499 Copyright ©2015 by The American Association of Petroleum Geologists. DOI:10.1306/13531947M1083650 18 The Structure of an Inverted Back-arc Rift: Insights from a Transect across the Eastern Cordillera of Colombia near Bogota Antonio Teixell and Juan-Camilo Ruiz Departament de Geologia, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain (e-mails: antonio.teixell@uab.cat; jcruiza@gmail.com) Eliseo Teson and Andres Mora Instituto Colombiano del Petroleo-Ecopetrol, km 7 Vía a Piedecuesta, Bucaramanga, Colombia (e-mails: eteson@gmail.com; resmora30@googlemail.com) ABSTRACT Geologic maps, seismic proiles, and structural ield 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 for- mer 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 dur- ing the pre-orogenic rifting, later squeezed and welded during the Andean shortening, to- gether with syntectonic sedimentation and halokinetic sequence development. A sequential restoration of the cross section to selected time steps based on a wealth of tectonics-sedimen- tation 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 accom- modated by thrusting in the outer margins of the former rift system. The eastern Cordillera of Colombia exempliies a pattern of tectonic evolution of inverted rifts in which deformation Teixell, Antonio, Eliseo Teson, Juan-Camilo Ruiz, and Andres Mora, 2015, The structure of an inverted back-arc rift: Insights from a transect across the Eastern Cordillera of Colombia near Bogota, in C. Bartolini and P. Mann, eds., Petroleum geology and potential of the Colombian Caribbean Margin: AAPG Memoir 108, p. 499–516. 13880_ch18_ptg01_499-516.indd 499 10/27/15 11:00 AM