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We present stable hydrogen-isotope analyses of volcanic glass (δ Dg) and radiometric ages (U-Pb zircon, U-Th calcite, AMS14C) from deformed sedimentary deposits in the vicinity of the intermontane Pocitos Basin in the central Puna of the Andean Plateau at about 24.5°S. Our results demonstrate 2-km surface uplift since the middle to late Miocene and protracted shortening that persists until the present day, while other sectors of the Puna show evidence for tectonically neutral and/or extensional settings. These findings are at odds with previous studies suggesting near-modern elevations (4 km) of the Puna Plateau since the late Eocene and formation of the intermontane Miocene Arizaro-Pocitos Basin associated with gravitational foundering of a dense lithosphere. Geophysical and geochemical data support the removal of continental lithosphere beneath the Puna, but the timing and mechanisms by which this removal occurs have remained controversial. We hypothesize that intermontane basin formation in the central Puna is the result of crustal shortening since about 20 Ma, followed by rapid surface uplift, likely related to lithospheric delamination. [ABSTRACT FROM AUTHOR]

Stable-isotopic and sedimentary records from the orogenic Puna Plateau of NW Argentina and adjacent intermontane basins to the east furnish a unique late Cenozoic record of range uplift and ensuing paleoenvironmental change in the south-central Andes. Today, focused precipitation in this region occurs along the eastern, windward flanks of the Eastern Cordillera and Sierras Pampeanas ranges, while the orogen interior constitutes high-elevation regions with increasingly arid conditions in a westward direction. As in many mountain belts, such hydrologic and topographic gradients are commonly mirrored by a systematic relationship between the oxygen and hydrogen stable isotope ratios of meteoric water and elevation. The glass fraction of isotopically datable volcanic ash intercalated in sedimentary sequences constitutes an environmental proxy that retains a signal of the hydrogen-isotopic composition of ancient precipitation. This isotopic composition thus helps to elucidate the combined climatic and tectonic processes associated with topographic growth, which ultimately controls the spatial patterns of precipitation in mountain belts. However, between 25.5 and 27°S present-day river-based hydrogen-isotope lapse rates are very low, possibly due to deep-convective seasonal storms that dominate runoff. If not accounted for, the effects of such conditions on moisture availability in the past may lead to misinterpretations of proxy-records of rainfall. Here, we present hydrogen-isotope data of volcanic glass ( δ Dg), extracted from 34 volcanic ash layers in different sedimentary basins of the Eastern Cordillera and the Sierras Pampeanas. Combined with previously published δ Dg records and our refined U–Pb and (U–Th)/He zircon geochronology on 17 tuff samples, we demonstrate hydrogen-isotope variations associated with paleoenvironmental change in the Angastaco Basin, which evolved from a contiguous foreland to a fault-bounded intermontane basin during the late Mio–Pliocene. We unravel the environmental impact of Mio–Pliocene topographic growth and associated orographic effects on long-term hydrogen-isotope records of rainfall in the south-central Andes, and potentially identify temporal variations in regional isotopic lapse rates that may also apply to other regions with similar topographic boundary conditions. [ABSTRACT FROM AUTHOR]

Abstract: 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 (δ 18O) data from Subandean foreland deposits of the Chaco Basin (Bolivia) show a rapid (<200ka) transition towards higher δ 18O and 87Sr/86Sr values at ∼8.5Ma that we interpret to reflect a change in precipitation patterns along the Eastern Cordillera and the Subandean fold-thrust belt. In agreement with δ 13C 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 δ 18O 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 ∼8.5Ma and associated rearrangement of the drainage systems. Collectively, the data presented here strongly suggest that between 12 and 8Ma 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. [Copyright &y& Elsevier]

The Cordillera Blanca detachment in the highest elevations of the Peruvian Andes has been accommodating synconvergent extension since the late Miocene. Stable isotope analysis of synkinematic mica from its exhumed footwall shear zone provides new constraints on deep meteoric-hydrothermal circulation during ductile deformation and regional paleoelevation. Muscovite and biotite that deformed and/or grew synkinematically in the shear zone have δ2H values of -131‰ to -58‰ and -149‰ to -98‰ (versus Vienna standard mean ocean water, VSMOW), respectively. The δ2H value difference between coexisting muscovite and biotite is consistent with equilibrium fractionation of the same fluid at the same temperature. Calculated δ2H values of water (-107‰ to -78‰) in equilibrium with these micas are indistinguishable from those of present-day, deeply circulated (9-11 km) hot spring waters emanating from the fault. Such low-δ2H fluids indicate circulation of meteoric water to the depths of the brittle-ductile transition that cannot be explained by other mechanisms. Average recharge paleoelevation for water entering the shear zone based on hydrogen isotopes was 3400 + 500/-700 m (1σ). This is near, but ~500 m below, the present-day mean elevation of the catchments feeding modern hot springs of 3965 ± 880 m, and ~700 m below the 4200 + 700/-900 m mean recharge elevation derived from δ2H values of modern surface and thermal water. The consistency between modern and ancient fault-assisted hydrothermal systems and elevation suggests that high topography, steep relief, and meteoric-hydrothermal circulation have persisted throughout the history of the Cordillera Blanca detachment system. [ABSTRACT FROM AUTHOR]

• 34 new volcanic glass H-isotope data from across the E Puna Plateau margin in NW Argentina. • 29 zircon U-Pb ages from intercalated tuffs refine basin chronologies. • δ Dg trends show significant paleoenvironmental change during the Miocene–Pleistocene. • Km-scale surface uplift in the Pastos Grandes and Toro basins during the late Miocene. • Plio-Pleistocene hinterland aridity triggered by regional tectonics, not global climate change. The Andes constitute an important orographic barrier in the southern hemisphere, impacting atmospheric circulation, the amount and distribution of rainfall, and Earth-surface processes in a highly asymmetric manner. In the Central Andes of NW Argentina, the Andean Plateau (Puna) and the intermontane basins of the adjacent Eastern Cordillera constitute geological archives that furnish spatiotemporal information on surface uplift and associated paleo-environmental change. Today, rainfall in NW Argentina is focused along the windward flanks of the Eastern Cordillera, while its intermontane basins and the Puna constitute high-elevation regions with strongly reduced rainfall. The present-day influence of these topographic characteristics on precipitation patterns is reflected by systematic changes in the hydrogen stable isotope ratios of meteoric water. Proxy archives from basin strata record past environmental conditions and provide insight into the nature of topographic growth through time. We present 34 new hydrogen isotopic compositions of volcanic glass (δ Dg), extracted from tuffs interbedded with the sedimentary deposits of three different regions across the eastern Andean margin between ca. 24°S and 25°S latitude. Combined with new zircon U-Pb geochronology, our data show clear spatiotemporal variations in δ Dg correlating with topographic growth and ensuing orographic effects during the Miocene and Pleistocene. In particular, variations in δ Dg indicate that the Pastos Grandes (Puna) and the Toro (Eastern Cordillera) basins had attained modern elevations by ca. 8 and 6 Ma, respectively. A strong positive shift in the δ Dg record of the Toro Basin after 840 ka coincides with deposition of thick fluvial/alluvial conglomerates suggesting orographically induced aridification due to progressive range uplift to the east. Other δ Dg records from the Eastern Cordillera show similar, but diachronous events emphasizing the importance of regional tectonics for environmental change with implications for the formation of orogenic plateaus. [ABSTRACT FROM AUTHOR]

Surface uplift of large plateaus may significantly influence regional climate and more specifically precipitation patterns and temperature, sometimes complicating paleoaltimetry interpretations. Thus, understanding the topographic evolution of tectonically active mountain belts benefits from continued development of reliable proxies to reduce uncertainties in paleoaltimetry reconstructions. Lipid biomarker-based proxies provide a novel approach to stable isotope paleoaltimetry and complement authigenic or pedogenic mineral proxy materials, in particular outside semi-arid climate zones where soil carbonates are not abundant but (soil) organic matter has a high preservation potential. Here we present δ D values of soil-derived n -alkanes and mean annual air temperature (MAT) estimates based on branched glycerol dialkyl glycerol tetraether (brGDGT) distributions to assess their potential for paleoelevation reconstructions in the southern central Andes. We analyzed soil samples across two environmental and hydrological gradients that include a hillslope (26–28°S) and a valley (22–24°S) transect on the windward flanks of Central Andean Eastern Cordillera in NW Argentina. Our results show that present-day n -alkane δ D values and brGDGT-based MAT estimates are both linearly related with elevation and in good agreement with present-day climate conditions. Soil n -alkanes show a δ D lapse rate ( Δ ( δ D ) ) of − 1.64 ‰ / 100 m ( R 2 = 0.91 , p < 0.01 ) at the hillslope transect, within the range of δ D lapse rates from precipitation and surface waters in other tropical regions in the Andes like the Eastern Cordillera in Colombia and Bolivia and the Equatorial and Peruvian Andes. BrGDGT-derived soil temperatures are similar to monitored winter temperatures in the region and show a lapse rate of Δ T = − 0.51 ° C / 100 m ( R 2 = 0.91 , p < 0.01 ), comparable with lapse rates from in situ soil temperature measurements, satellite-derived land-surface temperatures at this transect, and weather stations from the Eastern Cordillera at similar latitude. As a result of an increasing leeward sampling position along the valley transect lapse rates are biased towards lower values and display higher scatter ( Δ ( δ D ) = − 0.95 ‰ / 100 m , R 2 = 0.76 , p < 0.01 and Δ T = − 0.19 ° C / 100 m , R 2 = 0.48 , p < 0.05 ). Despite this higher complexity, they are in line with lapse rates from stream-water samples and in situ soil temperature measurements along the same transect. Our results demonstrate that both soil n -alkane δ D values and MAT reconstructions based on brGDGTs distributions from the hillslope transect ( Δ ( δ D ) = − 1.64 ‰ / 100 m , R 2 = 0.91 , p < 0.01 and Δ T = − 0.51 ° C / 100 m , R 2 = 0.91 , p < 0.01 ) track the direct effects of orography on precipitation and temperature and hence the combined effects of local and regional hydrology as well as elevation. [ABSTRACT FROM AUTHOR]

Globally, changes in stable isotope ratios of oxygen and hydrogen ( δ O 18 and δ D ) in the meteoric water cycle result from distillation and evaporation processes. Isotope fractionation occurs when air masses rise in elevation, cool, and reduce their water-vapor holding capacity with decreasing temperature. As such, δ O 18 and δ D values from a variety of sedimentary archives are often used to reconstruct changes in continental paleohydrology as well as paleoaltimetry of mountain ranges. Based on 234 stream-water samples, we demonstrate that areas experiencing deep convective storms in the eastern south–central Andes (22–28° S) do not show the commonly observed relationship between δ O 18 and δ D with elevation. These convective storms arise from intermontane basins, where diurnal heating forces warm air masses upward, resulting in cloudbursts and raindrop evaporation. Especially at the boundary between the tropical and extra-tropical atmospheric circulation regimes where deep-convective storms are very common ( ∼ 26 ° to 32° N and S), the impact of such storms may yield non-systematic stable isotope-elevation relationships as convection dominates over adiabatic lifting of air masses. Because convective storms can reduce or mask the depletion of heavy isotopes in precipitation as a function of elevation, linking modern or past topography to patterns of stable isotope proxy records can be compromised in mountainous regions, and atmospheric circulation models attempting to predict stable isotope patterns must have sufficiently high spatial resolution to capture the fractionation dynamics of convective cells. [ABSTRACT FROM AUTHOR]

The surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening, leading to their gradual rise. Recent studies of the Andes indicate that their elevation remained relatively stable for long periods (tens of millions of years), separated by rapid (1 to 4 million years) changes of 1.5 kilometers or more. Periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable, dense lower lithosphere after long-term thickening of the crust and lithospheric mantle. [ABSTRACT FROM AUTHOR]