Abstract

Geologic maps, seismic profiles, and structural field data are used to constrain a structural cross section of the eastern Cordillera of Colombia, the Medina–Utica transect, from the Llanos to the middle Magdalena Valley forelands. The Medina–Utica transect illustrates the geometry and kinematic evolution of a continental back-arc region that evolved from rifting to compressional mountain building because of changes in the dynamics of the subduction zones nearby. As other orogens formed by rift inversion, the eastern Cordillera shows intense thrust deformation in the foothill margins, associated with the reactivation of the main former extensional margins, and an orogen interior with comparatively less deformation and relief (the Sabana de Bogota Plateau). While the orogen margins are dominated by basement-involved thick-skinned thrusting, the Sabana de Bogota is interpreted as a salt-detached fold belt with a basal decollement in lower Cretaceous evaporite at a depth of ca 4 km (2.4 mi). Anticlines in the Sabana de Bogota are interpreted to have formed as diapiric salt walls during the pre-orogenic rifting, later squeezed and welded during the Andean shortening, together with syntectonic sedimentation and halokinetic sequence development. A sequential restoration of the cross section to selected time steps based on a wealth of tectonics-sedimentation and thermochronological data enables to track the evolution of orogenic deformation. The total shortening in the Medina–Utica transect is calculated as 82 km (50.9 mi) (27% of the predeformed length). Compressional deformation in the area started probably near the Cretaceous-Tertiary boundary, manifested by localized folding and faulting in the Sabana de Bogota and foothills, much guided by the weak salt horizons. Evidence for gentle folding and thrusting persists through the Paleogene at low rates of <0.5 mm/a. By the Neogene the mountain belt was in full accretion, and rapid shortening at rates near 3 mm/a was accommodated by thrusting in the outer margins of the former rift system. The eastern Cordillera of Colombia exemplifies a pattern of tectonic evolution of inverted rifts in which deformation commences in the basin interiors in a distributed way, strongly controlled by weak stratigraphic units, and evolves to a point when intense shortening is shifted into the weak faults at the rift margins. This gives rise to prominent mountain topography, rapid erosion, and fast sedimentation in the forelands.