ALONG-STRIKE SHORTENING RATES ACROSS THE EASTERN FOOTHILLS OF THE COLOMBIAN ANDES: EXAMPLES FROM THE LLANOS BASIN

We use neotectonic field mapping and structural controls from seismic profiles to estimate the style and rate of recent shortening across the eastern foothills of the Colombian Andes in the region of the Llanos basin. From west to east, the primary active structures include the east-directed Guicarimo thrust fault that folds Cretaceous, Paleogene, Neogene and Quaternary sediments, and the east-directed Cusiana thrust fault. The Cusiana fault cuts Neogene and Quaternary sediments – both structures are seismically active and are considered to sole at depth into a common decollement. Uplifted, folded and entrenched Quaternary alluvium are cut by the Cusiana thrust fault with resulting entrenchment mainly occurring across folds and faults suggesting abandonment of the terrace surfaces is likely tectonically controlled. Seismic data indicates that the subsurface geometry of the Cusiana thrust fault is listric and transitions from a flat on flat relationship in the hinterland to a hanging wall ramp on footwall ramp in the foreland. Kinematic modeling using several algorithms provides an added check on our estimated displacements from profiles across warped terraces related to the active fault systems. Surface exposure ages are determined on the uplifted terrace surfaces using terrestrial cosmogenic 10Be depth profiles in quartz-rich fluvial sand in weakly eroded sites with well-developed soils. The fault displacements and associated surface exposure ages provide preliminary along-strike fault slip rates across the active thrust front.