Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling

AbstractThe southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5°) in the north to relatively steep (~ 30°) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29° S–39° S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (~ 55 km) and less dense (~ 2900 kg/m3) than in the forearc (~ 35 km, ~ 2975 kg/m3) and foreland (~ 30 km, ~ 3000 kg/m3). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.

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Lithospheric density structure of the Southern Central Andes and their forelands constrained by 3D gravity modelling

10.5194/egusphere-egu2020-3313 ◽

2020 ◽

Author(s):

Constanza Rodriguez Piceda ◽

Magdalena Scheck-Wenderoth ◽

Maria Laura Gómez Dacal ◽

Judith Bott ◽

Claudia Prezzi ◽

...

Keyword(s):

Central Andes ◽

Crustal Thickening ◽

Velocity Variation ◽

Mountain Range ◽

South American ◽

The South ◽

Complex Deformation ◽

Nazca Plate ◽

Southern Central ◽

The Relationship

The Andean orogeny is a ~7000 km long N-S trending mountain range developed along the South American western margin. The formation of this mountain range is driven by the subduction of the oceanic Nazca plate beneath the continental South American plate, being the only known present-day case of subduction-type orogeny. In this tectonic setting, the intrinsic physical properties of the overriding plate govern the formation of zones of crustal strength and weakness and control the localization and the style of deformation. Furthermore, the dynamics of the subducting oceanic lithosphere is strongly conditioned by the properties of the continental counterpart. The southern segment of the Central Andes (29&#176;S-39&#176;S) is a suitable scenario to investigate the relationship between the two plates for several reasons. It is characterized by a complex deformation pattern with variations in horizontal shortening, crustal thickening and mean topographic elevation. In addition, the subduction angle changes at 33&#176;S-35&#176;S latitude from flat in the North to normal in the South. To gain insight into this geodynamic system, a detailed characterization of the lithosphere is needed. Therefore, we constructed a 3D model of the entire segment of the Southern Central Andes that is consistent with the available geological, seismic and gravity data in order to assess the geometry and density variation within the lithosphere. The derived configuration shows a spatial correlation between density domains and known tectonic features. It is also consistent with other independent observations such as S wave velocity variation and surface deformation. The generated structural model allows us to reach the first conclusions about the relationship between the characteristics of the overriding plate and the crustal deformation and dynamics of the subduction system. It is also useful to constrain thermomechanical experiments and therefore contributes to discussions about the crustal thermal and rheological fields within the region.

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Campesinos and Conservation in the Central Andes: Indigenous Herding and Conservation of the Vicuña

Environmental Conservation ◽

10.1017/s0376892900035359 ◽

1986 ◽

Vol 13 (4) ◽

pp. 311-318 ◽

Cited By ~ 3

Author(s):

Wayne Bernhardson

Keyword(s):

Indigenous Peoples ◽

Land Tenure ◽

Wildlife Management ◽

Technical Assistance ◽

Central Andes ◽

Short Term ◽

Local Patterns ◽

Local Residents ◽

Economic Future

Efforts to protect the wild Vicuña, an endangered relative of the domesticated Llama and Alpaca of the central Andes, and to increase the species' numbers, have been more successful in achieving technical wildlife management goals than in complementing agro-pastoral activities of the indigenous peoples of the region. In both Pampa Galeras National Reserve in Peru and in Lauca National Park in Chile, lack of consultation with native pastoralists, whose lands the reserves occupy, reflects weak commitment to broader social and economic goals on the part of national authorities, as well as the inadequacy of international technical assistance in involving local communities in such projects.Specifically, those projects have neglected to consider the importance of local patterns of land tenure, which are only in small part communal, and of economic differentiation with those communities. At the same time, authorities have controlled or monitored access to such subsistence resources as pasture and fuel for local residents, while failing to offer short-term alternatives for economic opportunity. This is in contrast to the free hand enjoyed by energy and agricultural irrigation projects such as those which benefit Chile's Atacama port of Arica. Continuation of such policies jeopardizes long-term success of wildlife protection as well as the economic future of the region's human inhabitants.

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Multi‐Centennial‐Scale Variations of South American Summer Monsoon Intensity in the Southern Central Andes (24–27°S) During the Late Holocene

Geophysical Research Letters ◽

10.1029/2019gl084157 ◽

2020 ◽

Vol 47 (4) ◽

Cited By ~ 2

Author(s):

Sebastian T. Kock ◽

Karsten Schittek ◽

Bertil Mächtle ◽

Antonio Maldonado ◽

Heinz Vos ◽

...

Keyword(s):

Summer Monsoon ◽

Late Holocene ◽

Central Andes ◽

South American ◽

Monsoon Intensity ◽

Southern Central

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Early exhumation of the Frontal Cordillera (southern Central Andes at ~33.5°S) and implications for Andean mountain-building

10.5194/egusphere-egu2020-3346 ◽

2020 ◽

Author(s):

Robin Lacassin ◽

Magali Riesner ◽

Martine Simoes ◽

Tania Habel ◽

Audrey Margirier ◽

...

Keyword(s):

Central Andes ◽

Crustal Thickening ◽

Mountain Range ◽

Mountain Building ◽

Ongoing Debate ◽

First Order ◽

The Andes ◽

Minimum Age ◽

Southern Central ◽

Basement Structures

The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building&#8232; and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of&#8232; this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The timing of uplift and deformation of the Frontal Cordillera basement culmination of&#8232; the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. Constraining this timing and in particular the onset of uplift is a key point in the ongoing debate about the initial vergence of the crustal-scale thrusts at the start of the Cenozoic Andean orogeny. To solve for this, new apatite and zircon (U-Th)/He ages from granitoids of the Frontal Cordillera at ~33.5&#176;S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12&#8211;14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample (Riesner et al. 2019). The inverse modeling of the thermochronological data using QTQt software confirms these conclusions and point to a continuous cooling rate since onset of cooling. The minimum age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera question the classically proposed east-vergent models of the Andes at this latitude. Additionally, this study provides further support to recent views on Andean mountain-building proposing that the Andes-Altiplano orogenic system grew firstly over west-vergent basement structures before shifting to dominantly east-vergent thrusts.&#160; Riesner M. et al. 2019, Scientific Reports, DOI: 10.1038/s41598-019-44320-1

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The nature of the North-South change of the magnitude of tectonic shortening in Central Andes at Altiplano-Puna latitudes: a thermomechanical modeling approach.

10.5194/egusphere-egu2020-8177 ◽

2020 ◽

Author(s):

Michaël Pons ◽

Stephan Sobolev

Keyword(s):

Cross Sections ◽

Pure Shear ◽

Foreland Basin ◽

Central Andes ◽

South American ◽

Nazca Plate ◽

South American Plate ◽

The Andes ◽

Subduction System ◽

Roll Back

The Andean orogeny is a subduction-type orogeny, the oceanic Nazca Plate sinks under the continental South American Plate. While the subduction has been active since ~180 Ma, the shortening of the Andes initiated at ~50 Ma or less.In a oceanic-continental subduction system, the absolute velocity of the overriding-plate (OP) largely controls the style of subduction (stable, advancing, retreating), the geometry of the slab (dipping angle, curvature) and the style of deformation (shortening or spreading) within the OP. In the case of the Central Peru-Chile subduction, the South American plate is advancing westwards whereas the Nazca plate is anchored into the transition zone (~660 km). As a consequence, the trench is forced to retreat and the Nazca plate to roll-back. The dip of the slab decreases meanwhile the Andes experienced a maximum shortening of ~300 km at ~19-21&#176;S latitudes.Previous study have shown that the strain localizes within areas of low strength and low gravitational potential of energy. In central Andes, weakening mechanisms of the OP such as lithospheric delamination have intensified the magnitude of tectonic shortening and contributed to formation of the Altiplano-Puna plateau. The deformation between the plateau and the foreland occurs in the form of pure shear or simple shear and is expressed in terms of different tectonic styles in the foreland basin, thick-skinned (e.g the Puna) and thin-skinned (e.g the Altiplano), respectively. Nevertheless, the influence of the strength variations of the OP on the subduction dynamics in the case of the central Andes has been poorly explored so far. Our hypothesis is that lateral variations of OP strength result in variable rates of trench roll-back. To test it, we have built 2D high-resolution E-W cross sections along the Altiplano and Puna latitudes (12-27&#176;S) including the subduction of the Nazca plate. For that purpose, we used the FEM geodynamic code ASPECT. Our model includes visco-plastic rheology in addition to gabbro-eclogite phase transition. These preliminary results contribute to the discussion on the nature of the magnitude of shortening in a subduction system. They are also a first step to derive a 3D model of the entire region and to consider additional surface processes such as erosion, transportation and sedimentation.

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Long-term dispersion of river gravel in a canyon in the Atacama Desert, Central Andes, deduced from their 10Be concentrations

Scientific Reports ◽

10.1038/s41598-019-53806-x ◽

2019 ◽

Vol 9 (1) ◽

Cited By ~ 2

Author(s):

Sébastien Carretier ◽

Vincent Regard ◽

Laëtitia Leanni ◽

Marcelo Farías

Keyword(s):

Grain Size ◽

Direct Proof ◽

Sedimentary Basins ◽

Central Andes ◽

Short Term ◽

Localized Source ◽

Gravel Size ◽

Velocity Relationship ◽

The Mean

AbstractIntense storms or earthquakes in mountains can supply large amounts of gravel to rivers. Gravel clasts then travel at different rates, with periods of storage and periods of displacement leading to their downstream dispersion over millennia. The rate of this dispersion controls the long-term downcutting rate in mountainous rivers as well as the grain-size signature of climate and tectonic variations in sedimentary basins. Yet, the millennial dispersion rates of gravel are poorly known. Here, we use 10Be concentrations measured in individual pebbles from a localized source along a 56 km-long canyon in the Central Andes to document the distribution of long-term gravel transit rates. We show that an inverse grain-size velocity relationship previously established from short-term tracer gravel in different rivers worldwide can be extrapolated to the long-term transit rates in the Aroma River, suggesting some universality of this relationship. Gravel are also dispersed by large differences in the mean transport rates independent of gravel size, highlighting that some gravel rest at the river surface over tens of thousands of years. These different transport rates imply a strong spreading of the gravel plumes, providing direct proof for the long-term river buffering of sediment signals between mountainous sources and sedimentary basins. The inferred distribution of residence times suggests the first evidence of anomalous diffusion in gravel transport over long timespans.

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Lithospheric Structure of the Central Andes Forearc from Gravity Data Modeling: Implication for Plate Coupling

Lithosphere ◽

10.2113/2020/8843640 ◽

2020 ◽

Vol 2020 (1) ◽

pp. 1-13

Author(s):

Greg Gushurst ◽

Rezene Mahatsente

Keyword(s):

Continental Crust ◽

Buoyancy Force ◽

Gravity Data ◽

Central Andes ◽

High Density ◽

Gravity Models ◽

The West ◽

Nazca Plate ◽

Plate Coupling ◽

Nazca Ridge

Abstract Geodetic and seismological data indicates that the Central Andes subduction zone is highly coupled. To understand the plate locking mechanism within the Central Andes, we developed 2.5-D gravity models of the lithosphere and assessed the region’s isostatic state. The densities within the gravity models are based on satellite and surface gravity data and constrained by previous tomographic studies. The gravity models indicate a high-density (~2940 kg m-3) forearc structure in the overriding South American continental lithosphere, which is higher than the average density of the continental crust. This structure produces an anomalous pressure (20-40 MPa) on the subducting Nazca plate, contributing to intraplate coupling within the Central Andes. The anomalous lithostatic pressure and buoyancy force may be controlling plate coupling and asperity generation in the Central Andes. The high-density forearc structure could be a batholith or ophiolite emplaced onto the continental crust. The isostatic state of the Central Andes and Nazca plate is assessed based on residual topography (difference between observed and isostatic topography). The West-Central Andes and Nazca ridge have ~0.78 km of residual topography, indicating undercompensation. The crustal thickness beneath the West-Central Andes may not be sufficient to isostatically support the observed topography. This residual topography may be partially supported by small-scale convective cells in the mantle wedge. The residual topography in the Nazca ridge may be attributed to density differences between the subducting Nazca slab and the Nazca ridge. The high density of the subducted Nazca slab has a downward buoyancy force, while the less dense Nazca ridge provides an upward buoyancy force. These two forces may effectively raise the Nazca ridge to its current-day elevation.

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Basement composition and basin geometry controls on upper-crustal deformation in the Southern Central Andes (30–36°S)

Geological Magazine ◽

10.1017/s0016756816000364 ◽

2016 ◽

Vol 153 (5-6) ◽

pp. 945-961 ◽

Cited By ~ 5

Author(s):

JOSÉ F. MESCUA ◽

LAURA GIAMBIAGI ◽

MATÍAS BARRIONUEVO ◽

ANDRÉS TASSARA ◽

DIEGO MARDONEZ ◽

...

Keyword(s):

Central Andes ◽

Nazca Plate ◽

The Andes ◽

Structural Style ◽

Thrust Belts ◽

Southern Central ◽

History Of ◽

Basin Geometry ◽

Subduction System ◽

Different Parts

AbstractDeformation and uplift in the Andes are a result of the subduction of the Nazca plate below South America. The deformation shows variations in structural style and shortening along and across the strike of the orogen, as a result of the dynamics of the subduction system and the features of the upper plate. In this work, we analyse the development of thin-skinned and thick-skinned fold and thrust belts in the Southern Central Andes (30–36°S). The pre-Andean history of the area determined the formation of different basement domains with distinct lithological compositions, as a result of terrane accretions during Palaeozoic time, the development of a widespread Permo-Triassic magmatic province and long-lasting arc activity. Basin development during Palaeozoic and Mesozoic times produced thick sedimentary successions in different parts of the study area. Based on estimations of strength for the different basement and sedimentary rocks, calculated using geophysical estimates of rock physical properties, we propose that the contrast in strength between basement and cover is the main control on structural style (thin- v. thick-skinned) and across-strike localization of shortening in the study area.

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Long-term and short-term vertical velocity profiles across the forearc in the NE Japan subduction zone

Quaternary Research ◽

10.1016/j.yqres.2008.12.005 ◽

2009 ◽

Vol 71 (2) ◽

pp. 227-238 ◽

Cited By ~ 26

Author(s):

Tabito Matsu'ura ◽

Akira Furusawa ◽

Hidetaka Saomoto

Keyword(s):

Velocity Profile ◽

Vertical Velocity ◽

Late Quaternary ◽

Crustal Thickening ◽

Height Distribution ◽

Uplift Rate ◽

Short Term ◽

Fluvial Terraces ◽

Vertical Velocity Profile

AbstractWe estimated the long-term vertical velocity profile across the northeastern Japan forearc by using the height distribution of late Quaternary marine and fluvial terraces, and we correlated the ages of the two marine terraces with marine isotope stages (MIS) 5.5 and 5.3 or 5.1 by cryptotephra stratigraphy. The uplift rate, estimated as 0.11–0.19 m ka− 1 from the relative heights between the terrace surfaces and eustatic sea levels, was nearly equal to, or slightly slower than, the uplift rate farther inland (0.15–0.19 m ka− 1), as determined from the relative heights of fill terrace surfaces. In contrast, the short-term vertical velocity profile, obtained from GPS observations, showed that the forearc is currently subsiding at a maximum rate of 5.4 ± 0.4 mm yr− 1. Thus, the current short-term (geodetic) subsidence does not reflect long-term (geological) tectonic movement. Short-term vertical deformation is probably driven by subduction erosion or elastic deformation caused by interplate coupling, or both. However, long-term uplift is probably due not to moment release on the mega-thrust but to crustal thickening.

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Conceptual short-term memory (CSTM) supports core claims of Christiansen and Chater

Behavioral and Brain Sciences ◽

10.1017/s0140525x15000928 ◽

2016 ◽

Vol 39 ◽

Author(s):

Mary C. Potter

Keyword(s):

Working Memory ◽

Rapid Serial Visual Presentation ◽

Language Comprehension ◽

Short Term Memory ◽

Visual Presentation ◽

Long Term Memory ◽

Short Term ◽

Term Memory ◽

Use Of Knowledge

AbstractRapid serial visual presentation (RSVP) of words or pictured scenes provides evidence for a large-capacity conceptual short-term memory (CSTM) that momentarily provides rich associated material from long-term memory, permitting rapid chunking (Potter 1993; 2009; 2012). In perception of scenes as well as language comprehension, we make use of knowledge that briefly exceeds the supposed limits of working memory.

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