ALBERT

Description: The tropical Northern Andes of Colombia are one the world's most biodiverse places, offering an ideal location for unraveling the linkages between the geodynamic forces that build topography and the evolution of the biota that inhabit it. In this study, we utilize geomorphic analysis to characterize the topography of the Western and Central Cordilleras of the Northern Andes to identify what drives landscape evolution in the region. We supplement our topographic analysis with erosion rate estimates based on gauged suspended sediment loads and river incision rates from volcanic sequences. In the northern Central Cordillera, an elevated low-relief surface (2500 m in elevation, ~40 × 110 km in size) with quasi-uniform lithology and surrounded by knickpoints, indicates a recent increase in rock and surface uplift rate. Whereas the southern segment of the Central Cordillera shows substantially higher local relief and mostly well graded river profiles consistent with longer term uplift-rate stability. We also identify several areas of major drainage reorganization, including captures and divide migrations. These changes in the topography coincide with the proposed location of a slab tear and flat slab subduction under the northern Central Cordillera, as well as with a major transition in the channel slope of the Cauca River. We identify slab flattening as the most likely cause of strong and recent uplift in the Northern Andes leading to ~2 km of surface uplift since 8–4 Ma. Large scale drainage reorganization of major rivers is likely driven by changes in upper plate deformation in relation to development of the flat slab subduction geometry; however, south of the slab tear other factors, such as emplacement of volcanic rocks, also play an important role. Several biologic observations above the area of slab flattening suggest that surface uplift isolated former lowland species on the high elevation plateaus, and drainage reorganization may have influenced the distribution of aquatic species.

Description: The incision of kilometer-scale canyons into high-standing topography is often used to constrain the surface uplift history of mountain ranges, controlled by tectonic and geodynamic processes. However, changes in climate may also be responsible for canyon incision. This study deciphers the timing of incision of the ∼2.5-km-deep Cauca River Canyon in the Central Cordillera of the Northern Andes using the cooling (exhumation) history of rocks from the canyon walls and a regional analysis of channel steepness in rivers. Ten bedrock samples and one detrital sample were collected on the eastern border of the canyon between 300 m and 2300 m of elevation. Bedrock and detrital AFT data yield ages from 50 to 38 Ma, while two bedrock AHe ages from the valley bottom yield ages of 7−6 Ma. The AHe ages and inverse thermal history models reveal a previously unidentified late Miocene (ca. 7−6 Ma) pulse of exhumation that we interpret as the age of a single incision event that formed the Cauca River Canyon. We conclude that the Cauca River Canyon was carved as a response to rock uplift in the northern Central Cordillera and propagation of an erosion wave into the mountain range starting in the latest Miocene.

Description: The tropical Northern Andes constitutes a great area to explore the interaction of tectonics and climate on the evolution of orogenic topography, as they are tectonically active, have steep gradients in relief and precipitation, and were less affected by Quaternary glaciations than high-latitude mountains. This study combines new and published thermochronology along ∼500 km of the Eastern Cordillera in Colombia to explore what controls the rates of Miocene and Pliocene exhumation, including: (1) variations in late Cenozoic paleoclimate, (2) orographic precipitation or (3) variation in rock uplift associated with structural heterogeneities along the eastern flank of the range. New thermochronology data consists of 27 apatite and seven zircon (U-Th)/He ages and five apatite fission track ages. Thermal history models show that rock cooling as a result of erosional exhumation has occurred everywhere in the Eastern Cordillera since at least the Miocene at spatially and temporally variable rates. Exhumation rates vary by an order of magnitude between various fault-bounded blocks and there is no evidence for a spatially uniform increase in exhumation rates during the past ∼4–5 Ma that would indicate uniform climatic driven exhumation. The west (dry) to east (wet) gradient in precipitation rates across the Garzon Massif is not a major factor controlling the exhumation rates, as the exhumation rates are highest on the western drier flank. The greatest rates of exhumation of the Eastern Cordillera occur in the blocks associated with vigorous Cenozoic reactivation of inherited faults that had focused extension during Early Cretaceous rifting.

Description: Erosion rates are widely used to assess tectonic uplift and sediment export from mountain ranges. However, the scarcity of erosion rate measurements often hinders detailed tectonic interpretations. Here, we present 25 new cosmogenic nuclide-derived erosion rates from the Northern Andes of Colombia to study spatio-temporal patterns of uplift along the Central and Eastern Cordillera. Specifically, we combine new and published erosion rate data with precipitation-corrected normalized channel steepness measurements to construct high-resolution erosion rate maps. We find that erosion rates in the southern Central Cordillera are relatively uniform and average ∼0.3 mm/a. In the northern Central Cordillera rapidly eroding canyons dissect slowly eroding, low-relief surfaces uplifting since Ma, based on a block uplift model. We interpret that persistent steep slab subduction has led to an erosional steady-state in the southern Central Cordillera, whereas in the northern Central Cordillera, Late Miocene slab flattening caused an acceleration in uplift, to which the landscape has not yet equilibrated. The Eastern Cordillera also displays pronounced erosional disequilibrium, with a slowly eroding central plateau rimmed by faster eroding western and eastern flanks. Our maps suggest Late Miocene topographic growth of the Eastern Cordillera, with deformation focused along the eastern flank, which is also supported by balanced cross-sections and thermochronologic data. Spatial gradients in predicted erosion rates along the eastern flank of the Eastern Cordillera suggest transient basin-ward migration of thrusts. Finally, sediment fluxes based on our erosion maps, suggest that the Eastern Cordillera exports nearly four times more sediment than the Central Cordillera. Our analysis shows that accounting for spatial variations in erosion parameters and climate reveals important variations in tectonic forcing that would otherwise be obscured in traditional river profile analyses. Moreover, given relationships between tectonic and topographic evolution, we hypothesize that spatio-temporal variations in slab dip are the primary driver of the dynamic landscape evolution of the Northern Andes, with potentially superposed effects from inherited Mesozoic rift structures.