Southernmost South America Research Articles

Patagonian landscape modeling during Miocene to Present-day slab window formation

The subduction of seismic oceanic ridges often results in the formation of slab windows, which can affect not only the heat flow and retroarc volcanism, but also the exhumation and topographic evolution of the upper plate. An active and world-class example of a slab window is southern Patagonia, in southernmost South America, which is related to the subduction of the seismic oceanic South Chile Ridge between the middle–late Miocene and the Present day. How the subduction of the ridge and formation of the slab window have influenced the evolution of the Patagonian landscape, exhumation and topography is still under debate. Some works have proposed orogenic deformation mostly affecting the Pacific margin and hinterland areas, or an inherited early Miocene tectonic relief generated before the slab window formation. Others have preferred epeirogenesis hypotheses, such as dynamic uplift or isostatic rebound as a result of lithospheric thinning associated with asthenospheric or lithospheric mantle changes. In this work, we analyze the landscape evolution at medium (orogen-scale) and long wavelengths (embracing the whole of southern Patagonia, from coast to coast) using FastScape a landscape numerical model. This program was coupled with an optimization scheme (the Neighborhood Algorithm) suitable for nonlinear inverse problems. The “goodness” (fit to data) of our landscape evolution models was evaluated using: i) cooling ages, and ii) maximum elevations, in order to provide constraints on the uplift rates, erosion efficiency and effective elastic thickness. We then used the best values to compare two forward models representing medium- versus long-wavelength processes. Our results indicate that long-wavelength uplift geometry (including dynamic uplift and/or lithospheric rebound from thinning) involving areas from the Andes to the Atlantic coast was required from 12 Myr to the Present day in order to reproduce not only the youngest cooling ages but also Present-day topography.

Linking accretionary orogens with continental crustal growth and stabilization: Lessons from Patagonia

The origin of the continental crust and the tectonic significance of Paleozoic magmatic rocks of Patagonia (southernmost South America) remain one of the main enigmas in the history of the Gondwana supercontinent. Here, new whole-rock geochemistry together with coupled zircon U-Pb and Lu-Hf isotopic data of Devonian to Permian intrusive rocks of northern Patagonia are integrated with a revised geochemical and isotopic database of the region, providing a novel model for the tectonomagmatic evolution and related crustal growth mechanisms. The development of a Devonian retreating accretionary orogen associated with crustal thinning was succeeded by a late Carboniferous to Permian advancing orogen and crustal shortening, resulting from slab shallowing. The latter was related to the Gondwanide Orogeny, a major transpressional tectonic event that led to the maturation and stabilization of the continental crust of Patagonia due to widespread magmatism and crustal thickening, which culminated with Permian-Triassic slab break-off. Addition of juvenile mantle-derived magmas is more significant during the Devonian, whereas a progressively increase in crustal reworking is documented from the late Carboniferous to the Permian due to crustal thickening. Therefore, this non-collisional model favors an in situ middle to late Paleozoic crustal growth of Patagonia during changing dynamics of accretionary orogens at the proto-Pacific margin of southwestern Gondwana.

Tree-ring δ18Ocellulose variations in two Nothofagus species record large-scaleclimatic signals in the South American sector of the Southern Ocean

In recent decades, southernmost South America (50–56° S) has experienced marked climate change (regional warming, decreased precipitation, reduced snow cover, and increased frequency of heat waves) related to variations in atmospheric circulation over varied timescales. In this paper, we develop oxygen isotopes from tree-ring cellulose (δ18OTRC) as a proxy for climate and atmospheric circulation in order to extend the regional meteorological record to deeper time intervals. Our work focuses on Nothofagus forests in two areas: (i) deciduous N. pumilio forest in the steppe transition zone near Punta Arenas and (ii) humid evergreen N. betuloides forest in the Navarino Island region. To investigate the potential for reconstructing palaeoclimate, δ18OTRC variations were correlated with local climate parameters as well as regional (Amundsen Sea Low, ASL) and hemispheric (Antarctic Oscillation, AAO) atmospheric circulation modes for the last 60 years. N. betuloides δ18OTRC variations show an overall positive trend, indicating isotopic enrichment over the study period, whereas no trend is recorded for the N. pumilio record. The strongest relationships with climate, together with the widest spatial representativeness, occur in the N. betuloides chronology during the growing season (spring to austral summer) and extend spatially from mid to high latitudes. In contrast, the sensitivity of the records is limited to summer months, and spatial correlations are much more limited. In addition, the N. betuloides record shows greater potential for reconstructing local climate features such as soil water (r = − 0.76), wind speed (r = 0.69), and precipitation (r = − 0.66), as well as regional (ASL, r = − 0.80) and hemispheric (AAO, r = 0.77) patterns of extratropical atmospheric circulation. Overall, we conclude that the N. betuloides record represents the most valuable tree-ring climate proxy for southernmost South America over past centuries.

Historical ecology and current abundance of the translocated Chilla or Grey fox Lycalopex griseus on the large Tierra del Fuego Island shared by Argentina and Chile

AbstractIn 1951, a batch of 24 young animals of both sexes of Chilla or Grey fox Lycalopex griseus from continental Magallanes region, Chile – and perhaps also from adjacent continental Argentina – were released at Onaisin (65 km ESE Porvenir town, 53°18′S) on Tierra del Fuego Island in southernmost South America, to control a European rabbit (Oryctolagus cuniculus) irruption that was considered detrimental to sheep (Ovis aries) ranching. Up to now, no attention has been paid to the temporal course of that introduction. Here we provide a historical account of the presence of foxes on the Tierra del Fuego Archipelago, starting with Charles Darwin account of 1839 until present. We also review the regulatory framework concerning hunting of those foxes and tally their culling for export of pelts. Finally, we provide estimates of the abundance of Chilla foxes on Tierra del Fuego Island for the period 1999–2007 and for 2021, and highlight their current spread. These translocated foxes have become an abundant and functional part of both steppe and forest ecosystems on the Fuegian archipelago, even though they are killed by hunters, without spatial, temporal, or numerical limits. We conclude that interesting scientific opportunities are being missed regarding the population genetics of a species that may have left founding effects and genetic bottlenecks in Tierra del Fuego, and perhaps some peculiar continental markers among the island population. We also raise the question whether this introduced native species may be deemed invasive, calling for research to determine its impact in its new environment.

Role of large-scale climate oscillations in precipitation extremes associated with atmospheric rivers: nonstationary framework

ABSTRACT Atmospheric rivers (ARs) are filamentary regions of high-water vapour flux in the lower troposphere that contribute significantly to poleward moisture movement in mid-latitude regions. Key characteristics (frequency, duration, and intensity) of ARs have been explored to recognize the regions vulnerable to AR-flood. To investigate the association of ARs with large-scale climate oscillations (LSCOs), precipitation extremes (PEs) maximum 1-day precipitation (Rx1day), maximum consecutive 5-day precipitation (Rx5day), precipitation amount from very wet days (R95pTOT) are explored in a non-stationary framework of generalized extreme value distribution, taking the Arctic Oscillation, North Atlantic Oscillation, El Niño Southern Oscillation, and Pacific Decadal Oscillation (PDO) as covariates. In almost 30% of regions around the globe, May-June-July-August-September (MJJAS) season PDO was found to be the relatively most influential covariate for capturing PEs. The west coast of North America and of Europe, southernmost South America, central East Asia, New Zealand, and Australia have been identified as the most critical regions associated with AR linked with PE-associated LSCOs.

New records of Theropoda from a Late Cretaceous (Campanian-Maastrichtian) locality in the Magallanes-Austral Basin, Patagonia, and insights into end Cretaceous theropod diversity

The end Cretaceous mass extinction was marked by a dramatic change in biodiversity, and the extinction of all non-avian dinosaurs. To understand the diversity of dinosaur clades prior to this event, as well as recovery by avian dinosaurs (birds), we need a better understanding of the global fossil record. However, the fossil record from southern localities, particularly southernmost (>60°S) South America, has only recently begun to be described. Discoveries from Patagonia are important to accurately assess global trends in dinosaur diversity, particularly during the latest Cretaceous before the Cretaceous/Paleogene (K/Pg) mass extinction event. Here we describe new theropod dinosaur specimens, representing both associated material and isolated elements, from Upper Cretaceous (Campanian-Maastrichtian) deposits of the Río de las Chinas Valley, Magallanes-Austral Basin, Chile. These discoveries include the southern-most known occurrences of several theropod clades outside of Antarctica, including megaraptorids, unenlagiines, enantiornithines and ornithurines. Notably, these remains provide much needed time-constrained records of smaller theropods, including birds, which are less often recovered from Upper Cretaceous deposits. While fragmentary, these fossils are the first records of theropods from Chilean Patagonia, and provide insight into the distribution of avian and non-avian theropods in southern high latitude ecosystems prior to the K/Pg mass extinction event. Sampling from this region is still sparse, and more fossils from age-controlled sections are needed to accurately assess global extinction dynamics through the end Cretaceous.

Tectonic Subsidence Modeling of Diachronous Transition From Backarc to Retroarc Basin Development and Uplift During Cordilleran Orogenesis, Patagonian‐Fuegian Andes

AbstractBackstripped tectonic basin subsidence histories are critical for interpreting phases of lithospheric deformation and paleoenvironmental change from the stratigraphic record. This study presents new subsidence modeling of the Rocas Verdes Backarc Basin (RVB) and Magallanes‐Austral retroarc foreland basin (MAB) of southernmost South America to evaluate along‐strike changes in tectonic subsidence related to the Late Jurassic through Miocene history of the Southern Andes. We compiled composite stratigraphic sections for seven basin localities that span 47°–54°S from published sedimentological records of paleoenvironment, paleobathymetry, and geochronology. Modeling results resolve regional trends in basin tectonic subsidence, uplift, and sedimentation rate that influenced the depositional environment during five broad phases of RVB‐MAB development: (a) Late Jurassic tectonic subsidence and basin deepening associated with rift‐related backarc extension that postdated regional diachronous rift‐related magmatism. (b) Southward younging of Early to Late Cretaceous pronounced acceleration in tectonic subsidence interpreted as the initiation of flexural loading and development of the MAB foreland basin system. (c) Late Cretaceous (ca. 85–70 Ma) tectonic uplift within the central foredeep ∼49° to 52°S, coeval with a shift from slope to shelf deposition at these latitudes. (d) A protracted period of low‐magnitude basin uplift and relative tectonic quiescence during the Paleogene, with the exception of southernmost localities; and (e) Synchronous latest Oligocene‐early Miocene tectonic subsidence linked to basin deepening and transgression across the northern and central basin sectors. Backstripped tectonic subsidence analysis corroborates existing interpretations for orogenic development in the RVB‐MAB and sheds new light on complex polyphase basin histories where extension precedes convergence.

Holocene environmental changes in the fuegian forest and steppe, Argentina

Environmental changes were reconstructed from a multiproxy synthesis of over 30 localities from the Isla Grande de Tierra del Fuego and Isla de los Estados, southernmost South America. At a local scale, the results from the mountain forest and gently undulating steppe areas were integrated as well as those from the marine environments of the Beagle Channel and the Atlantic coasts. At a regional scale, the results were integrated with those published for the southernmost Andean and Extra-Andean Patagonia and the Antarctic Peninsula. This study focuses on the environmental evolution during the Late Glacial-Holocene transition, the Middle to Late Holocene transgressive-regressive hemicycle and wet-dry oscillations, the Medieval Climate Anomaly, the Little Ice Age, the tephra inputs from the Patagonian Andes, and the recent climatic warming. Most paleoenvironmental changes are related to variations in the latitudinal position and intensity of the Southern Westerly Winds (SWW) while others are associated with astronomical or endogenous forcings. At a strong intensity of the SWW, a greater contribution of humidity to the forest areas and an increase in the rainfall gradient create windy and arid conditions in the steppe. At a weak intensity of the SWW, lower humidity input in the forest areas and the advection of air masses from the Atlantic Ocean promoted humid and slightly windy conditions in the steppe. Similar environmental trends are observed between terrestrial and marine environments in the center and south of Tierra del Fuego, Isla de los Estados and the Antarctic Peninsula, and between the Fuegian steppe and Extra-Andean Patagonia. The paleoclimatic evidence reveal high environmental variability in the last 10,000 years for this sector of the Southern Hemisphere.

The role of physicochemical and biochemical processes on carbonate precipitation within the Laguna Timone maar in the Pali Aike Volcanic Field, southernmost extra-Andean Patagonia

Carbonate deposits and sedimentary records from lakes in southern Patagonia provides an excellent contribution in the regional environmental register. The Laguna Timone is situated within a maar of the Pali Aike Volcanic Field, a Quaternary volcano-tectonic complex in southernmost South America and represents one of hundreds of “pools” of brines developed after explosive volcanic eruptions in a periglacial environment. The lake constitutes an endorheic hydrological system where processes leading to carbonate precipitation took place under extreme physicochemical conditions and biological influences that can be explored. Laguna Timone is recharged by groundwater and sporadic episodes of precipitation (ca. 200 mm/year) and high evaporation rates are controlled by regimes of strong wind (westerlies) and seasonal solar radiation. Carbonate precipitation was studied in microbialite fragments of tufa deposits and carbonate crust located in the edge of the lake, and the mineralogy of all samples is calcite. The clay fraction of the sediment underlying the carbonates was characterized and HRTEM analysis shows that authigenic smectites have influence on calcite crystal precipitation. The positive δ18O values (2.28 ‰) in thin crust layers are associated with evaporation processes. In contrast, the negative δ18O values (−6.52 ‰) in the microbialites show meteoric and/or groundwater influences. The δ13C (−0.43 ‰ to 2.50 ‰) values indicate physico-chemical and biochemical processes controlling the precipitation. The carbonate precipitation involves the interrelations of hydrogeological properties, climate and biological influences. Laguna Timone provides a natural laboratory for understanding mineral precipitation processes that register continuous climatic and environmental archives.

Two Nothofagus Species in Southernmost South America Are Recording Divergent Climate Signals

Recent climatic trends, such as warming temperatures, decrease in rainfall, and extreme weather events (e.g., heatwaves), are negatively affecting the performance of forests. In northern Patagonia, such conditions have caused tree growth reduction, crown dieback, and massive die-back events. However, studies looking at these consequences in the southernmost temperate forest (Nothofagus betuloides and Nothofagus pumilio) are much scarcer, especially in southernmost South America (SSA). These forests are also under the influence of the positive phase of Antarctic Oscillation (AAO, also known as Southern Annular Mode, SAM) that has been associated with increasing trends in temperature, drought, and extreme events in the last decades. This study evaluated the growth patterns and the climatic response of eight new tree-ring chronologies from Nothofagus species located at the upper treeline along different environmental gradients in three study areas: Punta Arenas, Yendegaia National Park, and Navarino Island in SSA. The main modes of the ring-width index (RWI) variation were studied using principal component analysis (PCA). We found that PC1 has the higher loadings for sites with precipitation values over 600 mm/yr, PC2 with N. betuloides sites, and PC3 with higher loadings for sites with precipitation values below 600 mm/yr. Our best growth-climate relationships are between N. betuloides and AAO and the most northeastern site of N. pumilio with relative humidity (which coincides with heatwaves and extreme drought). The climatic signals imprinted in the southernmost forests are sensitive to climatic variability, the climate forcing AAO, and the effects of climate change in the last decades.

Early rifting during marginal basin development: Petrography, microstructure, and detrital zircon U‐Pb geochronology of the Lapataia Formation, Argentine Fuegian Andes

AbstractAlthough being one of the most efficient tools to constrain the stratigraphy of sedimentary basins, detrital zircon geochronology may present limitations under certain conditions, and isotopic ages should be treated with caution before assigning a depositional age. This is the case of depositional environments that lack coeval volcanism, where there may exist a significant time gap between the age of the youngest detrital zircon available and the actual time of deposition of a sedimentary rock. In this paper, we put on a comprehensive analysis in which detrital geochronology is aided with other reliable geological constraints. As a case study, we evaluate the stratigraphic age and provenance of the Lapataia Formation from the Fuegian Andes of southernmost South America, a low‐grade metamorphic unit with a problematic stratigraphic definition in the context of the Rocas Verdes basin. Detrital ages show very wide age spectra, with a younger peak at ca. 267 Ma. However, based on detailed petrography and microstructural observations, as well as stratigraphic correlations at a regional scale, we conclude that the Lapataia Formation represents the earliest stages of Jurassic rifting during the opening of the Rocas Verdes basin. Erosion from older basement blocks in an environment lacking coeval volcanic activity explains the occurrence of Paleozoic and older grains, and the lack of younger zircons. This work offers a multifaceted approach, which may be essential to assist radiometric detrital geochronology at certain stages during basin evolution.

A contribution to the knowledge of the taxonomy of the subgenus Abrothrix (Angelomys) (Rodentia, Cricetidae) in southernmost South America

A contribution to the knowledge of the taxonomy of the subgenus Abrothrix (Angelomys) (Rodentia, Cricetidae) in southernmost South America

Paleomagnetic Constraints From South Georgia on the Tectonic Reconstruction of the Early Cretaceous Rocas Verdes Marginal Basin System of Southernmost South America

AbstractSouth Georgia exists as a microcontinent along the North Scotia Ridge ∼1,700 km east of Cape Horn. The tectonostratigraphic units of South Georgia have long been correlated with those of the Fuegian Andes of southernmost South America. Accordingly, South Georgia has been regarded as a continuation of the Late Jurassic–Early Cretaceous Rocas Verdes marginal basin system, formerly situated south of Burdwood Bank and east of Cape Horn. To test this, paleomagnetic analysis of samples from the Larsen Harbour Complex, Drygalski Fjord Complex, and Annenkov Island Formation of South Georgia showed that 21 sites yield a mean direction of D = 328.5°, I = −62.1° (a95 = 3.5°) and a paleomagnetic pole at 068.2°E, 67.2°N, A95 = 4.7°. The consistency of directions and strong polarity bias, plus indications of a negative differential tilt test, point to a secondary magnetization acquired in the Late Cretaceous. Comparison of predicted versus observed directions for South Georgia relative to stable South America indicate 27.2 ± 11.2° of counter‐clockwise rotation (and 10.5° ± 4.5° of northward tilting) since the acquisition of magnetization. These results are consistent with paleomagnetic studies from the Fuegian Andes and support a paleoposition of the South Georgia microcontinent south of Burdwood Bank as strongly indicated by the geologic evidence. Partitioning this rotation between oroclinal bending during the Rocas Verdes basin collapse in the Late Cretaceous and left‐lateral translation along the North Scotia Ridge is not possible on paleomagnetic grounds, but the co‐linearity of Andean structures between the restored microcontinent and Tierra del Fuego indicates the former.

Holocene glacier history of northeastern Cordillera Darwin, southernmost South America (55°S)

AbstractIn the Cordillera Darwin, southernmost South America, we used 10Be and 14C dating, dendrochronology, and historical observations to reconstruct the glacial history of the Dalla Vedova valley from deglacial time to the present. After deglacial recession into northeastern Darwin and Dalla Vedova, by ~16 ka, evidence indicates a glacial advance at ~13 ka coeval with the Antarctic Cold Reversal. The next robustly dated glacial expansion occurred at 870 ± 60 calendar yr ago (approximately AD 1150), followed by less-extensive dendrochronologically constrained advances from shortly before AD 1836 to the mid-twentieth century. Our record is consistent with most studies within the Cordillera Darwin that show that the Holocene glacial maximum occurred during the last millennium. This pattern contrasts with the extensive early- and mid-Holocene glacier expansions farther north in Patagonia; furthermore, an advance at 870 ± 60 yr ago may suggest out-of-phase glacial advances occurred within the Cordillera Darwin relative to Patagonia. We speculate that a southward shift of westerlies and associated climate regimes toward the southernmost tip of the continent, about 900–800 yr ago, provides a mechanism by which some glaciers advanced in the Cordillera Darwin during what is generally considered a warm and dry period to the north in Patagonia.

A full-vector paleomagnetic secular variation record from 55,000 to 33,000 cal. years BP from Río Valdéz glaciolacustrine outcrop (Tierra Del Fuego, Argentina)

High-resolution paleomagnetic investigations were performed on Río Valdez outcrop at the Fagnano Lake, central Isla Grande de Tierra del Fuego, southernmost South America. Our aims were to develop a full-vector paleomagnetic secular variation (PSV) record and establish paleomagnetic chronostratigraphy for the outcrop based on radiocarbon dating and the correlation of the PSV record with other dated PSV records in the region. We detected two distinct anomalous directional intervals at 35,400 and 33,800 cal. years BP and at 41,500 and 39,000 cal. years BP, which could be associated with Mono Lake and Laschamp excursions. Rock magnetic investigations revealed slight changes in concentration, mineralogy, and magnetic grain size along the sedimentary sequence, but the reconstruction of past direction and relative paleointensity variations in the geomagnetic field remained feasible. The record provides new insights into the behavior of the geomagnetic field at high latitudes in the Southern Hemisphere, about which very little is currently known.

The large MIS 4 and long MIS 2 glacier maxima on the southern tip of South America

Robust glacial chronologies are needed to address the fundamental questions of when and why Ice Age climates begin and end. Well-preserved glacial deposits left by large ice sheet lobes adjacent to Estrecho de Magallanes (53°S; Chile) in southernmost South America provide a unique opportunity to reconstruct the timing and fine structure of the last two major glaciations, as well as the last termination. We present a new precise 10Be surface exposure dataset of 34 moraine boulders directly tied to a recently published, high resolution glacial geomorphic map of the deposits left by the Magallanes lobe.We find that the southern section of the Patagonian Ice Sheet was more extensive during Marine Isotope Stage 4 (MIS 4) than during MIS 2, representing the first direct dating of the MIS 4 glacier culmination in South America. Similar to the MIS 2 glacial maxima, within MIS 4 there were multiple advances that we date (n = 6 samples) to between 67.5 ± 2.1 and 62.1 ± 2.0 ka. A similarly timed MIS 4 advance was identified in New Zealand, indicating that this is a hemisphere-wide glacier-climate signal, which is further corroborated by South Atlantic and Pacific temperature proxy records. Inboard of the MIS 4 moraine complex, we date a sequence of geomorphically distinct MIS 2 moraines that represent separate major periods of glacial stability. The MIS 2 maximum extent occurred by 27.4 ± 0.8 ka (n = 4; arithmetic mean, with the standard error of the mean and 3% propagated production rate error) and was followed by at least four more full glacial culminations at 25.7 ± 0.8 (n = 3), 23.9 ± 0.8 (n = 5), 19.1 ± 0.7 (n = 3), and 18.1 ± 0.6 ka (n = 3), which represent periods when the glacier was in equilibrium with the climate for long enough to form major moraines. About 18 km inboard, this sequence is followed by smaller-scale recessional moraine crests, deposited on drumlinized terrain rather than a moraine drift, that we date to 18.0 ± 0.8 ka, indicating the glacier was in net retreat at this time. Tentative results from a 2D ice sheet model suggest that the Magallanes lobe may have reached mapped inner and outer MIS 2 moraines from a climate with approximately 4.5 °C and 5.5 °C cooler summers, respectively, assuming ∼25% less annual precipitation, relative to modern climate. We hypothesize that during the last glacial cycle, shifts in the subtropical and subantarctic fronts, and related ocean-atmosphere patterns, explain MIS 4 to 2 glacial behavior in the southern mid-latitudes.

Introduction to Journal of South American Earth Sciences special issue on “Magmatism of southernmost South America”

During the Neoproterozoic West-Gondwana assembly, the Araçuaí belt together with its African counterpart, the West Congo belt, formed a confined orogenic system between the São Francisco and Congo cratons. As a result, a widespread magmatism took place from ca. 630 to 480 Ma at the Araçuaí belt, finishing with a ca. 530 to 480Ma post-collisional period. The Alto Chapéu pluton is one of the numerous intrusions emplaced during this post-collisional event. In order to investigate the crystallization conditions involved in the evolution of this pluton, we conducted a study based on zircon chemistry, concerning the granitic composition of this magmatism at the early stages of the Araçuaí orogenic collapse. Coupled zircon saturation and Ti-in-zircon temperatures along with REE-in-zircon oxygen fugacity estimatives indicate high temperatures and relative reducing conditions intervening magmatic differentiation given by autocrystic zircon domains (maximum 814 ± 14 °C to average 749 ± 24 °C and ΔQFM -3.78 ± 1.02). Even more reducing and hotter isolated magma pulses at Alto Chapéu Pluton are indicated by antecrystic zircon domains (isolated peaks of 864 ± 28 °C and 920 ± 30 °C and ΔQFM reaching −5.11 ± 0.84 at antecrystic rim). Inherited grains interpreted to derive from host rock point to slightly lower temperatures, and also reducing conditions (maximum 756 ± 16 °C to averages of 679 ± 22 °C and ΔQFM -3.88 ± 1.09). Older inherited zircons are problematic to constraint and because of this zircon trace element chemistry can shed light to characteristics of lost melts. The chemistry of inherited zircon points out to the presence of former continental-like crust at least 70 Ma prior to the Araçuaí main granite production events. The wide time-lapse involving zircon inheritance and crystallization seems to be related to post-collisional nature in recording a variety of geochemical sources signature.

ГЕОДИНАМИКА И МАГМАТИЗМ ТРАНСФОРМНЫХ ОКРАИН ТИХООКЕАНСКОГО ТИПА: ОСНОВНЫЕ ТЕОРЕТИЧЕСКИЕ АСПЕКТЫ И ДИСКРИМИНАНТНЫЕ ДИАГРАММЫ

Transform margins represent lithospheric plate boundaries with horizontal sliding of oceanic plate, which in time and space replaced the subduction related convergent margins. This happened due to: spreading ridge–trench intersection (California; Queen Charlotte–Northern Cordilleran, West of the Antarctic Peninsula, and probably the Late Miocene–Pleistocene Southernmost South America) or ridge death along continental margin (Baja California); change in the direction of oceanic plate movement (Western Aleutian–Komandorsk; Southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted either from the spreading ridge collision (subduction) with a continental margin or slab tear formation, or slab break-off after subduction cessation due to other reasons. Igneous magmatic series formed in consequence of these events show diversity of tholeiitic (sub-alkaline), alkaline or calc-alkaline, high-alumina and adakitic rocks. The comprehensive geochemical dataset (more than 2400 analyses) on igneous rocks of the model transform and convergent geodynamic settings allowed to substantiate the most informative triple diagrams for the petrogenic oxides TiO2 × 10 – Fe2O3Tot – MgO and trace elements Nb – La– Yb. Mostly approved for the rock compositions with SiO2 < 63 wt. %, the new plots are capable of distinguishing igneous rocks formed above zones of subduction at an island arc and continental margin (related to convergent margins), from those formed in the tectonic setting of transform margins along continents or island arcs.

Late Pleistocene glaciolacustrine MIS 3 record at Fagnano Lake, Central Tierra del Fuego, southern Argentina

Abstract A late Pleistocene glaciolacustrine record was studied at Fagnano Lake (54°35´S, 67°20´W), central Isla Grande de Tierra del Fuego, southernmost South America. Two profiles from the Río Valdéz outcrop were collected for isotopic, geochemical, sedimentological, and geophysical analyses. The sedimentological characteristics, such as rhythmites, presence of dropstones, absence of fossil record, and scarce presence of organic matter, suggest deposition in an ice-contact lake, possibly dammed by the Fagnano paleoglacier. Organic matter of C3 plant origin suggests certain cold and wet conditions. A chronology of the late Pleistocene outcrop was obtained from five 14C ages resulting in an age-depth model. The time span covered 49.01 cal ka BP to 32.14 cal ka BP. Based on the thickness of the deposit and the calculated average sedimentation rate, the glacial environment could have been present in the study area prior to the last glacial maximum, in agreement with the Inútil-San Sebastián paleoglacier. Both glaciers flowed from the same mountain ice sheet in the Darwin Cordillera, which makes it possible to infer a different behavior of this ice cap from those of the Patagonian Andes, perhaps forced by different atmospheric dynamics and proximity to the wet and cold subantarctic air masses.

Radiocarbon dates of fossil record assigned to mylodontids (Xenarthra - Folivora) found in Cueva del Milodón, Chile

The Cueva del Milodón (CdM, southern Chile), has been explored repeatedly since the end of the 19th century. Many institutions in the world house specimens from CdM, standing out the mylodontid sloths. Until now, 36 14C measurements were performed on these remains using equivalent methodologies, Gas Proportional Counter (GPC), Liquid Scintillation Counting (LSC) and Accelerator Mass Spectrometry (AMS). The goals of this work are to review published dates, report two new radiocarbon dates made by us, and discuss the occurrence of the mylodonts in southernmost South America in a glacial context and its possible coexistence with humans. Among the published 16 direct (hair, skin, bone) and 20 indirect (excrement) 14C dates, 13 and 18 are considered here as reliable, respectively. Dates considered unreliable presented several issues (transcription errors, duplicate dates, misidentified samples, etc.), being excluded from the analysis to avoid further misunderstandings. Two new 14C dates performed by us on pieces housed in the Museo de La Plata (skin - AMS) and (bone - LSC) were added, making a total of 33 valid dates. A remarkable amplitude in the radiocarbon ages of the mylodont remains in CdM is evidenced, showing a range of values from ∼14,000 BP to ∼9800 BP, since upper Pleistocene (“Tarantian”) to early Holocene (Greenlandian). However, the direct dates of maximum confidence available show a strong modal representation in the Upper Pleistocene. Climate evidence available for this period indicates a complex interplay of deglaciation and brief cold reversals, but suggests that mylodonts could take advantage of their dense fur and thick skin, large body mass and suitability to live in caves to prosper in such cold and humid environments. Finally, evidence supporting coexistence of mylodonts and humans is briefly discussed.