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Drainage-divide migration, controlled by rock-uplift and rainfall patterns, may play a major role in the geomorphic evolution of mountain ranges.
However, divide-migration rates over geologic timescales have only been estimated by theoretical studies and remain empirically poorly constrained.
Geomorphological evidence suggests that the Sierra de Aconquija, on the eastern side of the southern Central Andes, northwest Argentina, is undergoing active westward drainage-divide migration. The mountain range has been subjected to steep rock trajectories and pronounced orographic rainfall for the last several million years, presenting an ideal setting for using low-temperature thermochronometric data to explore its topographic evolution.
We perform three-dimensional thermal-kinematic modeling of previously published thermochronometric data spanning the windward and leeward sides of the range to explore the most likely structural and topographic evolution of the range.
We find that the data can be explained by scenarios involving drainage-divide migration alone, or by scenarios that also involve changes in the structures that have accommodated deformation through time.
By combining new Be-10-derived catchment-average denudation rates with geomorphic constraints on probable fault activity, we conclude that the evolution of the range was likely dominated by west-vergent faulting on a high-angle reverse fault underlying the range, together with westward drainage-divide migration at a rate of several km per million years.
Our findings place new constraints on the magnitudes and rates of drainage-divide migration in real landscapes, quantify the effects of orographic rainfall and erosion on the topographic evolution of a mountain range, and highlight the importance of considering drainage-divide migration when interpreting thermochronometer age patterns.
El plateau Andino es el segundo plateau orogénico más grande del mundo y se ubica en los Andes Centrales, desarrollado en un sistema orogénico no colisional. Se extiende desde el sur del Perú (15°S), hasta el norte de Argentina y Chile (27°30´S). A partir de los 24°S y prologándose hacia el sur, el plateau Andino se denomina Puna y está caracterizado por un sistema de cuencas endorreicas y salares delimitados por cordones montañosos. Entre los 26° y 27°30´S, la Puna encuentra su límite austral en una zona de transición entre una zona de subducción normal y una zona de subducción plana o “flat slab” que se prolonga hasta los 33°S. Diversos estudios documentan la ocurrencia de un aumento del espesor cortical, y levantamiento episódico y diacrónico del relieve, alcanzando su configuración actual durante el Mioceno tardío. Posteriormente, el plateau habría experimentado un cambio en el estilo de deformación dominado por procesos extensionales evidenciado por fallas y terremotos de cinemática normal. Sin embargo, en el borde sur del plateau de la Puna y en las áreas delimitadas con el resto del orógeno, la variación del campo de esfuerzo no está del todo comprendida, reflejando una excelente oportunidad para evaluar cómo el campo de esfuerzo puede evolucionar durante el desarrollo del orógeno y cómo puede verse afectado por la presencia/ausencia de un plateau orogénico, así como también por la existencia de anisotropías estructurales propias de cada unidad morfotectónica.
Esta Tesis investiga la relación entre la deformación cortical somera y la evolución en tiempo y espacio del campo de esfuerzos en el sector sur del plateau Andino, durante el cenozoico tardío. Para realizar esta investigación, se utilizaron técnicas de obtención de edades radiométricas con el método Uranio-Plomo (U-Pb), análisis de fallas mesoscópicas para la obtención de tensores de esfuerzos y delimitación de la orientación de los ejes principales de esfuerzos, análisis de anisotropía de susceptibilidad magnética en rocas sedimentarias y volcanoclásticas para estimar direcciones de acortamiento o direcciones de transporte sedimentario, técnicas de modelado cinemático para llegar a una aproximación de las estructuras corticales profundas asociadas a la deformación allí registrada, y un análisis morfométrico para la identificación de indicadores geomorfológicos asociados a deformación producto de la actividad tectónica cuaternaria.
Combinando estos resultados con los antecedentes previamente documentados, el estudio revela una compleja variación del campo de esfuerzo caracterizado por cambios en la orientación y permutaciones verticales de los ejes principales de esfuerzos, durante cada régimen de deformación, durante los últimos ~24 Ma. La evolución del campo de esfuerzos puede ser asociada temporalmente a tres fases orogénicas involucradas con la evolución de los Andes Centrales en esta latitud: (1) una primera fase con un régimen de esfuerzos compresivos de acortamiento E-O documentado desde el Eoceno, Oligoceno tardío hasta el Mioceno medio en el área, coincide con la fase de construcción andina, engrosamiento y crecimiento de la corteza y levantamiento topográfico; (2) una segunda fase caracterizada por un régimen de esfuerzos de transcurrencia, a partir de los ~11 Ma en el borde occidental y compresión y transcurrencia a los~5 Ma en el borde oriental del plateau de la Puna, y un régimen de esfuerzo compresivos en Famatina y las Sierras Pampeanas interpretado como una transición entre la construcción orogénica del Neógeno y la máxima acumulación de deformación y el alzamiento topográfico del plateau de la Puna, y (3) una tercera fase donde el régimen se caracteriza por la transcurrencia en la Puna y en su borde occidental y en su borde oriental con las Sierras Pampeanas, después de ~5-4 Ma, interpretado como un régimen de esfuerzos controlados por el engrosamiento cortical desarrollado a lo largo del borde sur del plateau Altiplano/Puna, previo a un colapso orogénico. Los resultados dejan en evidencia que el borde del plateau experimentó el paso desde un régimen compresivo hacia uno transcurrente, que se diferencia de la extensión documentada hacia el norte en el plateau Andino para el mismo período. Cambios en los esfuerzos similares han sido documentado durante la construcción del plateau Tibetano, en donde un régimen de esfuerzo predominantemente compresivo cambió a un régimen de transcurrente cuando el plateau habría alcanzado la mitad de su elevación actual, y que posteriormente derivó en un régimen extensional, entre 14 y 4 Ma, cuando la altitud del plateau fue superior al 80% respecto a su actitud actual, lo que podría estar indicando que los regímenes transcurrentes representan etapas transicionales entre las zonas externas del plateau bajo compresión y las zonas internas, en las que los regímenes extensionales son más viables de ocurrir.
Im Rahmen der Dissertation wurden an Wässern und freien Gasen aus Thermalquellen sowie an weniger als 5 Millionen Jahre alten basischen Vulkaniten des zentralandinen Puna-Hochplateaus (NE-Argentinien) umfangreiche element- und isotopengeochemische Untersuchungen durchgeführt und die Edelgasgehalte und -isotopensignaturen in diesen Medien bestimmt. Damit soll ein Beitrag zum besseren Verständnis der jüngeren Subduktionsgeschichte im Bereich der südlichen Zentralanden geleistet, die Wechselwirkungen zwischen ozeanischer Unter- und kontinentaler Oberplatte sichtbar gemacht und die Edelgassystematik verbessert werden. Wie die Ergebnisse der Untersuchungen an Gasen aus den Thermalquellen der Puna-Region zeigen, ist der Anteil an Mantel-Helium in den Thermalquellen dieser Region mit bis zu 67 % wesentlich höher als in der westlich gelegenen vulkanisch aktiven Westkordillere und den anderen angrenzenden Gebieten. In einigen Quellen konnten sogar Anteile an Mantel-Neon nachgewiesen werden, was aufgrund von Überlagerungen mit Neon atmosphärischen und krustalen Ursprungs weltweit bisher nur vereinzelt gelungen ist. Für kontinentale Bereiche mit großer Krustendicke ist ein solch starker Mantelgasfluss äußerst ungewöhnlich und bedeutet, dass Mantelschmelzen bis in die Kruste aufgedrungen sind und tief reichende Wegsamkeiten existieren, so dass die Mantelgase aufsteigen können, ohne stark krustal beeinflusst zu werden. Dass im Bereich der Puna rezent Mantelmaterial in die Kruste aufsteigt, zu diesem Ergebnis kommen auch aktuelle seismologische Untersuchungen. Zudem wurden junge, vorwiegend monogenetische Basalte bis basaltische Andesite geochemisch auf ihre Haupt-, Neben- und Spurenbestandteile sowie ihre Gehalte an Seltenenerdenelementen hin untersucht. Auch wurden die Isotopenverhältnisse von Sr, Nd und Pb in den Gesteinen bestimmt und petrographisch-mineralogische Analysen der darin enthaltenen Olivine und Pyroxene durchgeführt. Wie die Resultate belegen, haben die Magmen bei ihrem Aufstieg durch die Erdkruste insbesondere Material aus der Oberkruste assimiliert und sind zudem durch Fluide aus der abtauchenden Platte beeinflusst worden. Damit konnte gezeigt werden, dass einfache geochemische Methoden allein nicht ausreichen, um die Mantelquelle der Magmen ermitteln oder Aussagen über die Asthenosphärendynamik in der Region machen zu können. Im Gegensatz dazu zeigen die Messungen der Edelgasisotopenverhältnisse in den Fluideinschlüssen der Olivine und Pyroxene, dass deren Edelgaszusammensetzung nicht durch Krustenkontamination beeinflusst wurde, weil die Magmen erst nach der Olivin- bzw. Pyroxen-Kristallisation Schmelzen aus der Oberkruste assimiliert haben. Darüber hinaus konnten durch die Edelgasisotopenmessungen die bisher höchsten magmatischen He- und Ne-Isotopenverhältnisse von ganz Südamerika nachgewiesen werden. Aus der unterschiedlichen Höhe der Messwerte ist zu schließen, dass die im Osten der Puna vorkommenden älteren Laven aus einem nichtkonvektiven (lithosphärischen) Mantel stammen, während die am vulkanischen Bogen und Westrand der Puna gelegenen jüngeren Laven, ihren Ursprung in einer konvektiven (asthenosphärischen) Mantelquelle haben. Zudem konnte gezeigt werden, dass der Mantelgasfluss in der Region in den letzten 5 Millionen Jahren stark zunahm und sich die Eruption von mantelstämmigen basischen Laven in dieser Zeit kontinuierlich in westliche Richtung zum aktiven Vulkanbogen hin verlagerte. Im daraus abgeleiteten Modell beruht dieser Prozess (1) auf einer an die Kontinentalverschiebung gekoppelten W-Drift des Kontinents und (2) auf einem mit der Versteilung der Unterplatte verbundenen Vordringen des subkontinentalen asthenosphärischen Mantels nach W, nach dem Ende der Subduktion des unterseeischen aseismischen Juan Fernández-Rückens in der Region. Zudem gibt es starke Argumente dafür, dass die asthenosphärischen Magmen aus einer fluidreichen Zone in 500 – 600 km Tiefe parallel zur subduzierten Platte aufsteigen und nicht, wie bisher angenommen, durch Schmelzbildung in Bereichen unter 200 km Tiefe, allein durch Entwässerung der abtauchenden Platte erzeugt werden. Zu diesem Resultat führt vor allem die Kombination der He-Isotopenverhältnisse mit Ergebnissen seismologischer Untersuchungen.
The region of the Andean back-arc of northwestern Argentina has been struck by several magnitude >= 6 crustal earthquakes since the first historically recorded event in 1692. One of these events corresponds to the Anta earthquake on 25 August 1948, with epicenter in the Santa Barbara System causing three deaths and severe damage in Salta and Jujuy provinces with maximum Modified Mercalli seismic intensities (MMI) of IX. We collected and digitized analog seismograms of this earthquake from worldwide seismic observatories in order to perform first-motion analysis and modeling of long-period teleseismic P-waveforms. Our results indicate a simple seismic source of M0 = 2.85 x 1019 N m consistent with a moment magnitude Mw = 6.9. We have also tested for the focal depth determining a shallow source at 8 km with a reverse focal mechanism solution with a minor dextral strike-slip component (strike 20 degrees, dip 30 degrees, rake 120 degrees) from the best fit of waveforms. Using magnitude size empirical relationships, the comparison of the obtained Mw 6.9 magnitude value and the ca. 10,000 km2 area of MMI >= IX from our seismic intensity map, which was obtained from newspaper and many historical reports, indicates a rupture length of 42 +/- 8 km for the Anta earthquake. We show our results in a 3D geological model around the epicentral area, which integrates modern seismicity, geological data, and information of a previously studied east-west cross section located a few kilometers south of the 1948 epicenter. The integration of all available information provides evidence of the re-activation of the Pie de la Sierra del Gallo fault during the 1948 Mw 6.9 shallow earthquake; this thrust fault bounds the Santa Barbara System along its western foothill.
Despite a long history of plate convergence at the western margin of the South American plate that has been ongoing since at least the Early Paleozoic, the southern Peruvian fore-arc displays little to no evidence of shortening. In the light of this observation, we assess the deformation history of the southern Peruvian fore-arc and its geodynamic implications. To accomplish this, we present a new structural and geo-thermochronological data set (zircon U-Pb, mica Ar-40/Ar-39, apatite and zircon fission-track and zircon (U-Th)/He analyses) for samples collected along a 400km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures 350 degrees C since the Neoproterozoic and was later intruded by Jurassic volcanic arc plutons. Along the coast, a peculiar apatite fission-track age pattern, coupled with field observations and a synthesis of available geological maps, allows us to identify crustal-scale tilted blocks that span the coastal Peruvian fore-arc. These blocks, bounded by normal faults that are orthogonal to the trench, suggest post-60Ma trench-parallel extension that potentially accommodated oroclinal bending in this region. Block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore-arc basin. Our data set allows us to estimate the cumulative slip on these faults to be less than 2km and questions the large amount of trench-parallel extension suggested to have accommodated this bending.
Controls on the deformation pattern (shortening mode and tectonic style) of orogenic forelands during lithospheric shortening remain poorly understood. Here, we use high-resolution 2D thermomechanical models to demonstrate that orogenic crustal thickness and foreland lithospheric thickness significantly control the shortening mode in the foreland. Pure-shear shortening occurs when the orogenic crust is not thicker than the foreland crust or thick, but the foreland lithosphere is thin (<70-80 km, as in the Puna foreland case). Conversely, simple-shear shortening, characterized by foreland underthrusting beneath the orogen, arises when the orogenic crust is much thicker. This thickened crust results in high gravitational potential energy in the orogen, which triggers the migration of deformation to the foreland under further shortening. Our models present fully thick-skinned, fully thin-skinned, and intermediate tectonic styles in the foreland. The first tectonics forms in a pure-shear shortening mode whereas the others require a simple-shear mode and the presence of thick (>similar to 4 km) sediments that are mechanically weak (friction coefficient <similar to 0.05) or weakened rapidly during deformation. The formation of fully thin-skinned tectonics in thick and weak foreland sediments, as in the Subandean Ranges, requires the strength of the orogenic upper lithosphere to be less than one-third as strong as that of the foreland upper lithosphere. Our models successfully reproduce foreland deformation patterns in the Central and Southern Andes and the Laramide province.
The Andean Plateau (Altiplano-Puna Plateau) of the southern Central Andes is the second-highest orogenic plateau on our planet after Tibet. The Andean Plateau and its foreland exhibit a pronounced segmentation from north to south regarding the style and magnitude of deformation. In the Altiplano (northern segment), more than 300 km of tectonic shortening has been recorded, which started during the Eocene. A well-developed thin-skinned thrust wedge located at the eastern flank of the plateau (Subandes) indicates a simple-shear shortening mode. In contrast, the Puna (southern segment) records approximately half of the shortening of the Altiplano - and the shortening started later. The tectonic style in the Puna foreland switches to a thick-skinned mode, which is related to pure-shear shortening. In this study, carried out in the framework of the StRATEGy project, high-resolution 2D thermomechanical models were developed to systematically investigate controls of deformation patterns in the orogen-foreland pair. The 2D and 3D models were subsequently applied to study the evolution of foreland deformation and surface topography in the Altiplano-Puna Plateau. The models demonstrate that three principal factors control the foreland-deformation patterns: (i) strength differences in the upper lithosphere between the orogen and its foreland, rather than a strength difference in the entire lithosphere; (ii) gravitational potential energy of the orogen (GPE) controlled by crustal and lithospheric thicknesses, and (iii) the strength and thickness of foreland-basin sediments. The high-resolution 2D models are constrained by observations and successfully reproduce deformation structures and surface topography of different segments of the Altiplano-Puna plateau and its foreland. The developed 3D models confirm these results and suggest that a relatively high shortening rate in the Altiplano foreland (Subandean foreland fold-and-thrust belt) is due to simple-shear shortening facilitated by thick and mechanically weak sediments, a process which requires a much lower driving force than the pure-shear shortening deformation mode in the adjacent broken foreland of the Puna, where these thick sedimentary basin fills are absent. Lower shortening rate in the Puna foreland is likely accommodated in the forearc by the slab retreat.
We present a new three-dimensional density model of the Central Andes characterizing the structure and composition of the lithosphere together with a geodynamic simulation subjected to continental intraplate shortening. The principal aim of this study is to assess the link between heterogeneities in the lithosphere and different deformation patterns and styles along the orogen-foreland system of the Central Andes. First, we performed a 3D integration of new geological and geophysical data with previous models through forward modelling of Bouguer anomalies. Subsequently, a geodynamic model was set-up and parametrized from the previously obtained 3D structure and composition. We do not find a unambigous correlation between the resulting density configuration and terrane boundaries proposed by other authors. Our models reproduce the observed Bouguer anomaly and deformation patterns in the foreland. We find that thin-skinned deformation in the Subandean fold-and thrust belt is controlled by a thick sedimentary layer and coeval underthrusting of thin crust of the foreland beneath the thick crust of the Andean Plateau. In the adjacent thick-skinned deformation province of the inverted Cretaceous extensional Santa Barbara System sedimentary strata are much thinner and crustal thickness transitions from greater values in the Andean to a more reduced thickness in the foreland. Our results show that deformation processes occur where the highest gradients of lithospheric strength are present between the orogen and the foreland, thus suggesting a spatial correlation between deformation and lithospheric strength.
We describe the petrographic characteristics, whole-rock geochemistry and mineral chemistry of rocks from the Pucarilla-Cerro Tipillas Volcanic Complex with emphasis on the rocks belonging to the middle Miocene Luingo caldera, located in the south-eastern portion of the Central Volcanic Zone (CVZ) of the Andes. We modelled the petrogenesis of the Luingo caldera rocks as a mixture of ca. 20% crustal magmas and 80% of mantle magmas by AFC with recharge processes. A comparison of Luingo geochemical data with the composition of Miocene-Pliocene volcanic rocks from the broad area, points to major thickening events during the middle Miocene for the western portion and during the upper Miocene for the eastern portion of the Southern CVZ. In the eastern sector (similar to 66 degrees W) the mantle source appears to change from a spinel-lherzolite type for the middle Miocene to a garnet-lherzolite type for the upper Miocene-Pliocene magmas. The areal distribution of the volcanic products led to the recognition of approximately equivalent areas covered by volcanic rocks both in the eastern and in the western Puna borders. This indicates a broad arc, which was structurally controlled at the proto-Puna/Puna margins, whose geochemical differences are related with variations in crustal thicknesses and heterogeneous mantle sources from west to east.
The eastern flanks of the Central Andes are characterized by deep convection, exposing them to hydrometeorological extreme events, often resulting in floods and a variety of mass movements. We assessed the spatiotemporal pattern of rainfall trends and the changes in the magnitude and frequency of extreme events (ae<yen>95th percentile) along an E-W traverse across the southern Central Andes using rain-gauge and high-resolution gridded datasets (CPC-uni and TRMM 3B42 V7). We generated different climate indices and made three key observations: (1) an increase of the annual rainfall has occurred at the transition between low (< 0.5 km) and intermediate (0.5-3 km) elevations between 1950 and 2014. Also, rainfall increases during the wet season and, to a lesser degree, decreases during the dry season. Increasing trends in annual total amounts characterize the period 1979-2014 in the arid, high-elevation southern Andean Plateau, whereas trend reversals with decreasing annual total amounts were found at low elevations. (2) For all analyzed periods, we observed small or no changes in the median values of the rainfall-frequency distribution, but significant trends with intensification or attenuation in the 95th percentile. (3) In the southern Andean Plateau, extreme rainfall events exhibit trends towards increasing magnitude and, to a lesser degree, frequency during the wet season, at least since 1979. Our analysis revealed that low (< 0.5 km), intermediate (0.5-3 km), and high-elevation (> 3 km) areas respond differently to changing climate conditions, and the transition zone between low and intermediate elevations is characterized by the most significant changes.