The Neiva and Girardot basins of the upper Magdalena Valley are en echelon late Cenozoic structural depressions between the Central and Eastern Cordillera of the Colombian Andes. The basins contain three depositional sequences up to 9,000 m thick resting on Paleozoic crystalline basement: (1) a Triassic-Jurassic nonmarine clastic and minor carbonate sequence, (2) a middle Cretaceous to Paleogene marine to nonmarine clastic and minor carbonate sequence, and (3) a thick Neogene nonmarine molasse sequence.

The Andean orogeny in the upper Magdalena Valley occurred in two phases: (1) late Oligocene-early Miocene (Neiva basin), and (2) late Miocene-Pliocene (Neiva and Girardot basins). The first phase was dominated by basement-cored domes and basement-driven thrusts displaced toward the east from the Central Cordillera, while the second phase was dominated by basement-driven thrusting toward the west from the Eastern Cordillera. The basement-rooted thrusts splay upward within the Cretaceous and Paleogene strata and form complex shallow to moderate-dipping thrust sheets or terminate within the sedimentary cover to form a series of tight folds. From middle Miocene to late Pliocene, a thick molasse sequence accumulated on the earlier structures in both basins. During the younger phase of defo mation, the molasse was deformed by continued movement on basement-rooted faults.

The basement-driven structures in the upper Magdalena Valley are probably the result of transpressional movements along the eastern margin of the Andean volcanic arc in the Central Cordillera.

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