Altiplano-Puna Plateau Research Articles

Seismic interpretation and cenozoic tectonic evolution of the Pozuelos Basin, Andean plateau, Argentina

The Andean basin formation across the Altiplano-Puna plateau has been mostly associated with: (i) interconnected depozones within a simple eastward-migrating foreland, (ii) isolated depocenters in a broken foreland or (iii) intermontane basins within a rising plateau setting. Particularly, the regions located further east from the western orogenic belts are key to disentangle these proposals. The Pozuelos Basin is today an internally-drained depositional area located at ~22°22′S in the easternmost Andean plateau (an area known as the Puna). While this basin might be key to understanding the formation of basins at these latitudes, between belts Puna and Cordillera Oriental, during the Andean orogeny, only a few studies have been done since most of their records are in subsurface. In order to address the Cenozoic formation and evolution of the Pozuelos Basin, and support some of the Andean basin formation hypotheses, we conducted seismic data interpretation, complemented with field surveys. Data allowed the correlation of seismic reflectors, five identified tectono-sequences (two basements –non-stratified and stratified– packages and three stratified Cenozoic packages) and subsurface structures with the local outcropping geology. In contrast to other areas from the Puna and Altiplano, the Pozuelos basin seems to have been affected by basement thrusting and deformation along its eastern boundary, previous to the late Miocene plateau formation. Despite that in the basin configuration was clearly an isolated and intermontane setting since the late Miocene, seismic and stratigraphic data suggest that during the Late Oligocene - Middle Miocene the Pozuelos basin would have behaved as well as a disconnected depositional zone. Some features like sequence encroachments and stratigraphic arrangements in outcrops drove us to interpret an intermontane broken foreland.

Geology and structural controls of the Ag–Sn–Zn Pirquitas deposit, northwestern Argentina

Pirquitas is a polymetallic silver, tin, and zinc deposit, located in the Altiplano–Puna plateau, northwestern Argentina. The mineralization occurs in veins and hydrothermal breccia hosted in Ordovician meta–sedimentary rocks, but there is a lack of data on the structural geology of the deposit. In this paper we use detailed field mapping accompanied by structural analysis and mineralogy (petrography and XRD) to infer: (i) the mineralization structural controls, (ii) the influence of regional structures on the deposit formation, and (iii) characteristics of the Pirquitas deposit as compared with those from the Bolivian Tin–Silver Belt.The main stage of deformation observed at Pirquitas is associated with the Late Ordovician–Early Silurian Oclóyic orogeny. However, the ore deposition occurs as recurrent hydrothermal pulses related to the reactivation of Ordovician structures during the upper Miocene Quechua orogeny, which was partially contemporaneous with volcanic activity. The NW–SE and WNW–ESE ore–bearing veins of the Pirquitas deposit represent, respectively, second–order R– and T–fractures associated with major regional–scale, sinistral, brittle shear zones such as the Lípez and the Olacapato–El Toro faults.Similarities in mineralogy, structural control and association with magmatic activity indicate that Pirquitas is the southernmost deposit of the Bolivian Tin–Silver Belt. As such, subvolcanic intrusions associated with the Granada Volcano and the Coranzuli Caldera that are aligned along regional structures should be investigated by future exploratory works looking for porphyry to epithermal mineralization in the region.

Dust deposition tracks late-Holocene shifts in monsoon activity and the increasing role of human disturbance in the Puna-Altiplano, northwest Argentina

The Puna-Altiplano plateau represents a regionally significant dust source, which is critically located at the nexus between the tropical and sub-polar synoptic systems that dominate the South American climate. Dust emissions in this region would therefore be expected to be sensitive to changes in these systems, in particular the strength and position of the South American Summer Monsoon (SASM). Here, we present a late-Holocene multi-proxy study where changes in dust flux, reconstructed from a high-altitude peat mire, are examined in light of climate variability and human impacts. Results show that for most the 4300 cal. yr BP record, dust flux sensitively tracked changes in SASM activity. Prior to 2600 cal. yr BP relatively high dust flux implies dry conditions prevailed across the Puna-Altiplao in association with reduced SASM activity. The chemistry of dust deposited at this time matched the large endorheic basins on the Puna, which host ephemeral lakes and terminal fans, indicating these were actively supplying dust to the airstream. After 2600 cal. yr BP, SASM activity increased while dust flux decreased and the dust chemistry changed, collectively implying the shutting down of the Puna-Altiplano as a significant dust source. Dust flux increased after 1000 cal. yr BP during the ‘Medieval Warm Period’, associated with a return to drier conditions and reactivation of dust sources across the endorheic basins of the Puna. Natural variability in dust flux was dwarfed, however, by the very significant increase in flux after 400 cal. yr BP following Spanish Colonisation and associated changing landuse practices. This finding attests to the globally significant role of humans on dust emissions.

The paleoclimatic message from the polymodal grain-size distribution of late Pleistocene-early Holocene Pampean loess (Argentina)

Wind-blown dust deposits are considered one of the most important terrestrial archives for past climate change studies. In the Southern Hemisphere, the Pampean loess is the most extensive paleo-dust record, whose origin is still a matter of debate. In this paper, grain-size was carried out at three high-resolution loess profiles deposited during the late Pleistocene-early Holocene and for present-day dust collected on the Pampean plain. Based on comparing loess records with in situ present-day dust, this work aims to provide constraints on the climatic conditions that allowed deposition of the Argentinean loess mantle. Unmixing methods allow differentiating three grain-size subpopulations/end members in the Pampean loess which are comparable to end members found in present-day dust. The provenance and transport conditions observed for present-day dust were used to infer that the fine silt components of Pampean loess were transported by high-level air stream associated with the deflation of the Puna-Altiplano Plateau. On the other side, the coarse-silt sediments were carried by dust storms associated with high transport energy events taking place in proximal dust sources. In the central Pampas, the increased abundance of coarse-silt sediments during the Antarctic Cold Reversal (ACR) may indicate the existence of an increased frequency of dust storm episodes. During the beginning of the Holocene, the rise in fine-silt loess could be linked to the activation of distant sources associated with lake desiccation in the Puna-Altiplano Plateau.

Atmospheric dust dynamics in southern South America: A 14-year modern dust record in the loessic Pampean region

Mineral aerosols profoundly impact global climate. Modeling of the dust cycle is the main tool used to gauge this effect. However, the scarcity of in situ modern dust flux measurements is the main reason why validation of existing models is hampered. We present the first long-term (14-year) record of dust flux in the Pampas, southern South America, home to the largest loess deposit in the Southern Hemisphere. Measured 14-year mean deposition (40 g m−2 year−1) and horizontal (362 g m−2 year−1) fluxes imply that current models underestimate the power of the central Pampas as a dust sink. Based on cross-spectral analysis, both wet and, to a lesser extent, dry deposition are found to play significant roles in atmospheric dust extraction. Dust is sourced regionally from the South American Arid Diagonal and from the shores of Mar Chiquita lake (~260 km), which we find to be the main contributor of dust particles >30 µm. Cross-spectral and satellite image analyses show that surface wind speed and precipitation at the Puna-Altiplano Plateau are controlling factors for horizontal dust flux in the Pampas. El Niño Southern Oscillation probably plays a role in controlling interannual horizontal dust flux periodicities. Finally, preliminary comparisons between modern vertical dust fluxes and loess accumulation rates point to the Pampas as a more powerful dust sink during the last deglaciation and Antarctic Cold Reversal (18–12.5 ka).

Geological-geomorphological and geochemical control on low arsenic concentration in the Lerma valley groundwater between the two high arsenic geologic provinces of Chaco-Pampean plain and Puna

Argentina is known for having one of the most extensive areas with high arsenic (As) concentration in groundwater in the world. These areas correspond to two geological provinces, the Altiplano-Puna plateau and the Chaco-Pampean plain. In this large territory, there are some specific environments where the As concentration in groundwater is lower, and in some cases within the recommended limits for drinking water. In our study, we analyze and interpret the low As concentrations reported for the Lerma valley, the easternmost intermontane basin of the Cordillera Oriental, located between the aforementioned high‑arsenic areas. The groundwater from this valley is used for the consumption of >600.000 inhabitants in the city of Salta and nearby towns. The incipient geological development of the valley since the late Miocene and the subsequent tectonic and climatic evolution favored low As concentrations with respect to the Altiplano-Puna plateau and the Chaco-Pampean plain. The high-energy sedimentary environments that characterized the area during Plio-Quaternary times and the composition of the sediments have controlled the characteristics of the multilayered aquifer. Moreover, the absence of geogenic As sources, climate, high rain infiltration rate, near neutral pH, redox conditions, and wells construction with screens settled in coarse productive layers favor groundwater of good quality. The geological and tectonic evolution of the Lerma valley could be extrapolated to other similar valleys in NW Argentina and can be a useful tool for exploration of good quality groundwater. This is of high importance in Latin American territories with high As concentration in groundwater such as Argentina.

Geomorphology and spatial distribution of monogenetic volcanoes in the southern Puna Plateau (NW Argentina)

Monogenetic volcanoes are the most common volcanic landforms on Earth. Several works have shown that the study of geomorphology and spatial distribution of such small volcanoes reveals important aspects related to the volcanic field, from eruption dynamics to tectonic processes. The Altiplano-Puna Plateau (15° to 27° S, Central Andes) comprises the largest non-collisional plateau on Earth. Characterized by intensive deformation, uplift, and volcanism, this region is marked by the presence of hundreds of mafic monogenetic volcanoes. Despite its discrete occurrence, the monogenetic volcanoes show a close relationship with large scale, lithospheric foundering events in the region. In this contribution we applied a Geographic Information System (GIS) approach in order to build the first complete catalog of monogenetic landforms present in the southern Puna Plateau (24° 30′ to 27° 00′ S, NW Argentina), exploring their evolution and relationship with geotectonic events. Combining spatial analysis with a geomorphological study of each volcano, it was possible to identify the presence of 285 monogenetic volcanoes in the region, which could be grouped in seven clusters. Southern Puna shows a predominance of cinder cones and lava domes, suggesting a small influence of water in the eruption dynamics. Despite its high clustering degree, southern Puna monogenetic vents present a low density (8.3 × 10−3 vents/km2) when compared to other monogenetic fields around the world. In addition, Puna Plateau cones show a close relationship with monogenetic cones associated with an extensional environment, considering their back-arc context. Based on relative and available absolute ages, it was possible to map the volcanism evolution through time. The results indicate that the first monogenetic volcanoes appeared in the central region of the Plateau (Sierra de Calalaste), mainly associated with large-scale NNE-SSW reactivated faults. The activity then expanded (Antofalla and Antofagasta regions) and, finally reached the borders of the Plateau (Pasto Ventura and Arizaro regions), which show volcanoes associated with NW-SE normal faults.

Revisiting the chronology and environmental conditions for the accretion of late Pleistocene-early Holocene Pampean loess (Argentina)

The present research work reviews the age models of three previously studied loess sections across 700 km in the loess belt of the Pampean region in central Argentina. The loess sections were investigated for the first time through high-resolution luminescence dating in order to determine the chronology of the loess deposits and its temporal correlation with important palaeoclimatic records of the Southern Hemisphere. Reliable estimations of mass accumulation rates (MARs) for loess were used to infer the temporal dust flux variation during the last glacial/interglacial transition in southern South America. The new age models have significant differences compared to the previously published ones of likely Last Glacial Maximum (LGM) ages. The three studied sections showed general bimodal grain-size distributions consistent with multiple sources contributing dust to the Pampas. It is observed that higher MARs values are associated with an increased presence of finer loess. Increases in loess accumulation are coeval with climatic transitions from wetter to drier periods in the Puna-Altiplano Plateau, and high MARs are related to synchronous climatic shifts to humid conditions in the Pampean plain. Contrasting with dust fluxes observed in the more distal palaeoarchives (i.e., the South Atlantic Ocean and the Antarctic ice sheet) the new data suggest increased dust accumulation in the Pampas during the Antarctic Cold Reversal (ACR) and the Holocene. This supports the idea that changes related to atmospheric transport efficiency can better explain dust flux variations observed over glacial/interglacial periods in distant palaeoarchives.

Alternating contraction and extension in the Southern Central Andes (35°–37°S)

The Andes are thought to be formed through discrete contractional stages separated by periods of little to no orogenic construction. This paper analyzes the intervals between the main contractional phases that built the Southern Central Andes between 35° and 37°S in order to determine whether they were characterized by neutral, contractional or extensional conditions. During an interruption in orogenesis between the Late Cretaceous and the Miocene shortening phases, two extensional stages are recorded through the opening of a series of intra- and retro-arc basins. U-Pb dating of detrital zircons in a sample collected from the Los Angeles unit, a syn-extensional volcano-sedimentary succession located at ∼35°409S along the Chile and Argentina international border, provided a maximum depositional age of 67.1 +2.4/−0.9 Ma. This age, in association with evidence of regional crustal thinning, suggests a previously unrecognized extensional phase during latest Cretaceous times. Limited shortening succeeded this extensional event and was followed by a second extensional episode during late Oligocene and earliest Miocene times. While the first extensional event was restricted to the core of the Late Cretaceous orogen, the second episode affected a wide area ranging between the present forearc and retroarc areas. A structural section across the Malargue fold-thrust belt at ∼36°S indicates inversion of normal faults where extension was focused and new thrust generation in areas not affected by extensional deformation. Our data reveal that the growth of the Southern Central Andes is the product of a complex alternation of contractional and extensional phases, with inherited structures playing a role in their tectonic evolution. A comparison with other Cordilleran orogenic systems such as the Puna-Altiplano plateau, the northern Peruvian Andes and the North American Sevier-Laramide orogenic belt, suggests that extensional deformation in the Southern Central Andes responds better to changes in plate kinematics, rather than to localized events within a continuous contractional setting.

Origin, distribution, and geochemistry of arsenic in the Altiplano-Puna plateau of Argentina, Bolivia, Chile, and Perú

Elevated concentrations of arsenic in water supplies represent a worldwide health concern. In at least 14 countries of South America, high levels have been detected relative to international standards and guidelines. Within these countries, the high plateau referred to as the “Altiplano-Puna”, encompassing areas of Argentina, Bolivia, Chile, and Perú, exhibits high arsenic concentrations that could be affecting 3 million inhabitants.The origins of arsenic in the Altiplano-Puna plateau are diverse and are mainly natural in origin. Of the natural sources, the most important correspond to mineral deposits, brines, hot springs, and volcanic rocks, whereas anthropogenic sources are related to mining activities and the release of acid mine drainage (AMD). Arsenic is found in all water types of the Altiplano-Puna plateau over a wide range of concentrations (0.01 mg·L−1 < As in water > 10 mg·L−1) which in decreasing order correspond to: AMD, brines, saline waters, hot springs, rivers affected by AMD, rivers and lakes, and groundwater. Despite the few studies which report As speciation, this metalloid appears mostly in its oxidized form (As[V]) and its mobility is highly susceptible to the influence of dry and wet seasons. Once arsenic is released from its natural sources, it also precipitates in secondary minerals where it is generally stable in the form of saline precipitates and Fe oxides. In relation to human health, arsenic adaptation has been detected in some aboriginal communities of the Puna together with an efficient metabolism of this metalloid. Also, the inefficient methylation of inorganic As in women of the Altiplano might lead to adverse health effects such as cancer. Despite the health risks of living in this arsenic-rich environment with limited water resources, not all of the Altiplano-Puna is properly characterized and there exists a lack of information regarding the basic geochemistry of arsenic in the region.

Dynamics of caldera collapse during the Coranzulí eruption (6.6 Ma) (Central Andes, Argentina)

The Coranzulí caldera (23°00′ S – 66°15′ W) is one of the least known caldera complexes in the eastern part of the Argentinian Altiplano-Puna plateau (Central Andes). It lies at the intersection of N-S, NW-SE and NE-SW fault systems and was formed about 6.6 Ma ago; during the eruption, four main crystal-rich dacitic ignimbrites were emplaced in different directions around the caldera. Caldera collapse was not homogeneous, rather it occurred along different sectors of the ring fault as subsidence progressed. The location of co-ignimbrite lag breccias and the composition of the dominant lithic fragments within the different ignimbrite flow units reveal how the caldera collapse developed. According to the succession of deposits, in particular the lack of an initial fallout and the presence of co-ignimbrite lag breccias associated with the different ignimbrite units, we interpret this caldera-forming eruption as a pulsating boiling-over event in which the caldera collapse developed immediately after the onset of the eruption, favored by a transtensive tectonic system. Within the central part of the caldera, there is Cerro Coranzulí, a resurgent dome that exposes a thick intracaldera ignimbrite succession covered by ~ 100-m-thick dacite lava flows that erupted at the end of caldera formation.

Syn-subduction crustal shortening produced a magmatic flare-up in middle Sanjiang orogenic belt, southeastern Tibet Plateau: Evidence from geochronology, geochemistry, and structural geology

The Permo-Triassic Jomda-Weixi-Yunxian continental margin arc belt in the southeastern Tibetan Plateau was formed by the subduction of the main Paleotethyan branch underneath the Cathaysian-affinity blocks along the Longmu Co-Shuanghu-Changning-Menglian suture. This belt consists of three segments with distinctive magmatism history. Available geochronological data revealed that the northern and southern segments of this belt comprise several volcanic successions intercalated with terrestrial clastics suggesting long-lived and pulsed magmatism. In contrast, the middle segment is made of a single volcanic pile of dacite/rhyolite and minor basalt with thickness of >1.6km. Numerous granodiorite batholiths and a few diabase dykes intruded the volcanic pile. Detailed zircon U-Pb dating indicates that the volcanic rocks and intrusive rocks were formed during a short time period of ~7millionyears (~244–251Ma), defining a magmatic flare-up. Petrographic and geochemical data including Sr-Nd isotopics demonstrate that the felsic magmas were derived from ancient crust, while the basalt and diabase are sourced from the mantle wedge induced by subduction of the Paleo-Tethys. Structural studies reveal syn-subduction crustal shortening and thickening of the middle segment. The thickened crust likely has prevented the subduction-induced basaltic magma from eruption; they alternatively were accumulated in the bottom of the crust yielding a magma chamber. The chamber heated, softened, and melted the crust, forming felsic magmas. When the crust was weakened to a critical threshold, the felsic magmas rose up and erupted to the surface, resulting in the magmatic “flare-up”. This study suggests that syn-subduction shortening and thickening of the overriding crust seems to have played a significant role in the development of magmatic flare-ups along a continental margin arc-system. Other well-studied examples include the Mesozoic Serra Nevada and the late Miocene to Holocene Altiplano-Puna Plateau, central Andes.

The origin of extensive Neoarchean high-silica batholiths and the nature of intrusive complements to silicic ignimbrites: Insights from the Wyoming batholith, U.S.A.

Extensive intrusions composed entirely of biotite granite are common in Neoarchean cratons. These granites, which have high silica and potassium contents, are not associated with intermediate and mafic phases. One such Neoarchean granite batholith, herein named the Wyoming batholith, extends more than 200 km across central Wyoming in the Granite and the Laramie Mountains. From field characterization, petrology, geochemistry, and Nd isotopic data we establish that the magnesian Wyoming batholith exhibits continental arc chemical and isotopic signatures. It is best interpreted as a large, upper crustal silicic batholith that likely formed when the subducting oceanic plate steepened or foundered, bringing mantle heat and mass to the base of the crust. Similar Cenozoic settings, such as the Altiplano-Puna plateau of the Andes and the volcanic provinces of the western United States, host large volumes of silicic ignimbrite. The magma chambers supplying these eruptions are inferred to be silicic, but the structural, petrologic, and geochemical details are unknown because the batholiths are not exposed. We suggest that the Wyoming batholith represents an analog for the plutonic complex underlying these ignimbrite systems, and provides an opportunity to examine the shallow magma chamber directly. Our work establishes that, aside from more leucocratic margins, the sill-like magma chamber is petrologically and chemically homogeneous, consistent with effective mixing by vertical convection. Nd isotopic variations across the batholith indicate that horizontal homogenization is incomplete, preserving information about the feeder system to the batholith and variations in magma sources. The late Archean Earth may present optimal conditions for the formation of extensive granite batholiths like the Wyoming batholith. By this time the majority of the planet’s continental crust had formed, providing the environment in which differentiation, distillation, and assimilation could occur. Moreover, the Neoarchean Earth’s relatively high radioactive heat production provided the power to drive these processes.

Spatiotemporal characteristics and synchronization of extreme rainfall in South America with focus on the Andes Mountain range

The South American Andes are frequently exposed to intense rainfall events with varying moisture sources and precipitation-forming processes. In this study, we assess the spatiotemporal characteristics and geographical origins of rainfall over the South American continent. Using high-spatiotemporal resolution satellite data (TRMM 3B42 V7), we define four different types of rainfall events based on their (1) high magnitude, (2) long temporal extent, (3) large spatial extent, and (4) high magnitude, long temporal and large spatial extent combined. In a first step, we analyze the spatiotemporal characteristics of these events over the entire South American continent and integrate their impact for the main Andean hydrologic catchments. Our results indicate that events of type 1 make the overall highest contributions to total seasonal rainfall (up to $$50\,\%$$ ). However, each consecutive episode of the infrequent events of type 4 still accounts for up to $$20\,\%$$ of total seasonal rainfall in the subtropical Argentinean plains. In a second step, we employ complex network theory to unravel possibly non-linear and long-ranged climatic linkages for these four event types on the high-elevation Altiplano-Puna Plateau as well as in the main river catchments along the foothills of the Andes. Our results suggest that one to two particularly large squall lines per season, originating from northern Brazil, indirectly trigger large, long-lasting thunderstorms on the Altiplano Plateau. In general, we observe that extreme rainfall in the catchments north of approximately $$20^{\circ }$$ S typically originates from the Amazon Basin, while extreme rainfall at the eastern Andean foothills south of $$20^{\circ }$$ S and the Puna Plateau originates from southeastern South America.

Thermal and geodynamic contributions to the elevation of the Altiplano–Puna plateau

The most remarkable feature of the Central Andes is the Altiplano–Puna plateau. This plateau is characterized by 3.5km average elevation, approximately 70km crustal thickness and very high heat flow. The upper mantle structure changes along strike below the plateau. The upper mantle below the Puna becomes hotter, and the lithosphere becomes thinner and weaker. These features suggest that thermal isostasy could play a role in the compensation of the Altiplano–Puna. Thermal isostasy is the geodynamic process whereby regional variations in the lithospheric thermal regime cause changes in elevation. Elevation changes result from variations in rock density in response to thermal expansion. The aim of this study is to estimate the thermal and geodynamic contributions to the elevation. While the thermal component of the Altiplano elevation would be of 1km, the thermal contribution to the southern Puna elevation would be of 1.5km. However, in the case of the southern Puna a portion of the actual topography (∼20%) cannot be explained considering only compositional and thermal effects, suggesting additional geodynamical support. The obtained results suggest that the thermal state of the lithosphere would play a significant role in the elevation of the Central Andes, and may be responsible of some of the geological differences displayed by the Altiplano and the Puna.

Neogene paleoelevation of intermontane basins in a narrow, compressional mountain range, southern Central Andes of Argentina

The topographic growth of mountain ranges at convergent margins results from the complex interaction between the motion of lithospheric plates, crustal shortening, rock uplift and exhumation. Constraints on the timing and magnitude of elevation change gleaned from isotopic archives preserved in sedimentary sequences provide insight into how these processes interact over different timescales to create topography and potentially decipher the impact of topography on atmospheric circulation and superposed exhumation. This study uses stable isotope data from pedogenic carbonates collected from seven different stratigraphic sections spanning different tectonic and topographic positions in the range today, to examine the middle to late Miocene history of elevation change in the central Andes thrust belt, which is located immediately to the south of the Altiplano-Puna Plateau, the world's second largest orogenic plateau. Paleoelevations are calculated using previously published local isotope-elevation gradients observed in modern rainfall and carbonate-formation temperatures determined from clumped isotope studies in modern soils. Calculated Neogene basin paleoelevations are between 1 km and 1.9 km for basins that today are located between 1500 and 3400 m elevation. Considering the modern elevation and δ18O values of precipitation at the sampling sites, three of the intermontane basins experienced surface uplift between the end of deposition during the late Miocene and present. The timing of elevation change cannot be linked to any documented episodes of large-magnitude crustal shortening. Paradoxically, the maximum inferred surface uplift in the core of the range is greatest where the crust is thinnest. The spatial pattern of surface uplift is best explained by eastward migration of a crustal root via ductile deformation in the lower crust and is not related to flat-slab subduction.

Geodetic imaging of late postseismic lower crustal flow in Tibet

Integral to understanding past and present seismotectonic activity in orogenic zones is whether any part of the lower crust is weak enough to flow, since a weak zone at depth influences the style of deformation that occurs. Thick crust such as found beneath the Tibetan Plateau and the Altiplano–Puna Plateau in the Andes leads to elevated temperatures at depth, which potentially allows the lower crust to flow on human time scales, as well as on the time scales of plateau formation and evolution. Here we use the late postseismic response of the crust to large earthquakes as a tool for investigating rheological structure beneath the Tibetan Plateau. Eighteen years (1992–2010) of InSAR data on the southern Tibetan Plateau show a late postseismic relaxation response to two large earthquakes in 1951 and 1952. The relaxation occurs throughout a thick viscoelastic lower crust and possibly extends into the upper mantle. The lower crust has viscosity 6–10×1019 Pas, with an associated Maxwell relaxation time of 63 years. Flow within a thin mid-crustal channel can be ruled out. These late-stage postseismic observations represent the first clear evidence of present-day lower crustal flow, and are consistent with geophysical evidence for fluid-weakened lower crust across much of the Tibetan Plateau, as required by models of plateau formation. Active deformation of continental plateaus in other parts of the world, notably the Altiplano–Puna Plateau in South America, may also be strongly influenced by a weak lower crust.

Magnetic fingerprint of the late Holocene inception of the Río de la Plata plume onto the southeast Brazilian shelf

Measurement of changes in the assembly of magnetic particles in sediment records is useful both for paleoclimatic and paleoceanographic reconstructions and for assessing provenance and pathways of sediment transport. In this study we provide detailed rock magnetic analysis along with scanning electron microscopy (SEM), Ti/Ca ratio and particle size distribution of the decarbonated fraction of sediments to trace terrigenous source changes during the last 6calkyrBP in core 7610 collected on the SE Brazilian continental shelf. Magnetic mineralogy data indicate a gradual shift between two distinct sources of sediments: (1) dust from southern South America (Patagonia, central western Argentina and Puna-Altiplano plateau) and (2) sediments eroded from the weathered volcanics of Paraná Basin. These materials reached the continental margin off Argentina and Uruguay by different pathways and were transported northward by the Brazilian Coastal Current (BCC). The chronology of events related to the environmental record of core 7610 indicates that between 6 and 4.7calkyrBP dry conditions over continental South America resulted in enhanced eastward eolian transport of the Argentinean loess and a limited discharge of the RdlP river load into the Atlantic Ocean. At this time, the sedimentary magnetic fraction that reached the SE Brazilian shelf reflect predominantly loessic sources transported from the Pampean terraces to the Argentinean and Uruguay shelves and then distributed northward by the shelf current system. From 4.7 to 2calkyrBP the climate became gradually wetter over the continent and the contribution of oxidized phases from the weathered Paraná Basin (mainly from the basaltic and acidic volcanics) started to become important. This change in climate overlaps in time a with the late Holocene sea level fall recorded along the SE South American coast. Later, from 2 up to 0.9calkyrBP even wetter conditions were prevalent in S and SE South America, and the RdlP sedimentary load became the main source of sediments transported to the SE Brazilian shelf by the BCC. The magnetic record of core 7610 reported here together with other paleoenvironmental tracers (palynology, geochemistry and stable isotopes) thus indicates that the inception of the RdlP plume onto the SE Brazilian shelf started as early as 4.7calkyrBP and has been strengthened since then mostly due to changes in precipitation over South America and to a smaller extent by sea-level changes.

Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes

Convective removal of continental lithospheric roots has been postulated to be the primary mechanism of recycling lithospheric mass into the asthenosphere under large plateaux such as the Altiplano-Puna in the central Andes. Convective instabilities are especially likely to develop where there is extensive intermediate arc-like magmatism in the upper plate, as the residual masses complementing these magmatic products are typically denser than the underlying mantle. Mafic volcanic rocks erupted on the central Andean Altiplano-Puna plateau during the past 25 m.y. contain evidence of this process. Here we use equilibration temperatures, age data, and geochemical constraints—primarily based on transition metals—to show that the most important source materials by mass for this mantle-derived magmatism are pyroxenites from the lower parts of the lithosphere, with only minor contributions from mantle peridotite. Pyroxenites are denser than typical upper mantle whether they are garnet bearing or not, and are therefore likely to contribute to destabilizing parts of the continental lithosphere. The pattern of melting is consistent with the process of foundering/dripping of small-scale (

Spatiotemporal trends in erosion rates across a pronounced rainfall gradient: Examples from the southern Central Andes

The tectonic and climatic boundary conditions of the broken foreland and the orogen interior of the southern Central Andes of northwestern Argentina cause strong contrasts in elevation, rainfall, and surface-process regimes. The climatic gradient in this region ranges from the wet, windward eastern flanks (~2m/yr rainfall) to progressively drier western basins and ranges (~0.1m/yr) bordering the arid Altiplano–Puna Plateau. In this study, we analyze the impact of spatiotemporal climatic gradients on surface erosion: First, we present 41 new catchment-mean erosion rates derived from cosmogenic nuclide inventories to document spatial erosion patterns. Second, we re-evaluate paleoclimatic records from the Calchaquíes basin (66°W, 26°S), a large intermontane basin bordered by high (>4.5km) mountain ranges, to demonstrate temporal variations in erosion rates associated with changing climatic boundary conditions during the late Pleistocene and Holocene. Three key observations in this region emphasize the importance of climatic parameters on the efficiency of surface processes in space and time: (1) First-order spatial patterns of erosion rates can be explained by a simple specific stream power (SSP) approach. We explicitly account for discharge by routing high-resolution, satellite derived rainfall. This is important as the steep climatic gradient results in a highly nonlinear relation between drainage area and discharge. This relation indicates that erosion rates (ER) scale with ER~SSP1.4 on cosmogenic-nuclide time scales. (2) We identify an intrinsic channel-slope behavior in different climatic compartments. Channel slopes in dry areas (<0.25m/yr rainfall) are slightly steeper than in wet areas (>0.75m/yr) with equal drainage areas, thus compensating lower amounts of discharge with steeper slopes. (3) Erosion rates can vary by an order of magnitude between presently dry (~0.05mm/yr) and well-defined late Pleistocene humid (~0.5mm/yr) conditions within an intermontane basin. Overall, we document a strong climatic impact on erosion rates and channel slopes. We suggest that rainfall reaching areas with steeper channel slopes in the orogen interior during wetter climate periods results in intensified sediment mass transport, which is primarily responsible for maintaining the balance between surface uplift, erosion, sediment routing and transient storage in the orogen.