Neogene to Quaternary tectono-geomorphic evolution and paleo-hydrology of the South Central Andes, NW Argentina
Final Report Abstract
Two of the most controversial issues concerning the late Cenozoic evolution of the Andean orogen are (1) the timing of uplift of the orogenic Andean Plateau and its eastern bordering ranges of the Eastern Cordillera, and (2) ensuing changes in climatic and surface-process conditions in the adjacent intermontane basins. This inherently arid, high-elevation region is impacted by the South American Monsoon (SAM) that transports Atlantic-derived moisture via the Amazon southward along the eastern flanks of the Andes. The Eastern Cordillera separates the internally drained, arid Andean Plateau in the interior of the range from semi-arid intermontane basins and the humid sectors of the eastern flank of the Andes. With elevations between 4 and 6 km the eastern flanks of the Andes thus form an efficient orographic barrier with westward-increasing elevation and asymmetric rainfall. This is mirrored by pronounced gradients in the efficiency of surface processes that erode and re-distribute sediment from the uplifting ranges. The Andes are thus a hemispheric-scale rainfall barrier with respect to large atmospheric circulation systems. Although the overall pattern of deformation and uplift in this sector of the southern central Andes shows an eastward migration of deformation, a well-developed deformation front does not exist and uplift and associated erosion and sedimentary processes are highly disparate in space and time. In addition, periodic deformation within intermontane basins, and continued diachronous foreland uplifts associated with the reactivation of inherited basement structures furthermore make a rigorous assessment of the spatiotemporal uplift patterns difficult. We addressed these issues by analyzing a rich stratigraphy of intermontane basin fills that contain abundant volcanic ashes that were used to decipher tectono-sedimentary events through U-Pb geochronology and geochemical correlation; the volcanic ashes are also recorders of the hydrogen-isotope composition of ancient meteoric waters that can be compared to the isotopic composition of modern meteoric water and thus help to understand orographically controlled changes in rainfall and environmental conditions through time. In addition, we analyzed the hydrogen and carbon compositions of molecular lipid biomarkers in sedimentary strata and oxygen and carbon isotopes obtained from pedogenic carbonates. Our results document a very asymmetric distribution of rainfall and isotopic condition; tectonically controlled changes of environmental conditions on timescales of several 105 yrs caused the progressive aridification of the mountain belt in an eastward, yet diachronous fashion. First, in contrast to common belief the stable isotope analysis of >230 stream-water samples revealed that areas experiencing deep convective storms today do not show the often observed patterns of isotopic fractionation and the expected co-varying relationships between oxygen and hydrogen with increasing elevation. These convective storms are formed over semi-arid intermontane basins in the transition between the broken foreland and the Andean Plateau. Here, convective rainfall dominates the precipitation budget and no systematic stable isotope-elevation relationship exists. Regions to the north, in the transition between the broken foreland and the Subandean thrust belt, the impact of convection is subdued, with lower degrees of storminess and a stronger expected isotope-elevation relationship. This finding of present-day fractionation trends of meteoric water is thus of great relevance for paleoenvironmental studies in attempts to use stable isotope relationships in the reconstruction of paleoelevations in NW-Argentina and elsewhere. This information is pivotal for a correct assessment of paleo-proxy records contained in the basin strata. Second, our new U-Pb zircon geochronology and paleocurrent reconstructions of intermontane sediments revealed that several of the present-day intermontane basins were originally sustained as an integral part of a largely unrestricted depositional foreland system until early to mid-Pliocene time. However, due to ongoing shortening these basins became progressively decoupled from the foreland by range uplifts to the east that forced easterly moisture-bearing winds to precipitate at increasingly eastward locations. This is supported by variations in the hydrogen stable isotope composition of volcanic glass from the Neogene to Quaternary sedimentary record, which can be related to spatiotemporal changes in topography and associated orographic effects. Third, we focused on paleohydrological Mio-Pliocene (10-2 Ma] basin sedimentary records, which revealed fundamental environmental changes during Andean uplift and orographic barrier formation. Our isotopic studies on pedogenic nodules and organic compounds in the sedimentary sequences helped to derive a precipitation- evapotranspiration record that identified the onset of a precipitation regime related to the southward shift of the South American Monsoon at latitudes as far south as 24°S after 9 Ma. Such conditions are also revealed by a rich vegetation and vertebrate record that documents that humid conditions existed until 7 Ma, followed by orographic barrier uplift to the east of the intermontane basins of the Cordillera Oriental. These environmental conditions were superseded by rapid aridification in intermontane basins, highlighting the effects of eastward-directed deformation and barrier formation. A transition in vegetation cover from a humid C3 forest ecosystem to semi-arid C4-dominated vegetation was coeval with continued basin uplift to modern elevations.
Publications
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2013, Neotectonic basin and landscape evolution in the Eastern Cordillera of NW Argentina, Humahuaca Basin (~24°S): Basin Research, v. 25, p. 554-573
Pingel, H., Strecker, M.R., Alonso, R.N., and Schmitt, A.K.
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2014, Can stable isotopes ride out the storms? The role of convection for water isotopes in models, records, and paleoaltimetry studies in the central Andes: Earth and Planetary Science Letters, v. 407, p. 187-195
Rohrmann, A., Strecker, M.R., Bookhagen, B., Mulch, A., Sachse, D., Pingel, H., Alonso, R.N., Schildgen, T.F., and Montero-López, C.
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2014, Pliocene orographic barrier uplift in the southern Central Andes: Geology, v. 42, p. 691-694
Pingel, H., Alonso, R.N., Mulch, A., Rohrmann, A., Sudo, M., and Strecker, M.R.
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2016, Elevation-dependent changes in n-alkane 5D and soil GDGTs across the South Central Andes, Earth and Planetary Science Letters, v. 453, p. 234-242
Nieto-Moreno, V., Rohrmann, A. van der Meer, M., Sinnighe Damste, J., Sachse, D., Tofelde, S. Niedermeyer, E. Strecker, M.R., Mulch, A.
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2016, Miocene orographic uplift forces rapid hydrological change in the southern central Andes: Nature Scientific Reports, v. 6, p. 1-7
Rohrmann, A., Sachse, D., Mulch, A., Pingel, H., Tofelde, S., Alonso, R.N., and Strecker, M.R.
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2016, Surface uplift and convective rainfall along the southern Central Andes (Angastaco Basin, NW Argentina]: Earth and Planetary Science Letters, v. 440, p. 33-42
Pingel, H., Mulch, A., Alonso, R.N., Cottle, J., Hynek, S.A., Poletti, J., Rohrmann, A., Schmitt, A.K., Stockli, D.F., and Strecker, M.R.
