older rocks with an abnormal northwest strike. They are transgressed by thick, mostly volcanic Mesozoic rocks, with block fold tectonics. An intense young acid volcanism characterizes the Central ranges together with strong seismic activity that is missing in the Southern Andes. The Central Andes are bordered by the Nasca plate, which is subdivided by the aseismic Nasca ridge. The latter projects into the Andes where the metamorphic Coastal belt disappears. (3) The Northern Andes have a marked virgation leading into the Caribbean are and display a Coastal and Western (Caribbean) Cordillera of more or less metamorphic Mesozoic oceanic rocks, in strong contrast to the Central Andes. Their southern border coincides with a major east-west lineament. This is outlined by the Carnegie–Galapagos ridge and a possible renewal of an old Amazon fracture trend aligned with the Romanche fracture zone which displaces the Mid-Atlantic ridge. The continental edge of the Cocos plate, bordering the Northern Andes, exposes oceanic sediments in marked contrast to the coastal metamorphic “basement” of the Central Andes, that borders the Nasca plate. The Andes, being a marginal chain, are influenced by the shields to the east, which display younger remobilization along fracture zones, and by the east Pacific plates in the west. The geological complexities of the Andes, with their marked volcanism and irregular but strong seismicity, suggest that the east Pacific plates have had a much more complicated pattern than has generally been supposed.