UNRAVELING THE INCISION OF THE CAUCA RIVER CANYON OF THE NORTHERN ANDES: CONTRASTING COOLING HISTORIES REVEALED BY AFT AND AHE THERMOCHRONOLOGY

The incision of kilometer-scale canyons into high topography is often used to constrain the surface uplift history of mountain ranges, controlled by tectonic and geodynamic processes. However, changes in climate, such as orographic enhancement of precipitation during mountain growth or periods of time with increased rates of precipitation, may also result in the incision of canyons. This project aims to decipher the timing and, ultimately, the role of tectonic/climatic mechanisms on the incision of the ~2.5 km deep Cauca River Canyon in the Central Cordillera of the Northern Andes. We propose two testable hypotheses: (1) Tectonic hypothesis: canyon incision started since the Late Miocene (6-10 Ma) and it is synchronous with the documented onset of flat-slab subduction beneath the northern Central Cordillera. In this scenario, flat-slab subduction and greater coupling with the overriding plate caused plateau style uplift of the northern Central Cordillera and river incision occurred in response to increased relief; (2) Climate hypothesis: an acceleration of canyon incision has occurred since 4 Ma, decoupled with the timing of surface uplift of the northern Central Cordillera and would be a result of increased precipitation variability during the late Cenozoic. In this work, the cooling (exhumation) history of rocks from canyon walls is used to constrain the rate and timing of canyon incision. Eight bedrock samples (from 300 m to 2300 m elevation) were collected on the eastern border of the canyon. Preliminary AFT data yield ages from 42 to 53 Ma. Two bedrock AHe ages from the valley bottom yield ages of ~6 Ma. The AHe cooling ages are interpreted to represent the timing of incision of the Cauca River Canyon and suggest the tectonic hypothesis is correct. The absence of younger AFT ages in the Cauca River Canyon can be explained if the recent (since ~6 Ma) incision of the canyon occurred, but the magnitude of incision was insufficient to reveal younger AFT cooling ages below an exhumed Partial Annealing Zone. The Late Miocene canyon incision coincides with an increase in sedimentation rates recorded in the lower Magdalena Valley Basin to the north, and suggests that the basin has been the sink for the Cauca River sediments at least since the Late Miocene.