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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.

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The 1:500,000 coloured geological map of the traverse route combines observations from the Geotraverse, previous mapping, and interpretation of orbital images. The position of all localities visited by Geotraverse participants and basic geological data collected by them along the traverse route are shown on a set of maps originally drawn at 1:100,000 scale, reproduced on microfiche for this publication. More detailed mapping, beyond a single line of section, was achieved in five separate areas. The relationships between major rock units in these areas, and their significance, are outlined in this paper. Near Gyanco, (Lhasa Terrane) an ophiolite nappe, apparently connected with outcrops of ophiolites in the Banggong Suture about 100 km to the north, was under thrust by a discontinuous slice of Carboniferous—Permian clastic rocks and limestone, contrary to a previous report of the opposite sequence. At Amdo, a compressional left-lateral strike-slip fault zone has modified relationships along the Banggong Suture. Near Wuli, (northern Qiangtang Terrane) limited truncation of Triassic strata at the angular unconformity below Eocene redbeds demonstrates that most of the folding here is of Tertiary age. The map of the nearby Erdaogou region displays strong fold and thrust-shortening of the Eocene redbeds, evidence of significant crustal shortening after the India- Asia collision began. In the Xidatan-Kunlun Pass area, blocks of contrasting Permo—Triassic rocks are separated by east-trending faults. Some of these faults are ductile and of late Triassic — early Jurassic age, others are brittle and part of the Neogene—Quaternary Kunlun leftlateral strike-slip fault system. Some more significant remaining problems that geological mapping might help to solve are discussed briefly, including evidence for a possible additional ophiolitic suture within the Qiangtang Terrane.

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