Oblique transpression in the western thrust front of the Colombian Eastern Cordillera

New kinematic data reveal that the main faults of the western foothills of the Colombian Eastern Cordillera are a series of left-lateral to oblique thrusts that are offset by steeply dipping, northwest-trending, left-lateral, strike-slip faults. Kinematic data were collected from the main structures that dominate the 350-km length of the western foothills of the Eastern Cordillera, north of Bogota (La Salina-Bituima, Cambras, Dos Hermanos-Alto del Trigo, and Bucaramanga faults). These data indicate that transpression is responsible for the Cenozoic formation of the folds and thrusts that deform the Mesozoic and Cenozoic sedimentary sequences of the Middle Magdalena Valley. Kinematic and structural data suggest that the La Salina-Bituima fault changed from a reverse fault to a fault with a left-lateral, strike-slip sense of displacement during middle to late Miocene times. As it propagated, a transpressive zone was generated in the western foothills of the Eastern Cordillera, with the development of arcuate, oblique-reverse secondary faults on the leading edge of the Magdalena basin (e.g. Cambras, Dos Hermanos). This development implies that during the Neogene (?), the tectonics of the western foothills were dominated by nonplanar deformation with shortening accompanied by significant left-lateral, strike-slip displacements. These new data indicate that the oblique convergence vector imposed by convergence among the Nazca, Caribbean, and South American plates is not fully partitioned in space but instead must be distributed in a diffuse zone of transpressional deformation along the western margin of the Eastern Cordillera and its associated foreland basin. (C) 2004 Elsevier Ltd. All rights reserved.