This study presents a comprehensive characterization of the Lower Pennsylvanian Caleta Loa Metamorphic Complex (CLMC), located in the Coastal Cordillera of northern Chile at around 21°30′ S. Through integrated petrographic, geochemical, thermodynamic, and geochronological analyses, the research aims to constrain the pressure-temperature (P-T) conditions of migmatite formation, refine the protolith identity, and assess the regional geodynamic context of the complex. Thermodynamic modelling using P-T pseudosections and mineral composition reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates of approximately 3.3-3.4 kbar and 650 °C. These results are consistent with a high geothermal gradient and suggest a crustal depth of ~13 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no older than ca. 350 Ma (Early Mississippian), consistent with a genetic correlation with the nearby El Toco Formation, a low-grade metasedimentary unit in the forearc of the SW Gondwana margin. Structural data show evidence of compressional deformation, including top-to-the-west shear indicators and westward verging folds, which are interpreted as syn-anatectic features. These observations, combined with the metamorphic and geochronological data, point to a tectono-thermal event in the SW Gondwana forearc during the Early Pennsylvanian (ca. 318-316 Ma), associated with the so-called ‘Toco Orogeny’ and potentially linked to contemporaneous inland magmatic activity. The existence of this event supports a scenario of renewed subduction and/or the end of a magmatic lull along the proto-Pacific Gondwana margin. The current juxtaposition of the CLMC and the El Toco Formation on either side of a major fault suggests significant uplift and exhumation of the former prior to the Late Triassic.
The polycotylids, mostly short-necked plesiosaurs, were a clade of marine reptiles with an almost cosmopolitan geographical distribution in the Cretaceous Period. In austral continents, like South America, Antarctica and Australia, the presence of this group has remained relatively poorly documented, where apart from some rare specimens, most records are limited to fragmentary material. Reports of this group from South America are infrequent, particularly from Chile. Therefore, a critical review of the fossil record of Upper Cretaceous polycotylids from Chile is presented here. Previous putative reports of the group from the Quiriquina Formation (upper Maastrichtian) of the Arauco Basin (central Chile) were exclusively supported by misidentified axial elements belonging to elasmosaurid plesiosaurs, and are thus dismissed here. On the contrary, the presence of Polycotylidae in upper Campanian-lower Maastrichtian levels of the Dorotea Formation, in the Magallanes Basin (southern Chile), is confirmed. The pattern of taxonomic diversity, including polycotylids together with abundant, mostly non-aristonectine elasmosaurs in uppermost Cretaceous rocks of the Magallanes Basin, is similar to that observed in coeval localities of Argentinean Patagonia and Antarctica, but different from that observed in the Quiriquina Formation and the upper levels of the Dorotea Formation, where aristonectine elasmosaurids dominate the plesiosaur fauna. Polycotylids of equivalent age have also been described in other regions of the Weddellian Province, such as New Zealand. Polycotylidae seem to disappear from the fossil record during the late Maastrichtian in high southern latitudes of former Gondwana territories.
Seismites constitute a distinct category of sedimentary rocks that preserve soft-sediment deformation structures and associated seismogenic features, serving as critical stratigraphic archives for reconstructing paleoseismic events and regional tectonic evolution. Accurately identifying seismic sediment deformation structures and distinguishing these earthquake-induced features remains challenging. The Chiclayo region in northern Peru is located at the collision front between the Nazca and South American plates, a place with intense seismic activity. Comprehensive geological studies and section measurements were conducted in the Mesozoic La Leche Formation, identifying abundant brittle and soft-sediment deformation structures classified here as seismites. Five seismic events were determined. The paleogeographic environment of the La Leche Formation corresponds to a Late Jurassic littoral-shallow marine shelf within an initial back-arc basin. Synsedimentary fault activity between Peru’s northern coastal plain and western coastal range triggered basin margin collapses. This study provides new insights into Mesozoic plate tectonics in the eastern Pacific and modern seismic impacts on geological environments.
Oceanic ridge subduction is known to generate anomalous magmatism before and during the opening of the slab window. High Sr/Y intermediate magmas and slightly alkaline basalts with transitional arc-to-intraplate (OIB-like) signatures have been reported worldwide in such tectonic settings. Southernmost South America has been affected by the subduction of several spreading ridges from the late Oligocene to the present. Here we present new geochronological, petrographic, geochemical, and isotopic data to evaluate the petrogenesis of two Lower Miocene volcanic units from the Fuegian archipelago at ~56° S, formed during subduction of the Nazca-Phoenix spreading ridge. Comparison with igneous suites related to subduction of the Chile Ridge (currently being subducted beneath central Patagonia at ~46.5° S) reveals notable similarities in timing relative to slab-window formation and in chemical signatures. We interpret the high Sr/Y chemistry as resulting from amphibole fractionation and delayed plagioclase crystallization at depth from calc-alkaline melts, with input of terrigenous material via mantle contamination or lower crust assimilation. The slightly alkaline magmas reflect lower degrees of partial melting due to a different thermal structure of the slab-mantle wedge.
Two isolated bones of sea turtles from Algarrobo, in central Chile, are described. A fragment of a flat element of Ypresian age is identified as part of a right hypoplastron, referable to an indeterminate Pan-Chelonioidea. A second, large fragment from middle-to-upper Eocene levels belonging to the second or fourth left costal of a carapace, preserves distinctive features such as a considerable uniform thickness (5-7 mm), and a prominent ventral rib ridge, plus scute sulci remarkably similar to extant Cheloniidae such as Chelonia mydas or Lepidochelys spp. Based on these features, it is identified as an indeterminate Pan-Cheloniidae. The studied material represents the first occurrences of sea turtles in the Eocene of central Chile. The different ages of the specimens show the persistence of taxa related to Pan-Chelonioidea throughout the Eocene in the southeastern Pacific, helping to fill the austral geographic gap of the group during the Paleogene.
The early Paleozoic geology of New Zealand and evidence for the wider continental mass of Zealandia are reviewed in this contribution, especially with regard to the record of detrital zircon chronology. U-Pb age populations are reported from three new samples of siliciclastic and volcaniclastic sandstones in late Cambrian to Ordovician Swanson Formation, which constitutes the basement in the Ford Mountains and Edward VII Peninsula of western Marie Byrd Land, West Antarctica. Comparison with previously published data shows the same prominent age groups: 480-700 Ma (Early Ordovician to late Neoproterozoic) and 900-1,100 Ma (early Neoproterozoic to late Mesoproterozoic). These are related respectively to ultimate sources formed during earliest Gondwana and late Rodinia supercontinental assembly. They conform to a common and widespread pattern seen in early Paleozoic basement throughout the Western Province of Zealandia and which continues over much of the Campbell Plateau of New Zealand, as well as other parts of West Antarctica and South America. South Zealandia is conjectured to have extensive Mesoproterozoic and late Neoproterozoic- Cambrian igneous/metamorphic basement which supplied sediment to earliest Paleozoic basins over a wide region, including western Marie Byrd Land, which is considered as an integral part of Zealandia. It may also have provided late Neoproterozoic and Cambrian sediment to contemporary sedimentary basins in adjacent North Victoria Land (East Antarctica) and the central Transantarctic Mountains. Eastward continuation of any Zealandia continental margin into the Weddell Sea area is very uncertain but essentially comparable early Paleozoic sediment provenance in southern South America at least confirms the continuity of Rodinia and Gondwana assembly processes.
New petrographic and structural studies in NE Cordillera Darwin have impacts on the stratigraphic distinction between basement and cover of the Rocas Verdes Basin. Our results reveal that metasedimentary and metavolcanic rocks mapped in the hanging wall and footwall of the first-order Glaciar Marinelli Thrust were affected by the same episodes of Meso-Cenozoic deformation as the cover rocks of the Fuegian Andes, with no evidence of pre-Jurassic deformation. The key to determining the deformation sequence relies on correct interpretation of the primary sedimentary layering in fine-grained metasedimentary rocks. Our results suggest that the rocks in the study area are part of the sedimentary and volcanic fill of the Rocas Verdes Basin. Two new U-Pb age determinations indicate that early Paleozoic and older detrital zircon grains prevail in these cover rocks, consistent with provenance from an eroded Paleozoic basement during the early development of the rift basin. Careful petrographic studies are required to assess the validity of detrital zircon maximum depositional ages in the area.impact on previous stratigraphic determinations regarding the distinction between basement and cover of the Rocas Verdes basin. Our results reveal that the metasedimentary and metavolcanic rocks mapped in the hanging wall and footwall of the first-order Glaciar Marinelli thrust were affected by the same episodes of Mesozoic-Cenozoic deformation than the cover rocks of the Fuegian Andes, with no evidence of pre-Jurassic deformation. The key to determine the sequence of deformation relies on the correct distinction of the primary sedimentary layering in fine-grained metasedimentary rocks. These results indicate that the rocks in the study area are part of the sedimentary and volcanic fill of the Rocas Verdes Basin. Two new U-Pb age determinations indicate that early Paleozoic and older detrital zircon grains prevail in these cover rocks, consistent with the provenance from an eroded Paleozoic basement during the early development of the rift basin. Careful petrographic studies are thus required to assess the validity of detrital zircon maximum depositional ages in the area.
The understanding of ancient continent-continent collisions requires information on the type of accretion (frontal or oblique) of the colliding plates. The shape of the P-T path of a rock, which was metamorphosed during ongoing continent-continent collision, has so far been ignored to obtain this information. However, the consideration of corresponding P-T paths related to Phanerozoic orogens suggests that the shape of the burial path has the potential to distinguish between frontal (60-90° angle between collisional front and plate convergence direction) and oblique (
It is a great pleasure for us to present this issue celebrating the work of our colleague and friend Francisco (Pancho) Hervé. Response to the call for papers was tremendous – from Pancho’s long-standing colleagues, his many active ex-students and outside investigators inspired by his teaching, publications and lectures (and, not least, his open and engaging personality). Regretfully, a small number of papers had to be held over to the following issue......
Active slip partitioning between the subduction megathrust and the upper plate is investigated in the oblique-convergence setting of the Nazca-South American plate boundary between 33° and 47° S. This segment has two major along-strike bends: the Maipo Orocline (~34° S) and the Arauco Peninsula (~38° S), whereas south of 38° S, lies the intra-arc Liquiñe-Ofqui Fault System (LOFS). Here we examine long- and short-term upper-plate deformation by combining a harmonized catalog of about 2,300 fault-slip measurements in the forearc and arc regions, from which we derive P-T axes using kinematic inversions, along with an integrated seismological database for upper-plate events (1976-2025), including global and local networks. These data are categorized by forearc, arc, and back-arc regions. We identify four distinct tectonic segments based on the spatial distribution of P and T axes in the long-and short-term: (1) 33°-34° S, showing both ~E-W and ~N-S subhorizontal shortening in the forearc and arc areas, suggesting active radial shortening; (2) 34°-37° S, dominated by mostly blind, seismogenic, margin-parallel dextral faults along with NW- and NE-trending structures running at a high angle with respect to the plate margin (called transverse faults here); (3) 37°- 41° S, where margin-orthogonal subhorizontal shortening in the submerged forearc coexists with nearly margin-parallel shortening in the emerged forearc, with contemporaneous dextral slip along the LOFS and ~E-W shortening accommodated by transverse NE-trending dextral and NW-trending sinistral seismically active faults; and (4) 41°-47° S, a region governed mainly by the geometry and kinematics of the LOFS strike-slip duplex. Kinematic indicators on mesoscopic faults (mostly slickenfibers) and a lack of pseudotachylytes suggest a considerable aseismic component to upper-plate fault slip, implying that morphotectonic slip rates may overestimate seismic hazard. Future detailed geodetic data may help better constrain the relative contributions of aseismic and seismic slip on the upper-plate faults in this Andean segment.
This study reevaluates the chronostratigraphic extent and depositional conditions of the President Beaches Formation at the Byers Peninsula, Livingston Island, South Shetland Islands (Antarctica). Our aim is to constrain its age and improve its regional correlation using new palynological evidence. Dinoflagellate cyst assemblages enabled the recognition of the mid–late Valanginian Senoniasphaera tabulata Interval Zone, identified by the presence of its index-species and the coeval occurrence of characteristic dinocyst taxa such as Canningia reticulata, Cerbia tabulata, Kaiwaradinium scrutillinum, Spiniferites sp., Oligosphaeridium complex, Circulodinium vermiculatum, and Heslertonia heslertoniensis. These data extend the scope of the President Beaches Formation, shifting its upper boundary compared to earlier findings. Examination of dinocysts indicates low-energy neritic conditions for the investigated succession, characterized by normal marine availability and relatively oligotrophic water masses. An elevated C/M ratio (predominance of saccate pollen grains among sporomorphs) further suggests an offshore setting with significant terrestrial input. Palynofacies data support a mid-to-distal shelf depositional environment, consistent with previous lithofacies evidence. The diversity of sporomorphs indicates the hinterland flora primarily comprised pteridophyte-spore generating plants and coniferous woodlands, with nearly equal amounts of Araucariacites and Podocarpidites. A warm temperate climate with high humidity is also proposed for the mid- to late Valanginian time in this area.
The Ordovician Aguada de la Perdíz Formation exposed in the Puna de Antofagasta of northern Chile is one of the oldest non-metamorphosed sedimentary units on continental Chilean territory. Graptolites indicate a late Floian to early Dapingian age of the formation at the Aguada de la Perdíz type locality and small nearby outcrops in Chile. Correlative outcrops on the Argentinian side at Huaitiquina, Filo Pircas, Sierra de Guayaos and Lever Mucar range from the late Floian to the Darriwilian. Graptolites are associated with scarce occurrences of brachiopods and conodonts in some outcrops. In the Argentinian Puna the correlative units were assigned to the Coquena Formation. The lithological assemblage in all outcrops comprises of bimodal lavas, breccias and mostly reworked felsic tuffs. The main part of the successions consists of volcaniclastic turbidites and lutite flow deposits. The lithological assemblage reflects deposition in a marine volcaniclastic apron located on the eastern flank of the Famatinian magmatic arc. In view of the common characteristics of the respective outcrops on both sides of the political border we propose to group them in the Aguada de la Perdíz Formation thus respecting precedence of the first definition of the formation by Garcia et al. (1962). New U-Pb detrital zircon ages of a sample (n=124) from the Huaitiquina locality range between 3530 Ma and 550 Ma and reflect a common polycyclic provenance from older Amazonian mountain belts. It is noteworthy that the youngest U-Pb zircon age belongs in the Ediacaran and predates the biostratigraphically defined depositional age by c. 80 Myr. This means that the synsedimentary Famatinian felsic volcanism, otherwise common in coeval units, is not reflected in the detrital zircon age record at Huaitiquina. The absence of Famatinian ages may indicate that sediment delivery from the Famatinian arc bypassed this site and that erosion in the source region of the arc had locally dissected the volcanic edifices and had progressed to broach the pre-Pampean Neoproterozoic arc basement.
In, Ga, and Ge are considered critical metals due to the risks associated with their supply and the growing demand driven by emerging technologies. This contribution presents a statistical analysis, based on non-parametric techniques, of mineralogical data obtained through electron microprobe analysis (EMPA) and laser ablation (LA-ICP-MS), compiled from scientific literature. The study focuses on the presence and concentration of In, Ga, and Ge in high (HS), intermediate (IS), and low sulfidation (LS) epithermal deposits, as well as in polymetallic epithermal (EP) deposits and porphyry-epithermal transition zones (P-HS) in Argentina. The results show significant differences among deposit types, with the highest concentrations of In, Ga, and Ge found in IS-type deposits, where minerals such as roquesite, sakuraiite, ishiharaite, omariniite, catamarcaite, and putzite act as the main carrier phases. EP deposits exhibit high concentrations of In and Ge associated with Sn-bearing minerals and sphalerite. Meanwhile, HS and P-HS deposits stand out for their Ge potential, with low In and Ga contents. The correlation matrix indicates a moderate positive correlation between In and Ga (r = 0.55), a negative association between In and Ge (r = -0.04), and no significant correlation between Ge and Ga. This study demonstrates the usefulness of statistical tools in identifying concentration trends of these critical metals across different types of epithermal deposits, representing a valuable tool for their future exploration.
Respuesta al comentario sobre “Isoterma de Curie en Chile central: análisis del gradiente y flujo geotérmico en la zona continental” de Cáceres Araya (2025), Andean Geology 52 (2): 268-279, por Yáñez (2025), Andean Geology 52 (3): 412-419
South Georgia lies in a remote position in the circumpolar South Atlantic and forms one of the most isolated continental fragments on Earth. The basement geology of South Georgia is restricted to the southeast sector of the island and is termed the Drygalski Fjord Complex, which consists of metasedimentary rocks and localised paragneisses that form an accretionary complex cut by multiple dolerite dykes and gabbroic intrusive rocks. The basal Salomon Glacier Formation has a maximum likely depositional age of c. 270 Ma that is consistent with West Gondwana accretionary complexes from the Antarctic Peninsula and Patagonia, but also identical in depositional age to sedimentary successions from the Karoo Basin (South Africa) and East Antarctica. Any correlation to the sedimentary units from South Africa and East Antarctica can be ruled out as they lack a secondary age peak at c. 470 Ma, ubiquitous across Gondwana margin accretionary complexes, and are instead characterised by a secondary age peak at c. 530 Ma, associated with the recycled Cambrian sources of East Antarctica. The late Permian accretionary complex of South Georgia is closely correlated to units from the northern Antarctic Peninsula (Trinity Peninsula Group) and the southern Cordillera Darwin, and we favour a common origin on the Antarctic Plate before closure of the Rocas Verdes Basin and translation to the Scotia Plate.
The La Laja Hydrothermal System is located in a tectonically and seismically active area in the southern Central Andes, in the San Juan Province of Argentina. Here we present a geomorphological and geophysical characterization of this system, aimed at constructing geological-structural models to explain its origin, configuration, and main manifestations. The geomorphological analysis allowed the identification of travertine cones and terraces, mudflats, springs, and channels, geoforms that were studied using gravimetry, magnetometry, electrical tomography and audio-magnetotelluric methods. The combination of these techniques, which provide information at different depths and resolutions, together with surface geological data, made it possible to refine the interpretation and generate robust models at the sites of interest. Results suggest that the geothermal manifestations are distributed along a ~NE-SW structural zone, subparallel to the mountain front, and that their origin is tectonic. Two systems controlling the flow of thermal waters were identified: to the north, the El Museo Fault, where the structure acts as a conduit for hydrothermal fluid circulation; and another to the south, which involves a more complex tectonic deformation, including the development of faults, folds, and zones of weakness.
The Milna deposit (54°37’ S, 67°33’ W) is situated near Lago Fagnano, in the south-central part of Isla Grande de Tierra del Fuego. The presence in the deposit of laminations, dropstones, and convoluted structures, as well as a low organic matter content, suggest a lacustrine origin under relatively cold conditions, likely in direct contact with glacial ice. Radiocarbon ages obtained in this study define a depositional period from ca. 31.5 to ca. 11.5 ka cal BP, or even older. This new evidence points towards a glacial environment for the study area well prior to the Last Glacial Maximum; that is, older than ca. 25 ka cal BP.
This study enhances our understanding of Late Miocene-Pliocene climate in the Atacama Desert of the Antofagasta Region in northern Chile. Field relations, petrography, and geochemistry from two previously undescribed, fossil springs within the Calama Basin's Opache Formation demonstrate similarities between the springs, such as laminated crystalline carbonate, including freshwater aragonite botryoids, but differences in trace element and delta 13C values. One location contains stromatolites, oncolites, and sedimentary structures indicative of flowing water including ripple marks and microterracettes, whereas such features are generally absent at the other site. Both study areas are associated with fault systems that transferred groundwater from regional aquifers to the surface, bringing water rich in strontium, magnesium, iron, and manganese, to coprecipitate with CaCO3 as either aragonite or calcite. This correlation between springs and tectonics is becoming more widely recognized in other continental settings.
The southern margin of the Rio Chalia valley (also known as Sehuen or Shehuen), Province of Santa Cruz, Argentina, offers excellent outcrops of the Early-Middle Miocene Santa Cruz Formation (SCF). However, the information about the faunal content of these levels is scarce due to insufficient prospection. This contribution reports Litopterna specimens collected in 2018 and 2019 from six localities on the southern margin of the Rio Chalia valley, spanning a distance of similar to 30 km. Twenty-one specimens were collected, the majority belonging to Proterotheriidae (18), and three to Macraucheniidae. These specimens include mandibular fragments, maxillae, and postcranial elements. Among Proterotheriidae, Tetramerorhinus lucarius, Te. cingulatum, Thoatherium minusculum, Diadiaphorus majusculus, Anisolophus australis, and A. floweri were identified, while the Macraucheniidae record corresponds to Theosodon sp. The taxa recorded in the Rio Chalia area align with those recently reported for the SCF in other localities of the province, such as the Atlantic coast between Monte Leon and Rio Gallegos, Rio Santa Cruz, and Lago Posadas, as well as other sites across the extensive distribution of the SCF. These recent collections, with well-documented geographic and altitudinal reference, are valuable for verifying Ameghino's original descriptions and revisiting Santacrucian taxa.
