Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin
Active crustal shortening and surface uplift are vividly expressed in the foothills of the Eastern Cordillera of Colombia. Quaternary alluvial-fan and terrace deposits are folded and uplifted tens to hundreds of meters above the local base level, recording deformation and incision of the landscape. We used geomorphic observations and structural analysis from a 300 km 2 three-dimensional seismic-reflection survey over an actively growing anticline to investigate the relationship between finite shortening and surface uplift, and its effects on landscape evolution. A quantitative description of finite shortening of the pregrowth strata from excess-area and line-length calculations were compared with uplift and shortening estimates obtained from geomorphic strain markers dated using in situ 10 Be cosmogenic nuclide depth profiles. This combined approach provides the first estimates of Quaternary surface uplift and horizontal shortening rates along the foothills of the northeastern Colombian Andes. Finite shortening for the Tame anticline was calculated using two different approaches. Excess-area shortening varies from 530 ± 36 m (2σ) at its southern end, decreasing northward to 404 ± 72 m (2σ), whereas line-length shortening is typically an order of magnitude smaller. Using the maximum horizontal shortening, our results for the Tame anticline reveal a Quaternary uplift rate of 2.8 ± 1.5 mm/yr, and a shortening rate of 2.1 ± 1.2 mm/yr (2σ), with the older terraces located at the highest elevations and recording the highest rotation from the horizontal, indicating growth by limb rotation. Based on these rates, the initiation of folding is estimated to be 0.25 +0.09/–0.05 Ma (2σ). We hypothesize that the discrepancy between shortening estimates from line-length and excess-area calculations can be reconciled by accounting for internal deformation, likely developed during the initial stages of folding, which can be represented by strain hardening, for example. The shortening rates we obtain are comparable to geodetically derived shortening rates but are slightly lower than long-term geologic shortening rates. Slower shortening rates and the presence of a youthful structure at the thrust front suggest that the orogen has been migrating eastward since the late Pleistocene, probably due to reorganization of a supercritical thrust wedge farther west in the hinterland, to attain critical taper.