Late Miocene climate variability and surface elevation in the central Andes

Temporal and spatial variations in topography and oxygen stable isotope ratios in precipitation in the central Andes have stimulated widespread discussion about the competing roles of mantle and crustal processes and their feedbacks with global-scale climatic change in uplifting and shaping the central Andes. In general, one of the major obstacles in assessing the relative contributions of long-term (105–106a) tectonic processes and precipitation (as a proxy for climate) to the uplift history of the Andean orogen is the lack of integrated data sets that record late Miocene patterns of uplift and climate. Radiogenic (87Sr/86Sr), sedimentologic, and stable isotope (δ18O) data from Subandean foreland deposits of the Chaco Basin (Bolivia) show a rapid (< 200 ka) transition towards higher δ18O and 87Sr/86Sr values at ∼ 8.5 Ma that we interpret to reflect a change in precipitation patterns along the Eastern Cordillera and the Subandean fold-thrust belt. In agreement with δ13C studies on paleosol carbonates we attribute this change to a southward deflection of the South American low-level jet (LLJ) that currently exerts the dominant control over the seasonality and amount of precipitation along the Eastern flanks of the Andes. Deflection of the LLJ occurred most likely as the combined effects of readjustment of relief and topography within the Eastern Cordillera at 20–22°S and possibly associated surface uplift of the Altiplano. Contemporaneous rapid positive shifts in δ18O and 87Sr/86Sr of pedogenic carbonate in fluvial foreland deposits are consistent with a transition to more seasonal precipitation conditions and critical threshold elevations being attained that affected South American atmospheric circulation patterns. A four-fold increase in sedimentation rates in the foreland together with a shift to strongly radiogenic 87Sr/86Sr ratios in paleo-river water and sediment load as well as river incision into the well preserved San Juán del Oro paleo-surface directly reflects a rapid change in moisture distribution along the eastern flank of the central Andes at ∼ 8.5 Ma and associated rearrangement of the drainage systems. Collectively, the data presented here strongly suggest that between 12 and 8 Ma the amount, oxygen isotopic composition, spatial distribution and seasonality of precipitation along the eastern flank of the Bolivian Andes changed dramatically which in turn require changes in trans-Andean moisture flux and isotopic composition of precipitation on the uplifting plateau region.