Retrodeformable structural cross-sections were produced using published surface geological data, proprietary seismic and well data and gridded gravity data for the Eastern Cordillera, Llanos Basin and Middle Magdalena Basin of Colombia, South America. The McKenzie model of basin formation was tested for the Cretaceous thickness. The Eastern Cordillera Cretaceous basin was the result of lithospheric stretching and an associated regional thermal anomaly. Pre-Nfiddle Albian tectonic extension was followed by a period of regional thermal subsidence. Differential amounts of extension in the basin resulted in a contrasting morphology between the south (Bogota area) and the north (Tunja area). Greater extension in the Bogota area produced a broad flat-floored basin whereas in the Tunja area and further north lower tectonic extension generated two major grabens and a tectonic horst in the middle. Lower Cretaceous extension was sufficient to generate basaltic intrusions by thinning of the lithosphere and shallowing of the mantle. Neogene to Recent shortening of the Eastern Cordillera is estimated to be about 110 to 218 km for Profile 1 and 125 to 144 km for Profile 3-4. Most of this shortening directly involved the lower crust in the center of the range thickening the crust and forming the crustal root under the mountain.