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14-Jan-2014

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Contact: Kea Giles
kgiles@geosociety.org
Geological Society of America

Seafloor, sea-level, shear zones, subduction, sedimentation, and seismology

New Geology articles posted online ahead of print Jan. 10, 2014

Boulder, Colo., USA – Geology adds 19 new articles online, covering locations in China, the Atacama Desert, the Himalaya, Kilauea volcano, Australia, the Mediterranean basin, the Gulf of California, the southern Andes, the Gulf of Cadiz, the northern Red Sea, and offshore Japan. Oceanography is an emphasis in many of the papers, and several articles incorporate modeling studies. Three articles in this collection are open access.

Open Access Papers: 1. Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati-Benioff zone; 2. Land-ocean changes on orbital and millennial time scales and the penultimate glaciation; and 3. Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet.

Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Presence of an intra-lithospheric discontinuity in the central and western North China Craton: Implications for destruction of the craton
Ling Chen et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; lchen@mail.iggcas.ac.cn. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35010.1.

This study by Ling Chen and colleagues pertains to the vertical structural heterogeneities of cratonic lithosphere and their roles in the dynamic evolution of continents. They identified from dense seismic data a negative velocity discontinuity at ~100-km depth within the more than 160-km thick lithosphere beneath the central and western North China Craton (NCC). This together with similar observations in many other cratonic regions indicates that vertical structural heterogeneities are common in the lithosphere of cratons. The intra-lithospheric discontinuity may represent a long-existing mechanically weak layer within the overall strong cratonic lithosphere, and probably also existed beneath the eastern NCC before the Mesozoic. The presence of the horizontal weak layer may have facilitated simultaneous lithospheric modification both within and at the base of the lithosphere, eventually leading to the destruction of the eastern NCC in the Mesozoic. It, however unlikely, strongly affected the stability and evolution of the central and western NCC. This difference could be associated with the strong/weak influences of plate boundary processes, particularly the subduction of the Pacific plate under East Asia, on these regions. The different roles of the intra-lithospheric weak layer in the evolution of the NCC appear also applicable to the other stable and destabilized cratons in the world.


Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati-Benioff zone
Tom Garth and Andreas Rietbrock, Department of Earth, Ocean, and Ecological Sciences, University of Liverpool, Liverpool, Merseyside L69 3GP, UK; tomgarth@liverpool.ac.uk. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34730.1. OPEN ACCESS.

At the rim of many of Earth's oceans, ocean seafloor subducts beneath the less dense continents. This process causes large destructive earthquakes, explains the many earthquakes that occur hundreds of kilometers below Earth's surface, and also causes arcs of volcanoes (the ring of fire) above the subducted oceanic plate. These volcanoes and earthquakes are thought to occur as large amounts of water are carried down in the oceanic plate. Until now however, there has been little constraint on the amount of water that is carried particularly in the deeper part of the subducted plate. Tom Garth and Andreas Rietbrock show that fault zones that form in the deep oceanic trench offshore northern Japan persist to depths of up to 150 km in the subduction zone. These hydrated fault zones can carry large amounts of water, suggesting that subduction zones may carry much more water from Earth's ocean to the mantle than has previously been suggested. This supports the hypothesis that there are large amounts of water stored deep in the Earth.


Isotopically ultradepleted domains in the convecting upper mantle: Implications for MORB petrogenesis
Benjamin L. Byerly and John C. Lassiter, Jackson School of Geosciences, University of Texas at Austin, 1 University Station, C1160, Austin, Texas 78712-0254, USA; benbyerly@utexas.edu. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34757.1.

Mid-ocean ridge basalts form by partial melting of material in Earth's convecting upper mantle. Abyssal peridotites are always found with mid-ocean ridge basalts and thus assumed to represent the solid residue from melt extraction. If so, both peridotite and associated basalt should be in isotopic equilibrium. However, recent reports that the isotopic composition of many abyssal peridotites are significantly different than mid-ocean ridge basalts call into question the genetic relationship between them and also pose problems for current models for the composition of Earth's upper mantle. In this study, Benjamin L. Byerly and John C. Lassiter compare abyssal peridotites with other samples of the convecting upper mantle and demonstrate that they are indeed representative of the peridotitic upper mantle and are capable of generating mid-ocean ridge basalts. The wide range in the isotopic composition of abyssal peridotites is due to variable amounts of ancient melt extraction from the upper mantle. The common lack of isotopic equilibrium between mid-ocean ridge basalts and associated abyssal peridotites can be accounted for by a lithologically complex upper mantle and selective melting of two of these lithologic domains -- basalt recycled into the upper mantle and peridotite that have isotopic compositions that are similar to mid-ocean ridge basalt.


Seafloor spreading evolution in response to continental growth
N. Coltice et al., Laboratoire de Géologie de Lyon, Université Lyon 1, Ecole Normale Supérieure de Lyon, Université de Lyon, 69007 Lyon, France; and Institut Universitaire de France, 103, Bd Saint Michel, 75005 Paris, France; nicolas.coltice@univ-lyon1.fr. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35062.1.

Seafloor spreading is a cornerstone of plate tectonics theory, proposed in the 1960s. Scientists have reconstructed the evolution of the seafloor in the past 200 million years, driven by the need to estimate crucial geoscience data, like sea level or seafloor heat flow. Going beyond 200 million years was not possible because of lack of data, and because the models to study the dynamical origins of seafloor spreading evolution were not available. For ancient times, a fundamental unknown is how seafloor spreading operates before the continents have reached their actual size. This study identifies the mechanisms of seafloor spreading evolution related to the growth of continents, making use of very recent and state-of-the-art 3D spherical dynamic models of convection in the mantle that produce plate-like tectonics. When continents grow, seafloor spreading becomes more active, the production of new seafloor increasing, along with its fluctuations. Because continental freeboard has remained close to its present-day value for two billion years, the results presented in this study reinforce the hypothesis that more than 90% of continental growth is restricted to the Hadean-Archean eons.


Crustal structure in southeastern Egypt: Symmetric thinning of the northern Red Sea rifted margins
Ahmed Hosny, Seismology Department, National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Cairo, Egypt, ahhosny2000@yahoo.com; and Andrew Nyblade, Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34726.1.

From the abstract: Crustal structure in southeastern Egypt has been investigated to elucidate the nature of crustal thinning across the northern Red Sea. P-wave receiver function modeling for seven stations in southeastern Egypt yields typical Proterozoic crustal thicknesses of 35 to 38 km around Lake Aswan, and thinner crust (25 to 26 km) within 50 km of the Red Sea coast. Vp/Vs ratios are on average 1.78 and indicate an intermediate composition crust. These results, when combined with other estimates of crustal thickness in the region, reveal a symmetric pattern of crustal thickness beneath the conjugate margins of the northern Red Sea. Such a pattern is consistent with a pure shear model of extension, and suggests that the greater amounts of uplift and volcanism on the eastern side of the Red Sea compared to the western side may be the result of deeper flow in the mantle associated with the African superplume and not directly a consequence of the rifting process.


Incipient sediment motion across the river to debris-flow transition
Jeff P. Prancevic et al., California Institute of Technology, Division of Geological and Planetary Sciences, 1200 E. California Boulevard, MC 170-25, Pasadena, California 91125, USA; jprancev@caltech.edu. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34927.1.

River channels steeper than 5% grade make up most of the drainage network in mountainous landscapes and are the source of devastating debris flows, yet little work has been done to understand how and when sediment moves in these channels. Jeff P. Prancevic and colleagues conducted a series of experiments in a laboratory model river to identify the critical conditions for sediment motion in steep channels and whether this motion occurs as normal river transport of single grains or en masse failure of a collection of grains. In their experiments, normal river transport becomes progressively less effective at moving sediment with increased grade and gives way to en masse failure at about 40% grade. This transition is predicted well by previous models that depend on the respective Mohr-Coulomb friction angles of individual grains and a collection of grains. Use of these models and knowledge of friction angles allows us to predict where the process transition is likely to occur in natural rivers, aiding efforts to mitigate damage by debris flows and improving our understanding of how mountains erode.


Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean
Christian Heine, EarthByte Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia, christian.heine@sydney.edu.au; and Sascha Brune, Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 2.5, Geodynamic Modelling, Telegrafenberg, D-14473 Potsdam, Germany. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35082.1.

During the final stages of fragmentation of the Gondwana supercontinent in the Early Cretaceous, vast continental rift systems extended between present-day South America and Africa and within the African continent. The South Atlantic and West African rift systems were about to split the African-South American part of Gondwana North-South into nearly equal halves, generating a South- and Saharan Atlantic Ocean. In a dramatic plate tectonic twist, however, a competing rift along the present-day South American and African Equatorial Atlantic margins, won over the West African rift, causing it to become extinct, avoiding the break-up of the African continent and the formation of a Saharan Atlantic ocean. Our work elucidates the reasons behind the success and failure of these rift systems by coupling plate tectonic and advanced 3-D numerical models of continental lithosphere deformation. We find that obliquity acts as a selector between successful and aborted rift systems, explaining why the South and Equatorial Atlantic Ocean basins formed and other rifts became aborted. Our modeling also sheds lights on the dynamics of rifting, suggesting that feedback loops caused a significant acceleration of the South American plate once the Equatorial Rift System had sufficiently weakened the last remaining continental bridge between both plates.


Contourite processes associated with the Mediterranean Outflow Water after its exit from the Strait of Gibraltar: Global and conceptual implications
F.J. Hernández-Molina et al., Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK; fjhernan@uvigo.es. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35083.1.

F.J. Hernández-Molina and colleagues characterize the present and ancient processes related to the Mediterranean Outflow Water and its evolution in the Gulf of Cadiz after it was funneled through the Strait of Gibraltar. They identify morphological features along the middle slope by first time, including two large and impressive erosive channels, which determine a new and more detailed understanding of the overflow pathway of the Mediterranean Outflow Water and its deceleration upon exiting the Strait of Gibraltar. There is evidence for a present along-slope circulation, and an additional secondary circulation oblique to the main flow. However, a denser, deeper, and faster Mediterranean Outflow Water circulation that prevailed during past cold climate times is proposed. This paper determines that the Mediterranean Outflow Water enhanced during the Late Pliocene-Early Quaternary (3.2 to >2.0 Ma), coeval with global cooling, a sea-level fall and an increase in Thermohaline Circulation. So, a direct link between the Mediterranean Outflow Water and the global water masses circulation and climate is suggested. Results from this research have a broad interest to geologists, climatologists, oceanographers and petroleum geologists.


Land-ocean changes on orbital and millennial time scales and the penultimate glaciation
Vasiliki Margari et al. (corresponding author: P.C. Tzedakis), Environmental Change Research Centre, Department of Geography, University College London, London WC1E 6BT, UK; p.c.tzedakis@ucl.ac.uk. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35070.1. OPEN ACCESS.

Past glacials can be thought of as natural experiments in which different combinations of boundary conditions (such as the seasonal and latitudinal distribution and magnitude of incoming sunlight, the extent and distribution of continental ice sheets, the Earth's reflection of received insolation, and atmospheric greenhouse gas concentrations) influenced the character of climate change. These "experiments" can provide a more complete view of the range and underlying physics of natural climate variability. Although our understanding of the nature of changes during the last glacial is fairly advanced, our grasp of such changes during previous glacials has remained sketchy. Work by Vasiliki Margari and colleagues addresses this gap by providing a synthesis of centennial to multi-millennial changes in the coupled ocean-land system at the Iberian margin in the North Atlantic during the penultimate glaciation and how these relate to the framework of ice volume changes with particular reference to European ice-sheet dynamics. Their results have implications for our understanding of the sensitivity of the Atlantic ocean overturning circulation to freshwater fluxes. More specifically, they suggest that the regional distribution of ice sheets and location of freshwater discharges may influence the extent of ocean heat transport between hemispheres.


The geologic record of deep episodic tremor and slip
Nicholas W. Hayman and Luc L. Lavier, Institute for Geophysics, University of Texas, 10100 Burnet Road, R2200, Austin, Texas 78758, USA; hayman@ig.utexas.edu. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34990.1.

Earth's crust not only deforms through hazardous earthquakes or aseismic motion, but also through transient episodes of deformation that can be measured with global positioning systems and detected in subtle signals in seismometers. The best documented settings for such "slow slip events" and associated "tremor" are subduction zones, where transitions between stable and unstable frictional sliding are thought to lead to nearly episodic slip. Such slip may well load the areas of the slab interface that cause destructive earthquakes. However, deep in subduction zones, materials are thought to deform viscously rather than frictionally. Outcrops of a deep mountain belt in the southern Andes preserve excellent geological examples of such viscously deformed crustal materials. These ductile shear zones are made of mixtures of strong and weak materials, and are cut by syn-tectonic fractures. Using geological observations and an analytical expression for coupled viscous and plastic deformation, slow-slip events similar to those observed in modern subduction zones can be modeled. Combining geological observation and the model provides a nice explanation for deep slow slip events, and may be broadened to other areas where transient creep arises through mixtures of materials with contrasting strength.


Oblique rifting ruptures continents: Example from the Gulf of California shear zone
Scott E.K. Bennett and Michael E. Oskin, Department of Earth and Planetary Sciences, University of California–Davis, 1 Shields Avenue, Davis, California 95616, USA; sekbennett@usgs.gov; sekbennett@ucdavis.edu. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34904.1.

The example of the Gulf of California Shear Zone demonstrates that the degree of rift obliquity plays a fundamental role in localizing rift-related faulting and the process of continental rupture. The Gulf of California Shear Zone developed as a continuation of the southern San Andreas fault system and directly preceded the formation of the Gulf of California about 6 to 7 million years ago, via northwesterly oblique motion of Baja California away from mainland Mexico. New, high-precision paleomagnetic data from two extensive pyroclastic flow deposits, erupted from within the rift, show that approximately half (locally 0 to 75%) of clockwise vertical- axis rotation associated with the Gulf of California Shear Zone began prior to localization of rifting. Rotations are highest near the Baja California and Sonora shorelines, which constitute displaced fragments of the Gulf of California Shear Zone. This is consistent with independent geologic studies that suggest shear- zone faulting and block rotation initiated about 9-8 million years ago and plate tectonic studies that document an increase in rift obliquity about 8 million years ago.


Dominance of tectonics over climate in Himalayan denudation
Vincent Godard et al., Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545 Aix-en-Provence, France; godard@cerege.fr. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35342.1.

Denudation of continents is under the control of tectonic processes building topographic relief and precipitation providing water to erode the surface. However, the relative importance of these two factors is still a matter of debate, in particular in mountain ranges, where some studies have proposed that local high precipitation could focus denudation, which might affect underlying tectonic deformation. In this study we present a new dataset of denudation rates for the Himalayas of Nepal derived from measurements of rare isotopes (10Be) in river sediments. We observe that, despite important variations in precipitation across the studied region there are no corresponding systematic changes in denudation. On the other hand, we observe a strong gradient of denudation across the range, from the lowlands of the Lesser Himalaya to the high relief mountains of the Greater Himalaya, which mimics the inferred pattern of tectonic uplift in that area. We conclude that this similarity in patterns suggests a dominant control of Himalayan denudation by tectonic uplift, with a minor influence of climate. Such result questions the viability of models proposing that climate could be a major driver of tectonic deformation in actively deforming mountain ranges.


Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy)
Marcello Natalicchio et al., Dipartimento di Scienze della Terra, Universitŕ di Torino, 10125 Turin, Italy; marcello.natalicchio@unito.it; marcello.natalicchio@gmail.com. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34986.1.

Marcello Natalicchio and colleagues convey the results of the first detailed analysis of fluid inclusion salinities in Messinian gypsum crystals, providing new constraints for hydrologic models explaining widespread gypsum precipitation during the Messinian salinity crisis. This study demonstrates that at the northernmost offshoot of the Mediterranean Basin, gypsum did not form just from pristine evaporated seawater, but rather from a mix of seawater and Ca2+ and SO42- enriched non-marine waters probably derived from partial dissolution and recycling of coeval marginal marine deposits. Complex recycling processes should also be taken in account for explaining the genesis of other similar Messinian gypsum deposits across the Mediterranean Basin.


A Proterozoic Wilson cycle identified by Hf isotopes in central Australia: Implications for the assembly of Proterozoic Australia and Rodinia
R.G. Smits et al., New South Wales Institute of Frontiers Geoscience, University of Newcastle, Newcastle, New South Wales 2308, Australia; and Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia; russell.smits@adelaide.edu.au. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35112.1.

From the abstract: Current models for the assembly of Proterozoic Australia suggest that the North Australian craton West Australian craton, and South Australian craton had amalgamated by at least 1.6 billion years ago, with possible rafting and reattachment of the South Australian craton by about 1.3 billion years ago. In this scenario, the younger Grenvillian-aged Musgrave Province of central Australia, which separates all three cratons, has been considered postcollisional to intracratonic. However, new and recent U-Pb and Lu-Hf isotopic analyses of zircons from the Musgrave Province indicate continuous active-margin magmatic activity between 1.7 and 1.2 billion years ago. … This favors an Australia-Mexico configuration at 1.2 Ga, rather than the southwestern United States and East Antarctica or Proterozoic Australia-western United States (AUSWUS) models. The Musgrave-AFO marks a major, underestimated phase of Rodinian assembly.


Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet
Robert D. Storrar et al., Department of Geography, Durham University, South Road, Durham DH1 3LE, UK; r.d.storrar@durham.ac.uk. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35092.1. OPEN ACCESS.

Studies of relatively small outlet glaciers over a few years show that glacial meltwater can lubricate their beds and that this increases the rate of ice flow, which in turn can lead to increased sea level rise. However, the structure of the meltwater drainage system can evolve into a series of connected channels which drain meltwater more efficiently, precluding the acceleration of ice loss. Until now, the extent to which these processes operate when huge, continental-scale ice sheets melt over hundreds to thousands of years has remained unknown. This paper presents the first evidence of changes in meltwater drainage systems over continental and millennial scales. We show that as the North American ice sheet retreated, from about 13 to 7 thousand years ago, the drainage system evolved into numerous large subglacial channels, resulting in relatively stable ice flow during the demise of this large ice sheet. Ice sheet models do not currently incorporate switches in meltwater drainage systems, which is required for accurate predictions of future ice sheet dynamics and associated sea level rise.


Seismic evidence for a crustal magma reservoir beneath the upper east rift zone of Kilauea volcano, Hawaii
Guoqing Lin et al., Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA; glin@rsmas.miami.edu. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35001.1.

An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8-11 km depth beneath the upper east rift zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.


What controls the growth of the Himalayan foreland fold-and-thrust belt?
John Hirschmiller et al. (corresponding author: Djordje Grujic), Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada; dgrujic@dal.ca. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35057.1.

Empirical evidence of the impact of surface processes on the structure and shape of the Himalayan foreland fold-and-thrust belt is provided. Along-strike changes in convergence, strain rates, wedge shape, and precipitation in the Sub-Himalaya of the Himalayan orogen have been identified. These observations distributed along the length of the Himalayan arc reveal two important characteristics: (1) a distinct west-to-east increase in contraction rate correlates with convergence rates between the Indian and Eurasian plates, and (2) an eastward decrease in belt width corresponds to an eastward increase in rainfall rates. Tectonic model predictions suggest an increase in convergence rate induces higher rates of material accretion; thus, the Himalayan fold-and-thrust belt should widen eastward. Conversely, higher annual rainfall amounts and specific stream power appear to favor a narrower belt. We suggest the Himalayan fold-and-thrust belt morphology is controlled primarily by erosion, in accordance with the critical taper model. Surface material removal is controlled mainly through rainfall and runoff, and because the lithology, erodibility, and rock mechanical properties are relatively homogeneous in the Sub-Himalaya, erosion factors, in particular, precipitation and discharge, are deemed the principal controls on wedge width. We conclude that climate-induced erosion is the principal control on Himalayan fold-and-thrust belt morphology.


Is river avulsion style controlled by floodplain morphodynamics?
E.A. Hajek, Department of Geosciences, Pennsylvania State University, 511 Deike Building, University Park, Pennsylvania 16803, USA, hajek@psu.edu; and D.A. Edmonds, Department of Geological Sciences and Center for Geospatial Data Analysis, Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405, USA. Posted online 10 January 2014, http://dx.doi.org/10.1130/G35045.1.

River relocation, or avulsion, is an important process that distributes sediment, water, and nutrients across landscapes, and can be the cause of unexpected flooding hazards. Two distinct end-member avulsion styles have been observed: one where significant sedimentation occurs on a floodplain as a new river channel is built, and another where a new channel is excavated or eroded into the floodplain. This study explores the degree to which floodplain conditions can be used to predict how new channels form during river avulsion. By comparing generalized floodplain erosion and deposition rates in a given system, it may be possible to predict the likelihood of deposition- or erosional-style river avulsions. This broad rate comparison is shown to explain trends in a series of numerical models and is also consistent with data from ancient river deposits. These results demonstrate that understanding deposition and erosion patterns on floodplains may be useful for anticipating flooding and sedimentation during river avulsions.


Climate change and tectonic uplift triggered the formation of the Atacama Desert's giant nitrate deposits
Alida Pérez-Fodich et al. (Corresponding author: Martin Reich: mreich@ing.uchile.cl), Department of Geology, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile, and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Santiago, Chile. Posted online 10 January 2014, http://dx.doi.org/10.1130/G34969.1.

Nitrogen is abundant in the Earth's atmosphere but is highly depleted in the crust. Therefore, the giant nitrate deposits of the Atacama Desert are one of the most extraordinary, yet enigmatic mineral occurrences on Earth. This world-class, almost continuous ~700-km-long nitrate layer has puzzled researchers from different scientific disciplines since the 1800s, and its origin has remained elusive and highly controversial to this day. By focusing for the first time on the exotic iodine and chromium isotopic signature of the nitrates, we provide conclusive evidence to link the formation of these massive deposits to a unique convergence of large-scale groundwater flow coupled to long-term atmospheric accumulation, triggered by climate change (Atacama's desiccation) and tectonic uplift (rise of the Andes).

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