FLUVIAL SYSTEM RESPONSE TO GROWTH OF THE BOLIVIAN ANDES

Mountain building involves the coevolution of topography and climate and it is often the fluvial system that sets the orographic structure in uplifting landscapes. It is bedrock river systems that possess a record of the contrasting roles that tectonics and climate can play on affecting mountain form during orogenesis. The Bolivian Andes possess a strong latitudinal disparity in relief and orographic precipitation, but less variation in thrust belt architecture. Here, we combine existing and ongoing work that reconstructs the orogen-scale orographic development and explores its consequences encoded in the shape of the Bolivian fluvial system.

A regional climate model with rising surface elevations shows that near present-day patterns of mean orographic rainfall only become triggered once the topography grows to >75% modern elevations. A geo-thermochronology dataset that quantifies the spatiotemporal patterns of deformation and erosional exhumation suggests this establishment of near-modern orography occurred ~15-11 Ma. In the semiarid south, channels possess low concavities, and exhibit non-systematic downstream changes in steepness at multiple scales. The steepness patterns mostly correlate with the tectonic architecture of the deforming thrust belt. In the wet north, channels possess high concavities and mostly decrease in steepness systematically downstream.

These observations suggest several generalizations about the coevolution of topography, climate, and mountain river systems. (1) Small, threshold changes in surface elevation can cause large changes in orographic precipitation gradients. (2) Low magnitudes (<2 m/yr) and gradients (<Δ~1.5 m/yr) in rainfall means that thrust belt tectonics will continue to exhibit a dominant influence on channel steepness patterns even after ~10 million years. (3) Over ~10 million years of large magnitudes (>2 m/yr) and gradients (>Δ3-4 m/yr) in mean rainfall causes fluvial systems to exhibit only muted, non-systematic changes in steepness influenced by thrust belt tectonics. These insights provide quantitative hypotheses for the values and timescales of orographic growth that are required to impart a dominantly climatic vs. tectonic influence on fluvial systems in active orogens that can be tested with landscape evolution models.