The Cordillera Oriental of Colombia is built of Mesozoic and Tertiary strata, deformed chiefly during Tertiary orogenesis. These materials form a sedimentary cover, overlying a basement of lower Paleozoic (and Precambrian?) metamorphic and igneous rocks, and of little deformed and nonmetamorphic Upper Paleozoic rocks, which often behaved mechanically as part of the cover.

No metamorphism took place during the Tertiary orogenesis, and the main structural features of the chain are determined by the relationship between basement and cover. The basement crops out within the chain in two massifs, Santander and Quetame-Garzón, arranged en échelon.

The general structure of the chain is that of a raised block between the Magdalena Valley and Los Llanos of eastern Colombia. Tilting, differential lift, and faulting produced differences in height of the order of 10,000 m in the upper surface of the basement. The faults in the basement are reverse faults, often high-angle reverse faults (some of them reactivated normal faults of pre-Cretaceous age), some of which have been active until Quaternary times (Suárez fault). It is possible that strike-slip occurred along some faults, as has been suggested for the Santa Marta (= Bucaramanga) fault and for many faults in the Caribbean area. A rather narrow strip in the central part of the chain, between the two massifs shows a more tightly folded structure.

The sedimentary cover shows different reactions to basement tectonics: (1) passive behavior (undeformed or tilted cover on basement blocks); (2) simple draping structures (monoclinal flexures overlying basement faults; asymmetrical gentle folds); (3) more complex, tight structures, where the basement has a less rigid behavior; (4) independent behavior of the cover, due to gravity (collapse structures), to injection (salt of the “Sabana de Bogotá”), and to disharmonic relationships. Thus, the Cordillera Oriental of Colombia shows a series of different examples of the relationship between basement and cover structures.

The Cordillera Oriental constitutes the external zone of the Andean System in Colombia, in sharp contrast to the internal zones (eugeo-syncline), with their volcanism of Cretaceous age, their plutonism, and their metamorphism. Such mountain chains as the Appalachians, the Cantabrian Chain, the Alps and the Betic System are characterized by tangential tectonics (nappes), structures facing regularly toward the outside of the chain, and a general décollement of the cover. However, the Cordillera Oriental shows a fan structure, more related to the structure of such mountain chains as the Pyrenees.

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