Lago Buenos Aires Research Articles

The Volcanism of the Meseta del Lago Buenos Aires, Patagonia: the Transition from Subduction to Slab Window

Abstract The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts’ composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas.

Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina

The early-middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma
 (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral-Magallanes Basin of southern Patagonia, the Río Frías Formation in Chile, and the lower Collón Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal dissimilarities between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías
 and Collón Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both forested and open environments for the area occupied by the CBF rocks. However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.

Quaternary landscape evolution of patagonia at the Chilean Triple Junction: Climate and tectonic forcings

Glacial and fluvial landforms record the recent history of the Earth surface and have the potential to provide information about uplift and climate changes during Quaternary times. Patagonia hosts uniquely well-preserved moraine deposits, along with outwash and fluvial terraces. This offers the possibility of determining the timing of glacier advances since the Great Patagonian Glaciation (ca. 1.2 Ma) and allows us to investigate landscape response to climate and tectonic forcing.In this study, we present twenty-eight new in situ 10Be exposure ages for glacial deposits, outwash sediments, and fluvial terraces at the latitude of the Chile Triple Junction (∼46°30′S). We combine this database with a detailed mapping and a morphological analysis of the Deseado River terrace system. The glacial and outwash deposits document at least five major glacial advances in the Lago Buenos Aires valley before the deposition of the Moreno system during Marine Isotope Stage (MIS) 8. Key advances occurred during MIS 12 and MIS 16. Further ice expansions that resulted in Telken V and VI/VII outwash surfaces date to 780 ka and ca. 1.2 Ma respectively. Overall, our data show the difficulty of dating pre-LGM glacial deposits using surface exposure methods and reinforce the idea that outwash surfaces are better targets to determine the age of glacial sequences in the region.The Deseado River terrace system yields stratigraphically consistent ages that range from 1167 to 447 ka, which indicate that these terraces formed during Pleistocene times. Our analysis suggests that terraces T3, T7 and T8 may have been formed during cold stages, close to the glacial-interglacial transitions. We also calculated incision rates that reveal a strong initial pulse, from ca. 1200 to 900 ka, which was followed by significantly weaker river incision pulses.Finally, our geomorphological analysis confirms that the oldest fluvial terraces of the Deseado River system exhibit a NE-SW tilting through a broad area, next to the Atlantic Ocean. This was previously interpreted as the surface response to a long-wavelength dynamic uplift, a phenomenon that should be reassigned to the early Pleistocene.

Postglacial Patagonian mass movement: From rotational slides and spreads to earthflows

Many of the volcanic plateau margins of the eastern, formerly glaciated, foreland of the Patagonian Andes are undermined by giant landslides (≥108 m3). One cluster of such landslides extends along the margin of the Meseta del Lago Buenos Aires (MLBA) plateau that is formed mainly by Neogene-Quaternary basalts. The dry climate is at odds with numerous >2-km long earthflows nested within older and larger compound landslides. We present a hydrological analysis, a detailed geomorphic map, interpretations of exposed landslide interiors, and radiocarbon dating of the El Mirador landslide, which is one of the largest and morphologically most representative landslide.We find that the presence of lakes on top of the plateau, causing low infiltration rates, correlates negatively with the abundance of earthflows on compound landslides along the plateau margins. Field outcrops show that the pattern of compound landslides and earthflows is likely controlled by groundwater seepage at the contact between the basalts and underlying soft Miocene molasse. Numerous peat bogs store water and sediment and are more abundant in earthflow-affected areas than in their contributing catchment areas.Radiocarbon dates indicate that these earthflows displaced metre-thick layers of peat in the late Holocene (<2.5 ka). We conclude that earthflows of the MLBA plateau might be promising proxies of past hydroclimatic conditions in the Patagonian foreland, if strong earthquakes or gradual crustal stress changes due to glacioisostatic rebound can be ruled out.

Giant landslides in the foreland of the Patagonian Ice Sheet

Quaternary glaciations have repeatedly shaped large tracts of the Andean foreland. Its spectacular large glacial lakes, staircases of moraine ridges, and extensive outwash plains have inspired generations of scientists to reconstruct the processes, magnitude, and timing of ice build-up and decay at the mountain front. Surprisingly few of these studies noticed many dozens of giant (≥108 m3) mass-wasting deposits in the foreland. We report some of the world's largest terrestrial landslides in the eastern piedmont of the Patagonian Ice Sheet (PIS) along the traces of the former Lago Buenos Aires and Lago Puyerredón glacier lobes and lakes. More than 283 large rotational slides and lateral spreads followed by debris slides, earthflows, rotational and translational rockslides, complex slides and few large rock avalanches detached some 164 ± 56 km3 of material from the slopes of volcanic mesetas, lake-bounding moraines, and river-gorge walls. Many of these landslide deposits intersect with well-dated moraine ridges or former glacial-lake shorelines, and offer opportunities for relative dating of slope failure. We estimate that >60% of the landslide volume (∼96 km3) detached after the Last Glacial Maximum (LGM). Giant slope failures cross-cutting shorelines of a large Late Glacial to Early Holocene lake (“glacial lake PIS”) likely occurred during successive lake-level drop between ∼11.5 and 8 ka, and some of them are the largest hitherto documented landslides in moraines. We conclude that 1) large portions of terminal moraines can fail catastrophically several thousand years after emplacement; 2) slopes formed by weak bedrock or unconsolidated glacial deposits bordering glacial lakes can release extremely large landslides; and 3) landslides still occur in the piedmont, particularly along postglacial gorges cut in response to falling lake levels.

U–Th and &amp;lt;sup&amp;gt;10&amp;lt;/sup&amp;gt;Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

Abstract. The estimation of sediment transfer times remains a challenge to our understanding of sediment budgets and the relationships between erosion and climate. Uranium (U) and thorium (Th) isotope disequilibria offer a means of more robustly constraining sediment transfer times. Here, we present new uranium and thorium disequilibrium data for a series of nested moraines around Lago Buenos Aires in Argentine Patagonia. The glacial chronology for the area is constrained using in situ cosmogenic 10Be analysis of glacial outwash. Sediment transfer times within the periglacial domain were estimated by comparing the deposition ages of moraines to the theoretical age of sediment production, i.e., the comminution age inferred from U disequilibrium data and recoil loss factor estimates. Our data show first that the classical comminution age approach must include weathering processes accounted for by measuring Th disequilibrium. Second, our combined data suggest that the pre-deposition history of the moraine sediments is not negligible, as evidenced by the large disequilibrium of the youngest moraines despite the equilibrium of the corresponding glacial flour. Monte Carlo simulations suggest that weathering was more intense before the deposition of the moraines and that the transfer time of the fine sediments to the moraines was on the order of 100–200 kyr. Long transfer times could result from a combination of long sediment residence times in the proglacial lake (recurrence time of a glacial cycle) and the remobilization of sediments from moraines deposited during previous glacial cycles. 10Be data suggest that some glacial cycles are absent from the preserved moraine record (seemingly every second cycle), supporting a model of reworking moraines and/or fluctuations in the extent of glacial advances. The chronological pattern is consistent with the U–Th disequilibrium data and the 100–200 kyr transfer time. This long transfer time raises the question of the proportion of freshly eroded sediments that escape (or not) the proglacial environments during glacial periods.

High-resolution chronology for deglaciation of the Patagonian Ice Sheet at Lago Buenos Aires (46.5°S) revealed through varve chronology and Bayesian age modelling

Glaciolacustrine varves offer the potential to construct continuous, annually-resolved chronologies for ice-sheet deglaciation, and improved understanding of glacier retreat dynamics. This paper investigates laminated glaciolacustrine sediments deposited around the waning margins of the Patagonian Ice Sheet, following the local Last Glacial Maximum (LGM). Detailed macro- and microfacies analyses confirm an annual (varve) structure within these sediments. The correlation of annual layers (varves) across five sites in eastern Lago Buenos Aires yields a 994 ± 36 varve-year (vyr) chronology and thickness record. The floating chronology has been anchored to the calendar-year timescale through identification of the Ho tephra (17,378 ± 118 cal a BP) in the varve sequences. Using a Bayesian age model to integrate the new varve chronology with published moraine ages, the onset of deglaciation at 46.5°S is dated to 18,086 ± 214 cal a BP. New age estimates for deglacial events are combined with high-resolution analysis of varve thickness trends, and new lithostratigraphic data on ice-margin position(s), to reconstruct ice-margin retreat rates for the earliest ca. 1000 years of ice-sheet demise. Glacier retreat rates were moderate (5.3–10.3 m yr−1) until 17,322 ± 115 cal a BP, but subsequently accelerated (15.4–18.0 m yr−1). Sustained influxes of ice-rafted debris (IRD) after 17,145 ± 122 cal a BP suggest retreat rates were enhanced by calving after ice contracted into deeper lake waters. Ice persisted in eastern Lago Buenos Aires until at least 16,934 ± 116 cal a BP, after which the glacier started to retreat towards the Patagonian mountains.

Regional mid-Pleistocene glaciation in central Patagonia

Southern South America contains a glacial geomorphological record that spans the past million years and has the potential to provide palaeoclimate information for several glacial periods in Earth's history. In central Patagonia, two major outlet glaciers of the former Patagonian Ice Sheet carved deep basins ∼50 km wide and extending over 100 km into the Andean plain east of the mountain front. A succession of nested glacial moraines offers the possibility of determining when the ice lobes advanced and whether such advances occurred synchronously. The existing chronology, which was obtained using different methods in each valley, indicates the penultimate moraines differ in age by a full glacial cycle. Here, we test this hypothesis further using a uniform methodology that combines cosmogenic nuclide ages from moraine boulders, moraine cobbles and outwash cobbles. 10Be concentrations in eighteen outwash cobbles from the Moreno outwash terrace in the Lago Buenos Aires valley yield surface exposure ages of 169–269 ka. We find 10Be inheritance is low and therefore use the oldest surface cobbles to date the deposit at 260–270 ka, which is indistinguishable from the age obtained in the neighbouring Lago Pueyrredón valley. This suggests a regionally significant glaciation during Marine Isotope Stage 8, and broad interhemispheric synchrony of glacial maxima during the mid to late Pleistocene. Finally, we find the dated outwash terrace is 70–100 ka older than the associated moraines. On the basis of geomorphological observations, we suggest this difference can be explained by exhumation of moraine boulders.

Geographical and geological explorations of the La Plata Museum 1884–1905

Abstract The contributions of the La Plata Museum to the geology of Argentina began following the exploratory trips of its founder, F.P. Moreno, in the 1870s. The geological expeditions of the La Plata Museum started in the 1880s and covered the Andean region, although they were focused in the Patagonian Andes. They became more important from 1893 when they were related to explorations aimed at fixing the boundary between Argentina and Chile. Within 10 years the geographical and geological bases of large and almost unknown regions were established. R. Hauthal studied the area between the Last Hope Inlet and Lake Belgrano, defining its stratigraphy and publishing the first geological map of the Cordilleran region between c. 49° 30′ and 52° S. The geology between Lake Buenos Aires and the Rio Negro (41–47° S) was established by Santiago Roth. The stratigraphic succession, facies and structural changes through the Argentine–Chilean Cordillera at the latitude of Lake Nahuel Huapi and Lake Lacar were studied by L. Wehrli, while studies at the latitude of Neuquén and Mendoza (36–39° S) were carried out by C. Burckhardt. Mesozoic and Cenozoic fossils were studied either in the Museum or abroad. Supplementary material: Map of southern South America (Argentina and Chile) showing the distribution of towns in 1870 is available at https://doi.org/10.6084/m9.figshare.c.3283385.v1

Luminescence dating of glacial advances at Lago Buenos Aires (∼46 °S), Patagonia

Understanding the timing of past glacial advances in Patagonia is of global climatic importance because of the insight this can provide into the influence on glacier behaviour of changes in temperature and precipitation related to the Southern Westerlies. In this paper we present new luminescence ages determined using single grains of K-feldspar from proglacial outwash sediments that were deposited by the Patagonian Ice Sheet around Lago Buenos Aires (∼46 °S), east of the contemporary Northern Patagonian Icefield. The new luminescence ages indicate that major outwash accumulations formed around ∼110 ± 20 ka to 140 ± 20 ka and that these correspond to the Moreno I and II moraine ridges, which were previously dated using cosmogenic isotope dating to 150 ± 30 ka. Luminescence dating at Lago Buenos Aires has also identified outwash sediments that were deposited during glacial advances ∼30.8 ± 5.7 ka and ∼34.0 ± 6.1 ka (MIS 3) that are not recorded in the moraine record. Younger outwash accumulations were then deposited between ∼14.7 ± 2.1 and 26.2 ± 1.6 ka which correspond to the Fenix I – V moraine ridges. The combined chronology suggests that glacial advances occurred ∼110 ± 20 ka to 150 ± 30 ka (MIS 6), ∼30.8 ± 5.7 ka to ∼34.0 ± 6.1 ka (MIS 3), and ∼14.7 ± 2.1 to 26.2 ± 1.6 ka (MIS 2) at Lago Buenos Aires. Overall luminescence dating using single grains of K-feldspar has excellent potential to contribute towards the ever-increasing geochronological dataset constraining the timings of glacial advances in Patagonia.

Refertilization process in the Patagonian subcontinental lithospheric mantle of Estancia Sol de Mayo (Argentina)

Anhydrous mantle xenoliths equilibrated at 1003–1040°C from Estancia Sol de Mayo (ESM, Central Patagonia, Argentina) and entrained in post-plateau alkaline lavas belonging to Meseta Lago Buenos Aires have been investigated aiming at reconstructing the depletion and enrichment processes that affected this portion of the Patagonia lithospheric mantle. Xenoliths are characterized by a coarse-grained protogranular texture and are devoid of evident modal metasomatism. They show two texturally different clinopyroxenes: protogranular (cpx1) and texturally related to spinel (cpx2). Three different types of orthopyroxenes are also recognized: large protogranular crystals with exsolution lamellae (opx1); small clean and undeformed grains without exsolution lamellae (opx2) and small grains arranged in a vein (opx3). Major element composition of clinopyroxenes and orthopyroxenes highlights two different trends characterized by i) a high Al2O3 content at almost constant mg# and ii) a slight increase in Al2O3 content with decreasing mg#. Clinopyroxenes are enriched in LREE and are characterized by prominent to slightly negative Nb, Zr and Ti anomalies. No geochemical differences are observed between cpx1 and cpx2, while a discrimination can be observed between opx1 and opx2 (LREE-depleted; prominent to slightly negative Ti and Zr anomalies) and opx3 (prominent positive Zr anomaly). Partial melting modeling using both major and trace elements indicates a melting degree between ~5% and ~13% (up to ~23% according to major element modeling) for lherzolites and between ~20% and ~30% for harzburgites (down to ~5% according to trace element modeling). La/Yb and Al2O3, as well as Sr and Al2O3 negative correlations in clinopyroxenes point to a refertilization event affecting this lithospheric mantle. The agent was most probably a transitional alkaline/subalkaline melt, as indicated by the presence of orthopyroxene in the vein and the similar geochemical features of ESM clinopyroxenes and those from Northern Patagonia pyroxenites which are derived from transitional alkaline/subalkaline lavas.

Geochemical analysis of obsidian from archaeological sites in northwestern Santa Cruz Province, Argentine Patagonia

Obsidian raw material has been widely used in central and southern Patagonia from the beginning of the occupation of this region. In NW Santa Cruz province, Argentina, the geochemical analyses published to date suggest that Pampa del Asador was the main obsidian source. However, both the spatial and temporal limits of the distribution of obsidians from this and other sources in the region remain undefined. The principal goal of this article is to characterize obsidian artifacts from archaeological sites, located in a portion of NW Santa Cruz, from which there is no available geochemical information. ICP-MS analyses were obtained for 49 obsidian artifacts recovered from 24 excavated rockshelters and open-air surface scatters. The results indicate that all the samples of black obsidian came from the Pampa del Asador source, located ∼90–160 km to the south. Two samples of grey obsidian discovered on the top of Meseta del Lago Buenos Aires are chemically different from any other archeological obsidian yet analyzed in southern Patagonia. Their origin is still unknown, but they are most likely derived from a local Meseta del Lago Buenos Aires obsidian source. These results contribute to the knowledge about the use of obsidian in NW Santa Cruz, and provide constraints on the communication and material exchange between different environments and basins in this region.

The reconstruction and climatic implication of an independent palaeo ice cap within the Andean rain shadow east of the former Patagonian ice sheet, Santa Cruz Province, Argentina

This paper describes the reconstruction of the previously undocumented Meseta Cuadrada palaeo ice cap on south-west Meseta del Lago Buenos Aires, Santa Cruz Province, Patagonia. Based on theoretical surface profiles the reconstruction of the Meseta Cuadrada Palaeo Ice Cap indicates an ice mass covering at least 78km2 with a total ice volume around 9.2km3. The inferred equilibrium line altitude (ELA) of the palaeo ice cap (2031m asl) represents a drop of 286m compared to the ELA of the current Meseta Cuadrada glacier (~2317m asl). We explain this small change in ELA with reference to the flat hypsometry of the palaeo ice cap and an enhanced aridity to the west of the Patagonian Andes caused by the existence of the Last Glacial Maximum (LGM) Patagonian ice sheet. Calculated annual accumulation values of ca. 402 to 957mm/a at the ELA of the Meseta Cuadrada palaeo ice cap derived by a degree day model (DDM) during the last local glacial maximum extent are low compared with estimations of the current accumulation at the ELA of the remaining glacierized area of around 3789mm/a. This strongly supports the existence of increased aridity and seasonality east of the Patagonian Andes during the Last Glacial Maximum, provided both maximum extents were synchronous.

The chronology of the Last Glacial Maximum and deglacial events in central Argentine Patagonia

This paper evaluates the chronology of the last glacial cycle and deglaciation in the Lago Pueyrredón valley of central Patagonia, 47.5° S, Argentina. The valley was a major outlet of the former Patagonian Ice Sheet and the moraines that record its fluctuations are an important proxy record of climate change in southern South America. Such moraines are well-preserved in the Lago Pueyrredón valley owing in part to the semi-arid environment east of the mountain front. Here, we provide the first direct chronology for the age of the “Rio Blanco” moraine system by utilizing cosmogenic-nuclide surface exposure ages. Boulders on the moraines give 10Be exposure ages that indicate the Last Glacial Maximum (LGM) maximum extent occurred by 27–25 ka. Subsequent advances occurred at 23–22 ka, 20–18 ka, and ca. 18–17 ka. Initial deglaciation began after ca. 18–17 ka and was interrupted as evidenced by the Lago Columna moraines up-valley. Subsequently the outlet glaciers occupying both the Lago Pueyrredón basin (Chilean name: Lago Cochrane) and the Lago Buenos Aires basin (Chilean name: Lago General Carrera) to the north, rapidly retreated more than 80 km at around 16.5–15 ka. The timing of the LGM maximum extent and the onset of deglaciation occurred broadly synchronously throughout Patagonia. Deglaciation resulted in a series of interconnected glacier-dammed lakes in the region that initially drained toward the Atlantic Ocean and later drained to the Pacific Ocean as a consequence of disintegrating ice in the Andes.

First insights on Lake General Carrera/Buenos Aires/Chelenko water balance

Abstract. Lago General Carrera (Chile) also called Lago Buenos Aires (Argentina) or originally Chelenko by the native habitants of the region is located in Patagonia on the Chilean-Argentinean border. It is the largest lake in Chile with a surface area of 1850 km2. The lake is of glacial/tectonic origin and surrounded by the Andes mountain range. The lake drains primarily to the Pacific Ocean to the west, through the Baker River (one of Chile's largest rivers), and intermittently eastward to the Atlantic Ocean. We report ongoing results from an investigation of the seasonal hydrological cycle of the lake basin. The contribution by river input through snowmelt from the Andes is of primary importance, though the lack of water input by ungaged rivers is also critical. We present the main variables involved in the water balance of Lake General Carrera/Buenos Aires/Chelenko, such as influent and effluent river flows, precipitation, and evaporation, all this based mostly in in-situ information.

Subduction of the South Chile active spreading ridge: A 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/ 39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4–3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

Glacial dynamics in southernmost South America during Marine Isotope Stage 5e to the Younger Dryas chron: a brief review with a focus on cosmogenic nuclide measurements

AbstractThe timing of major glacial changes between Marine Isotope Stage (MIS) 5e and the Younger Dryas chron (YD) in southern South America is reviewed. Focus is placed on studies that use cosmogenic nuclide measurements. However, other dated records and modelling results are also discussed to provide the broader glacial geological context. Limits on the state of glacial geologic knowledge are examined and suggestions are made as to how some of these can be addressed in future studies. Studies show that on the east side of the Andes at Lago Buenos Aires (45° S) there is evidence indicating that following a MIS 6 glaciation ice was less extensive during MIS 4 than MIS 2. In contrast, on the west side of the Andes in the Chilean Lake District area, MIS 4 ice may have been more extensive than during MIS 2. Although Patagonia experienced an MIS 2 glaciation broadly in phase with that in other areas around the globe, there are differences in glacial histories between areas on millennial timescales. Climate reversals are evident in many records between ca. 17 and 10 ka. The timing of such Lateglacial activity, which included less extensive advances than during the local Last Glacial Maximum (LGM), varies somewhat between areas. This could be due to the relative influence of middle‐ to high‐latitude climatic regimes. At present, chronological uncertainties prevent confident assignment of cosmogenic ages for glacial events at the millennial timescale and defining associated climate variability between different study areas. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: geochronological constraints and new field evidence

AbstractRecent field work and review of radiometric data obtained from Neogene lavas and plutonic rocks exposed in the Eastern Central Patagonian Cordillera (46–48ºS), which overlie subducted segments of the South Chile Ridge, suggest important Late Miocene to Pleistocene morphological changes in relation to base level variations and/or tectonic events. We present new field observations from a region south of Lago General Carrera‐Buenos Aires, between the main Cordillera and the Meseta del Lago Buenos Aires, demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5–4 Ma and after 3 Ma. We also show that the 12 Ma basaltic flows of the Meseta del Lago Buenos Aires (∼2000 m a.s.l.) have been subjected to deep incision, with younger lavas dated at 1.2 Ma partially filling the valleys. These incisions are thought to reflect progressive eastward tilting of the entire meseta. Our new observations, together with additional features from Central Patagonia, strongly suggest that tectonic events led to a regional widespread morphological change after 5 Ma. The coincidence in time and space between the subduction of segments of the South Chile Ridge at 6 and 3 Ma causing opening of a slab window, and strong base level variations in the studied area, suggests a cause‐and‐effect relationship. In Central Patagonia, compressional tectonics ended well before extensional events reported here. Causes of uplift and further extension are probably completely disconnected. The uplift is purely tectonic in origin and occurred prior to the subduction of the South Chile Ridge. Extension should be a consequence of this subduction.

Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A 1-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO 2; 8.7–10.8 wt.% Na 2O + K 2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts (( 87Sr/ 86Sr) o = 0.70488–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these ∼ 3–4 Ma old alkaline intrusions occur aligned along a ∼ N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench.

Cosmogenic nuclide surface exposure dating of boulders on last-glacial and late-glacial moraines, Lago Buenos Aires, Argentina: Interpretive strategies and paleoclimate implications

In situ cosmogenic nuclides have become a powerful means to determine surface exposure ages of boulders on moraines and other landforms that are important to paleoclimate reconstructions. Potentially the largest and least quantifiable source of uncertainty of these surface exposure ages is the variable exposure histories of individual boulders. We use the mean square of weighted deviates (MSWD) statistic and cumulative frequency plots to identify groups of boulders that have statistically similar ages based on the number of analyses and their uncertainties. These samples most likely represent the true age of the moraine. We use these tools to interpret 49 10Be and 26Al surface exposure ages of erratic boulders on six last-glacial and late-glacial moraines at Lago Buenos Aires, Argentina (LBA; 71°W, 46.5°S). Seven of the 49 boulders are identified as anomalously young, and are interpreted to have been exhumed after moraine deposition. The remaining 42 samples indicate that glacial advances or still-stands of the ice margin occurred at 22.7±0.9, 21.4±1.9, 19.9±1.1, 17.0±0.8, 15.8±0.6, and 14.4±0.9 ka (weighted mean ages ±2 σ, analytical, erosion rate, and attenuation length uncertainties). This chronology of an outlet of the Patagonian Ice Cap is comparable to many records in the Northern Hemisphere despite a maximum in local summer insolation during this period. The implication is that climate during the Last Glacial Maximum was generally synchronous at the global scale. However, the late-glacial readvance to 95% of the extent of the largest advance during the Last Glacial Maximum at 14.4±0.9 ka is distinctively “Antarctic” in nature. It is contemporaneous with the Antarctic Cold Reversal, and precedes the Younger Dryas Chronozone. Further, our precise exposure ages highlight climatic asynchrony across southern South America between 23 and 14 ka. The timing of maximum ice extent and onset of deglaciation at LBA occurred ∼4 ka later than in the northern parts of the Chilean Lake District (41°S), but were synchronous with glacial advances or still stands in the Strait of Magellan (52°S). This regional asynchrony is likely related to the strength and position of the Southern Westerlies.