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The shortening history of the Andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cretaceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in Colombia reveal two provenance shifts during Cenozoic time. The fi rst shift occurs between early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dominated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can be linked to uplift of the Eastern Cordillera. Our results show that signifi cant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of Paleogene climate.

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Passive-roof duplexes are important features for accommodating shortening in active orogens, but their occurrences have been previously demonstrated only with significant subsurface data or after their exhumation. In this study, we describe a series of thin-skinned passive-roof duplexes along the Subandean front in Colombia and compare them with potential analogues in southern Peru. We suggest type localities for this structural style, which display conditions favorable for formation of these structures. It appears that passive-roof duplexes in the Subandean zones are mostly late Miocene features. Our main data for placing temporal and thermokinematic constraints on their development are 25 apatite fission-track (AFT) analyses, including track-length distributions. We show that these areas require high shortening, surface erosion, and downstream sedimentation rates at the time of their formation, as well as two distinct low-friction detachments. These features, which have been previously described by analog models, appear to have been conditioned by a phase of late Miocene topographic growth and denudation in the hinterland and subsequent increase in accommodation as well as sedimentation rates in the foreland.

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