Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin

Abstract

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-refl ection survey over an actively growing anticline to investigate the relationship between fi nite shortening and surface uplift, and its effects on landscape evolution. A quantitative description of fi nite 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 profi les. This combined approach provides the fi rst 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.

Constraints on the Cenozoic Deformation of the Northern Eastern Cordillera, Colombia

“…In the Llanos region (Figure b), the main structure is the Guaicaramo fault, which places Lower Cretaceous rocks over the Neogene foredeep deposits of the LB (Figure b). Our cross section follows the same geometry as those proposed by Bayona et al () and Veloza et al () and shows the blind Borde Llanero fault and associated Tame anticline displacing and deforming the foredeep sediments.…”

“…The topography coincides with strata that dip ~35° to the west (supporting information Figure S7), producing a cuesta‐type morphology (Figure h). The eastern Cocuy domain and Llanos region are hardly accessible, and the only available data are from regional geological maps (Fabre, ; Fabre et al, ; González et al, ), from the works of Bayona et al (), Tesón et al (), Teixell et al (), Mora, Parra, et al (), and Veloza et al (), and from Google Earth satellite images. The entire domain is characterized by outcrops of Lower Cretaceous rocks cross cut by high‐angle faults.…”

“…The cross section was constructed using bedding dips and fault attitudes that were either measured at the surface or constrained by interpolation on the available 1:100,000 scale geological maps (Fabre et al, ; Gómez et al, ; González et al, ; Pulido González, ; Vargas et al, ; Table S1c and Figure S7 in the supporting information). Limited seismic and well log data were available only for the easternmost part of the section (Bayona et al, ; Veloza et al, ). Uncertainties are related to the geometry of the deep structures and the interpretation of faults at depth.…”

“…Sedimentation and deformation in the LB have continued until the present day (Bayona et al, ; Duarte et al, ). Quaternary alluvial and alluvial‐fan deposits, locally deformed, cover the main structures in the proximal LB (Veloza et al, ).…”

“…The geodynamic flexural models of Sarmiento‐Rojas () and Bayona et al () indicate that tectonic loads located in the eastern structural domain are needed to generate the accommodation space for the accumulation of the thick Paleocene succession (>200‐m thick) reported in the subsurface of the LB (Bayona et al, ). The Borde Llanero fault is the easternmost fault along the section; it is a blind fault whose surface expression is an anticline resting on a small basement high (Veloza et al, ). In the eastern domain, the thickness of the Lower Cretaceous rocks decreases across the rift structures (Bayona et al, ; Sarmiento‐Rojas et al, ; Figure b).…”