Pediplains are classically identified as flat landscape surfaces in arid regions linked to tectonic quiescence, whereas deep incision of a pediplain is attributed to tectonic uplift. In the Atacama ...Desert, pediplains are generally used as morphotectonic markers to define the chronology of episodes of Late Cenozoic Andean uplift from their erosion and incision patterns and timings. The Atacama Pediplain (AP) extends over >12,000 km2 (26° to 28°S Lat) through the Central Depression and Precordillera of the southern Atacama Desert. In this study we perform geomorphologic and stratigraphic observations on the AP in the Salado Canyon area, combining new geochronological results derived from 40Ar/39Ar biotite ages from volcanic layers interbedded within the alluvial deposits, and 21Ne exposure ages on quartz-clasts on alluvial plains, to determine the chronology of the AP evolution. Results show that the evolution of the AP is a long-term and continuous process (from >20 to ~2.3 Ma) of alluvial deposition and subsequent alluvial plain formation developed by interplay between the climate variability of the Atacama Desert and Andean uplift. The AP alluvial deposition occurred in two spatially and temporally separated episodes of alluvial backfilling: 1) shortly before ~20.14 Ma and to prior to ~9.4 Ma, a timespan that allows for the drainage capture of the eastern Precordillera, and considerable landscape rearrangement; 2) post ~9.4 Ma, with a re-positioning of alluvial backfilling from the Precordillera towards the Central Depression. This occurs after the Mid Miocene onset of the hyperarid conditions in the Central Depression and is more likely due to late Miocene surface uplift. Exposure ages reveal the cessation of basin-scale deposition and the abandonment of the alluvial plains during ~5.24 to ~3.8 Ma linked to the incision of Salado Canyon. Subsequent climate conditions modulated the surfaces by the development of lag deposits until ~2.69 to ~2.3 Ma when hyperarid conditions reach a threshold that limits surficial activity. Additionally, the drainage capture of the Precordillera by headwards erosion of the Salado Canyon explains marked deep incision depth of this canyon without the need for an increase in surface uplift or a change in climate conditions. The AP is not a general marker of a single climatic or tectonic event/period but a composite paleosurface formed by a complex concatenation of extrinsic and intrinsic geomorphic processes over more than ~17 myr.
•Atacama Pediplain is a paleosurface composed by a mosaic of small alluvial plains.•Extrinsic and intrinsic aggradation and degradation control the Atacama Pediplain.•Shift of basin-scale aggradational processes is due to late Miocene surface uplift.•Drainage captures implicate large-scale landscape rearrangement.•We propose not to use the Atacama Pediplain as a marker for a single regional event.
ALMA, the Atacama Large Millimeter/submillimeter Array, situated high in the Chilean desert, is the largest ground-based telescope on Earth. This is an insiders' account of how this complex ...mega-project came to fruition from authors with intimate knowledge of its past and present. The separate roots of ALMA in the United States, Europe, and Japan are traced to their merger into an international partnership involving more than 20 countries. The book relates the search for a suitable telescope site, challenges encountered in organization, funding, and construction, and lessons learned along the way. It closes with a review of the most significant results from ALMA, now one of the most productive telescopes in the world. Written for a broad spectrum of readers, including astronomers, engineers, project managers, science historians, government officials, and the general public, the eBook edition is available to download as an Open Access publication on Cambridge Core.
The Cordillera de la Costa is located along the coastline of northern Chile, in the hyperarid Atacama Desert area. Chemical and isotopic analyses of several small coastal springs and groundwater ...reservoirs between 22.5 °S and 25.5 °S allow understanding groundwater origin, renewal time and the probable timing of recharge. The aquifers are mostly in old volcanic rocks and alluvial deposits. All spring waters are brackish, of the sodium chloride type due to intensive concentration of precipitation due aridity and for deep groundwater to additional water-rock interaction in slowly renewed groundwater and mixing with deep seated brines. The heavy δ18O and δ2H values in spring water are explained by recharge by the arrival of moist air masses from the Pacific Ocean and the originally lighter values in the deep wells can be associated to past recharge by air masses coming from the Atlantic Ocean. Current recharge is assumed almost nil but it was significant in past wetter-than-present periods, increasing groundwater reserves, which are not yet exhausted. To explain the observed chloride content and radiocarbon (14C) activity, a well-mixed (exponential) flow model has been considered for aquifer recharge. The average residence time of groundwater feeding the springs has been estimated between 1 and 2kyr, up to 5kyr and between 7 and 13kyr for deep well water, assuming that current recharge is much less than during the previous wetter period. The recharge period feeding the coastal springs could have been produced 1 to 5kyr BP, when the area was already inhabited, and recharge in the Michilla mine was produced during the 10 to 14.5kyr BP CAPE (Central Andean Pluvial Event) pluvial events of the central Andes. The approximate coincidence of turnover time with the past wet periods, as revealed by paleoclimate data, points to significant recharge during them.
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•Small springs have been recognized in the hyperarid coastal zone of the Atacama Desert.•The δ18O and δ2H values of spring waters are similar to coastal region rainfall.•The residence time of the waters from springs varies between 1 and 2kyr, up to 5kyr.•Waters from the deep wells are isotopically much heavier than those of springs.•Turnover time for deep waters varies between 7 and 13kyr, which overlaps the CAPE events.
In arid and semiarid regions, persistent hydrological imbalances illuminate the considerable gaps in our spatiotemporal understanding of fundamental catchment‐scale governing mechanisms. The Salar de ...Atacama basin is the most extreme example of groundwater‐dominated continental basins and therefore is an ideal place to probe these unresolved questions. Geochemical and hydrophysical observations indicate that groundwaters discharging into the basin reflect a large regional system integrated over 102–104 year timescales. The groundwater here, as in other arid regions, is a critical freshwater resource subject to substantial demand from competing interests, particularly as development of its world‐class lithium brine deposit expands. Utilizing a uniquely large and comprehensive set of H and O isotopes in water, we demonstrate that much of the presumed recharge area on the Altiplano‐Puna plateau exhibits isotopic signatures quite distinct from waters presently discharging within the endorheic Salar de Atacama watershed. δ18O values of predicted inflow source waters are 3.6‰ to 5.6‰ higher than modern plateau waters, and 3H data from 87 discrete samples indicate that nearly all of this inflow is composed of premodern recharge (i.e., fossil water). Under plausible conditions, these distinctions cannot be explained solely by natural variability in modern meteoric inputs or by steady state groundwater flow. We present a conceptual model revealing the extensive influence of transient draining of fossil groundwater storage augmented by regional interbasin flow from the Andes. Our analysis provides robust constraints on fundamental mechanisms governing this arid continental groundwater system and a framework within which to address persistent uncertainties in similar systems worldwide.
Plain Language Summary
Groundwater in the driest places on Earth is a vital resource for both humans and ecosystems, yet fundamental characteristics of this water such as where it originates and how it moves in the ground remain unresolved. This water often lies deep underground and flows across great distances and over long periods of time; as a result, it is quite difficult to study. Using the 1,000 water samples in the Salar de Atacama basin in northern Chile at the border of the driest desert on Earth, we trace the origin and traveltime of water across a large region. Groundwater in the Salar de Atacama region is fundamental to sustaining natural and human systems; therefore, developing a better understanding of how this water moves will be critical for their management, particularly as development of its world‐class lithium brine deposit expands. We find that “fossil water,” which entered the ground hundreds or thousands of years ago, makes up most of the water now flowing into the basin. Our analysis also defines the area that contributes water to the basin, much of which incorporates flow through mountains and from other higher‐elevation basins. By improving our understanding of how these large flow systems develop and function, this work will aid efforts to sustainably manage these critical freshwater resources for all who rely on them.
Key Points
Analysis of tritium in water discharging within Salar de Atacama basin shows that it is composed predominantly of water >60 years old
Water entering the Salar de Atacama basin is spatially distinct and decoupled from recharge on the Altiplano‐Puna plateau
Analysis of stable O and H isotope ratios in 900 water samples constrains the spatiotemporal dimensions of modern and fossil groundwaters
The last hydrometeorological extreme event that caused large floods in the southern Atacama Desert in March 2015 raised concern about how little was known about the fluvial dynamic of these arid ...basins. Understanding the response of intermittent and ephemeral rivers in drylands to the present context of global change is critical to preserve the ecological and human systems they support, to sustainably manage their scarce water resources and to develop flood risk management plans. We have studied the instrumental and historical record and explored the potential of the Copiapó River geological record in the comprehension of how extraordinary the 2015 flood was and how its fluvial dynamic relates with global climate oscillations. We have identified 36 flood events that have occurred in the last 400 years: 22 of them have been classified as ordinary rises of the river flow (discharges <30 m3/s), 11 as extraordinary floods in which the damage is confined to areas adjacent to the river (discharges 30–180 m3/s), and only 3 as catastrophic floods (discharges >180 m3/s), including the 2015 flood event. The incorporation of the historical and palaeohydrological data into the flood frequency analysis results in an increase of the magnitude of the flood quantiles in which large flood events occur with an average recurrence interval of 120 years. Most of the flood events were caused by heavy rains that are largely linked to the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation with a superimposed effect of the ENSO. Discharges >30 m3/s, i.e., extraordinary and catastrophic floods, occur with positive phases of the PDO and the ENSO. Further exploration of the fluvial geological record of the Copiapó River will help lengthening to thousands of years the flood record what will help improving communities' resilience by anticipating flood hazards in the current global change context, in which stronger rainfall events modulated by ENSO and ENSO-like conditions are expected.
•We study the instrumental and historical record of the Copiapó River floods.•Paleodischarges were reconstructed using palaeohydrological techniques.•Three catastrophic floods of thirty-six events have occurred in the last 400 years.•Large flood events occur with an average recurrence interval of 120 years.•Floods were mostly caused by heavy rains mainly linked to the PDO and the ENSO.
Extreme precipitation events and multi-annual droughts, especially in arid to semi-arid subtropical regions, are among the most critical El Niño Southern Oscillation (ENSO) and global climate change ...impacts. Here, we assess the variability of torrential rainfall during the Late Holocene and its projection into the 21st century at the southern edge of the hyperarid Atacama Desert.
The analysis of historical data since the beginning of the 20th century reveals that most (76.5%) alluvial disasters in the southern Atacama Desert (26–30°S) have resulted from extreme rainfall events occurring between March and September under El Niño conditions, and more frequently during the warm phase of the Pacific Decadal Oscillation. Particular rainfall events under these ocean-climate conditions are associated with the convective phase of the Madden-Julian Oscillation (MJO) near the central-equatorial Pacific, resulting in warmer sea surface temperature (SST) there and in the triggering of persistent/intense Pacific South America (PSA) tropical-extratropical teleconnection patterns which result in blocking of the westerly flow at high latitudes and the subsequent deviation of storm tracks towards central-northern Chile.
On a longer timescale, marine sediments from Tongoy Bay (30°S) reveal an increasing trend of stronger runoff by torrential coastal rain since ca. 3500 cal yr BP and even stronger heavy rainfall since ca. 1700 cal yr BP. Highly variable coastal sea surface temperatures in the same time span deduced from the sedimentary record can be explained by intensified southerly winds in connection with stronger alongshore pressure gradients and reduced coastal low-cloud cover. Both storm intensification and increased intensity of upwelling-favorable winds point to a variable climate conditioned by strengthened interannual ENSO and interdecadal ENSO-like variability during the Late Holocene.
Climate projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5) indicate a reduction in annual precipitation of 15–30% during the current century, together with an intensification of the storms, such as the alluvial disaster on March 25, 2015 in Atacama.
•PSA teleconnection pattern is a conditioning factor for most alluvial disaster in Atacama.•ENSO and ENSO-like conditions modulate the frequency of extreme rainfall events.•Marine sedimentary proxies point to the intensification of torrential rainfall since 3500 cal yr BP.•During the current century we expect increased aridification and heavier rainfall events.
Arid alluvial fan aggradation is highly episodic and fans often comprise active and inactive sectors. Hence the morphology and texture of fan surfaces are partly determined by secondary processes of ...weathering and erosion in addition to primary processes of aggradation. This introduces considerable uncertainty in the identification of formative processes of terrestrial and Martian fans from aerial and satellite imagery. The objectives of this study are (i) to develop a model to describe the sedimentological and morphological evolution of inactive fan surfaces in arid settings, and (ii) to assess the relative importance of primary processes of aggradation and secondary processes of weathering and reworking for surface morphology and sedimentology and for the stratigraphic record. We studied an alluvial fan characterized by a recently active sector and a long-abandoned, inactive sector along the coast of the hyperarid Atacama Desert. Here, rates of primary geomorphic activity are exceptionally low because of extreme aridity, while weathering rates are relatively high because of the effects of coastal fogs. Long-term processes of fan aggradation and reworking were determined through sedimentological facies analysis of stratigraphic sections. Ground surveys for textural and morphological patterns at the fan surface were integrated with remote-sensing by an Unmanned Airborne Vehicle (UAV). Discharges and sediment-transport capacities were calculated to estimate the efficiency of secondary runoff in reshaping the inactive fan sector. Stratigraphic sections reveal that the fan was dominantly aggraded by debris flows, whereas surface morphology is dominated by debris-flow signatures in the active sector and by weathering and runoff on the inactive sector. On the latter, rapid particle breakdown prevents the formation of a coarse desert pavement. Furthermore, relatively frequent local runoff events erode proximal debris-flow channels on the inactive sector to form local lag deposits and accumulate fine sediment in low-gradient distal channels, forming a well-developed drainage pattern that would suggest a runoff origin from aerial images. Nevertheless, reworking is very superficial and barely preserved in the stratigraphic record. This implies that fans on Earth and Mars that formed dominantly by sporadic mass flows may be masked by a surface morphology related to other processes.
•Effect of primary processes versus secondary processes assessed on alluvial fan•Stratigraphy, morphology and texture compared with ground survey and UAV images•Alluvial fan along the hyperarid Atacama coast aggraded mainly by debris flows•Morphology of inactive fan sectors dominated by fluvial erosion and weathering•Well-developed fluvial patterns on fan surface not indicative of formative process
Abstract This article explores how Francisco de Aguirre used the Copiapó Valley encomienda to negotiate political power during the transition from conquest to colonial rule in northern Chile. ...Simultaneously, we analyze the circumstances of how a native society was incorporated into the Spanish Empire after a decade of fighting and resistance on the fringes of the empire. The strategic use of the fear of native rebellions to close the road from Peru to Chile gave Aguirre enough power to negotiate an important political position, which in the future would clash with the colonial authorities. Copiapó Valley’s peripheral location in the southernmost Atacama Desert constituted a political gray zone for the colonial administration. This space contributed to consolidating power for Aguirre and enabled locals some negotiation power within the possibilities afforded by the colonial system.
Resumen Este artículo explora como Francisco de Aguirre utilizó la encomienda del Valle de Copiapó para negociar poder político durante la transición entre la conquista al dominio colonial en el norte de Chile. Simultáneamente analizamos las circunstancias de como una sociedad indígena fue incorporada dentro del imperio español luego de una década de lucha y resistencia en los márgenes del imperio. El uso estratégico del miedo a que rebeliones indígenas cerrasen el paso del camino de Perú a Chile le dio a Aguirre suficiente poder para negociar una posición política importante, la cual en el futuro entraría en conflicto con las autoridades coloniales. La ubicación periférica del Valle de Copiapó en la parte más austral del desierto de Atacama constituyó una zona gris para la administración colonial. Este espacio ayudó a consolidar el poder de Aguirre y le permitió a las poblaciones locales obtener algún poder de negociación dentro de las posibilidades que entregaba el sistema colonial.
Spatiotemporal patterns of deformation and exhumation in the central Andes are key parameters for reconstructing the kinematic history of the orogenic belt. Previous studies of the retroarc thrust ...belt document overall eastward propagation of deformation since the late Eocene, but the amount and timing of exhumation during the early phase of Andean orogeny remains largely unconstrained, particularly in the modern forearc region. In order to determine the timing and amount of exhumation prior to the late Eocene, we employed a multidating approach combining zircon U‐Pb geochronology with apatite fission track and apatite (U‐Th)/He thermochronology. We focus on the low‐temperature cooling history of the Cordillera de Domeyko thrust belt and synorogenic deposits in the Salar de Atacama basin. Our results show Late Cretaceous to Oligocene cooling and exhumation in the Cordillera de Domeyko. The distribution of cooling ages in the forearc indicates three periods of exhumation: ~86–65, ~65–50, and 50–28 Ma. The amount of cooling was variable in space and time but requires total exhumation of ~2.5–3.3 km of rocks above major structures in the thrust belt. Regional unconformities in the Salar de Atacama basin correlate with periods of eastward migration of the orogenic front at ~65 Ma and ~50–40 Ma. Pulses of deformation at the front of the thrust belt alternated with periods of out‐of‐sequence hinterland deformation and exhumation. Overall, our data show that shortening in the central Andes commenced during the Late Cretaceous (as early as ~86 Ma) and that deformation (shortening) and exhumation were coupled in space and time.
Key Points
The Cordillera de Domeyko experienced exhumation during the upper Cretaceous, Paleocene, Eocene, and Oligocene
The Cordillera de Domeyko experienced ~2.5‐3.3 km of exhumation above major structures
Regional unconformities in the Salar de Atacama basin correspond to periods of migration of the orogenic front at ~65 and between 40 and 50 Ma
The Salar de Atacama region (23°-24°S) in the Central Andes of northern Chile represents an anomalous segment of the Andes, where a very large forearc basin is present (Salar de Atacama) and the axis ...of the active volcanic chain is deflected to the east from its regional trend. Here we used magnetotelluric (MT) data to infer the distribution of partial melts and hydrothermal fluids in order to better understand the mechanism controlling the magmatism in this subduction zone.
Our 3-D inversion model shows a complex electrical resistivity structure, with several mid crustal conductors (<5 Ωm) imaged around the Salar de Atacama basin (SdAb). Although the forearc is dominated by high resistivities (>1000 Ωm), conductors were imaged below the Precordillera and in the northern SdAb, where major trench-parallel and transverse fault systems are present. We interpret these forearc conductors as evidence of active and fossil hydrothermal systems hosted by the Precordillera Fault System and the Calama-Olacapato-El Toro transverse fault. Conductors found along the arc are spatially coincident with active volcanoes likely representing partial melting in the continental crust. A large conductor was imaged in the backarc, indicating the presence of a significant volume of partial melts below the Altiplano-Puna plateau, as many previous geophysical studies have suggested. The model shows that the mid-lower crust and subcontinental mantle (i.e., >15 km depth) below the SdAb is dominated by resistivity values >100 Ωm., suggesting the presence of a relatively poorly hydrated continental crust and mantle wedge in this segment of the northern Chile subduction zone. These results, together with petrophysical interpretations derived from previous seismic and gravimetric studies, suggest that the lithosphere below the SdAb represents an impermeable block, which inhibits the upward flow of fluids and melts derived from deeper parts of the subduction system.
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•We present a 3-D electrical resistivity model of Central Andes lithosphere (23°–24°S).•The Salar de Atacama basin (SdAb) lithosphere represents a high-resistivity block.•Petrophysical properties suggest that the SdAb block is an impermeable region.•The SdAb impermeable block may explain the eastward deflection of the volcanic arc.
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