RESOLVING UPLIFT OF THE EASTERN CORDILLERA IN THE COLOMBIAN ANDES USING DETRITAL ZIRCON U-PB AGE SIGNATURES

Uplift of the Eastern Cordillera in the northern Andes has been linked to orographic climate change and genesis of large South American river systems (Amazon, Orinoco, and Magdalena rivers). The timing of initial uplift remains poorly constrained, with estimates ranging from Late Cretaceous to late Miocene time. New detrital zircon U-Pb ages from proximal fill of the Llanos foreland basin in Colombia reveal a pronounced mid-Cenozoic shift in provenance from an Amazonian craton source to an Andean fold-thrust-belt source. During Eocene time, zircons from the Guyana shield (1850-1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous-Paleocene (90-55 Ma), Jurassic (190-150 Ma), and limited Permian-Triassic (280-220 Ma) zircons recording uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Following this Eocene phase of dual eastern and western provenance, Oligocene-Miocene sandstones of the proximal Llanos foreland basin document significant uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of a Paleogene cover succession rich in both arc-derived detritus (dominantly 180-40 Ma) and shield-derived sediments (mostly 1850-950 Ma). Subsequent erosional unroofing of the Cretaceous section in the Eastern Cordillera is evidenced by elimination of Mesozoic-Cenozoic zircons and increased proportions of 1650-900 Ma zircons emblematic of Cretaceous strata. A late Oligocene to earliest Miocene shift from a craton source to a fold-thrust belt source corresponds with a conglomeratic unroofing sequence and a sharp increase in foredeep accumulation rates. These nearly simultaneous changes in zircon age spectra, clast compositions, and sediment accumulation rates are attributable to earliest Miocene uplift of the eastern flank of the Eastern Cordillera. These relationships suggest an early activation of the frontal thrust system, implying a long-term (up to 20 Myr) cessation of orogenic wedge advance, which we hypothesize was driven by structural inheritance and/or climate change.