Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline

The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper-gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper-gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U-Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426-0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi <  + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00126-023-01172-0.

3D geodynamic-geomorphologic modelling of deformation and exhumation at curved plate boundaries: Implications for the southern Alaskan plate corner

Plate corners with extreme exhumation rates are important because they offer a perspective for understanding the interactions between tectonics and surface processes. The southern Alaskan margin with its curved convergent plate boundary and associated zones of localized uplift is a prime location to study active orogeny. Here, we present the results of fully-coupled thermo-mechanical (geodynamic) and geomorphologic numerical modelling, the design of which captures the key features of the studied area: subduction of oceanic lithosphere (Pacific plate) is adjacent to a pronounced asymmetric indenter dipping at a shallow angle (Yakutat microplate), which in turn is bounded to the east by a dextral strike-slip shear zone (Fairweather fault). The resulting first-order deformation/rock uplift patterns show strong similarities with observations. In particular, relatively young thermochronological ages are reproduced along the plate-bounding (Fairweather) transform fault and in the area of its transition to convergence (the St. Elias syntaxis). The focused exhumation of the Chugach Core also finds its equivalent in model predicted zones of high rock uplift rates in an isolated region above the indenter. From these results, we suggest that the general exhumation patterns observed in southern Alaska are controlled by mutually reinforcing effects of tectonic deformation and surface erosion processes.

Assessment of intrinsic aquifer vulnerability at continental scale through a critical application of the drastic framework: The case of South America

An assessment of the intrinsic aquifer vulnerability of South America is presented. The outcomes represent the potential sensitivity of natural aquifers to leaching of dissolved compounds from the land surface. The study, developed at continental scale but retaining regionally a high resolution, is based on a critical application of the DRASTIC method. The biggest challenge in performing such a study in South America was the scattered and irregular nature of environmental datasets. Accordingly, the most updated information on soil, land use, geology, hydrogeology, and climate at continental, national, and regional scale were selected from international and local databases. To avoid spatial discrepancy and inconsistency, data were integrated, harmonized, and accurately cross-checked, using local professional knowledge where information was missing. The method was applied in a GIS environment to allow spatial analysis of raw data along with the overlaying and rating of maps. The application of the DRASTIC method allows to classify South America into five vulnerability classes, from very low to very high, and shows an overall medium to low vulnerability at continental scale. The Amazon region, coastal aquifers, colluvial Andean valleys, and alluvial aquifers of main rivers were the areas classified as highly vulnerable. Moreover, countries with the largest areas with high aquifer vulnerability were those characterized by extended regions of rainforest. In addition, a single parameter sensitivity analysis showed depth to water table to be the most significant factor, while a cross-validation using existing vulnerability assessments and observed concentrations of compounds in groundwater confirmed the reliability of the proposed assessment, even at regional scale. Overall, although additional field surveys and detailed works at local level are needed to develop effective water management plans, the present DRASTIC map represents an essential common ground towards a more sustainable land-use and water management in the whole territory of South America.

Rapid surface uplift and crustal flow in the Central Andes (southern Peru) controlled by lithospheric drip dynamics

The high flux magmatism, crustal shortening/extension and plateau formation in Cordilleran orogenic systems have been explained by removal of lithosphere (lower crust and the sub-arc mantle lithosphere) that develops beneath the magmatic arc and hinterland regions. However, the primary role of this process driving surface uplift, and crustal deformation is not well understood. Here, reconciling geodynamic model predictions with lithospheric structure and paleoelevation estimates, we suggest that viscous drip-type lithospheric removal from beneath the Central (Peruvian) Andes can explain several tectonic features: (1) “double humped” shaped/axisymmetric topographic profile and rapid surface rise (up to 1.2 km in ~ 4.31 Myrs); (2) thicker crust associated with the lower surface elevation of the Altiplano plateau (Lake Titicaca region) (negative residual topography) and higher topography and thinner crust of Western and Eastern Cordilleras (positive residual topography); and (3) faster wave speed (colder)/sub-Moho anomaly underlying the Altiplano, surrounded by slower speed anomalies on both western arc-forearc areas and parts of the eastern Cordillera and Sub-Andes. Our results emphasize the important role of lithospheric drip and associated mantle dynamics in the transient evolution of Andean orogeny controlling surface uplift and crustal flow and thickening.

Southward expanding plate coupling due to variation in sediment subduction as a cause of Andean growth

Growth of the Andes has been attributed to Cenozoic subduction. Although climatic and tectonic processes have been proposed to be first-order mechanisms, their interaction and respective contributions remain largely unclear. Here, we apply three-dimensional, fully-dynamic subduction models to investigate the effect of trench-axial sediment transport and subduction on Andean growth, a mechanism that involves both climatic and tectonic processes. We find that the thickness of trench-fill sediments, a proxy of plate coupling (with less sediments causing stronger coupling), exerts an important influence on the pattern of crustal shortening along the Andes. The southward migrating Juan Fernandez Ridge acts as a barrier to the northward flowing trench sediments, thus expanding the zone of plate coupling southward through time. Consequently, the predicted history of Andean shortening is consistent with observations. Southward expanding crustal shortening matches the kinematic history of inferred compression. These results demonstrate the importance of climate-tectonic interaction on mountain building.

Rollback, scissor-like closure of the Mongol-Okhotsk Ocean and formation of an orocline: magmatic migration based on a large archive of age data

Tracing the closure of oceans with irregular margins and the formation of an orocline are crucial for understanding plate reconstruction and continental assembly. The eastern Central Asian Orogenic Belt, where the Mongol-Okhotsk orocline is situated, is one of the world's largest magmatic provinces. Using a large data set of U-Pb zircon ages, we updated the timing of many published igneous rocks, which allowed us to recognize tightly ‘folded’ linear Carboniferous-Jurassic magmatic belts that wrap around the Mongol-Okhotsk suture and their migrations both sutureward and suture-parallel. The new successive magmatic belts reveal a rollback, scissor-like (or zipper-like) closure of the Mongol-Okhotsk Ocean that was fundamentally controlled by coeval subduction rollback and rotation of the Siberian and Mongolian-Erguna blocks. This study also demonstrates the complex mechanisms and processes of the closure of an ocean with irregular margins and the formation of a consequent orocline.

Evolutionary trade-offs between male secondary sexual traits revealed by a phylogeny of the hyperdiverse tribe Eumaeini (Lepidoptera: Lycaenidae)

Male butterflies in the hyperdiverse tribe Eumaeini possess an unusually complex and diverse repertoire of secondary sexual characteristics involved in pheromone production and dissemination. Maintaining multiple sexually selected traits is likely to be metabolically costly, potentially resulting in trade-offs in the evolution of male signals. However, a phylogenetic framework to test hypotheses regarding the evolution and maintenance of male sexual traits in Eumaeini has been lacking. Here, we infer a comprehensive, time-calibrated phylogeny from 379 loci for 187 species representing 91% of the 87 described genera. Eumaeini is a monophyletic group that originated in the late Oligocene and underwent rapid radiation in the Neotropics. We examined specimens of 818 of the 1096 described species (75%) and found that secondary sexual traits are present in males of 91% of the surveyed species. Scent pads and scent patches on the wings and brush organs associated with the genitalia were probably present in the common ancestor of Eumaeini and are widespread throughout the tribe. Brush organs and scent pads are negatively correlated across the phylogeny, exhibiting a trade-off in which lineages with brush organs are unlikely to regain scent pads and vice versa. In contrast, scent patches seem to facilitate the evolution of scent pads, although they are readily lost once scent pads have evolved. Our results illustrate the complex interplay between natural and sexual selection in the origin and maintenance of multiple male secondary sexual characteristics and highlight the potential role of sexual selection spurring diversification in this lineage.

Old divergence and restricted gene flow between torrent duck (Merganetta armata) subspecies in the Central and Southern Andes

AIM

To investigate the structure and rate of gene flow among populations of habitat-specialized species to understand the ecological and evolutionary processes underpinning their population dynamics and historical demography, including speciation and extinction.

LOCATION

Peruvian and Argentine Andes.

TAXON

Two subspecies of torrent duck (Merganetta armata).

METHODS

We sampled 156 individuals in Peru (M. a. leucogenis; Chillón River, n = 57 and Pachachaca River, n = 49) and Argentina (M. a. armata; Arroyo Grande River, n = 33 and Malargüe River, n = 17), and sequenced the mitochondrial DNA (mtDNA) control region to conduct coarse and fine-scale demographic analyses of population structure. Additionally, to test for differences between subspecies, and across genetic markers with distinct inheritance patterns, a subset of individuals (Peru, n = 10 and Argentina, n = 9) was subjected to partial genome resequencing, obtaining 4,027 autosomal and 189 Z-linked double-digest restriction-associated DNA sequences.

RESULTS

Haplotype and nucleotide diversities were higher in Peru than Argentina across all markers. Peruvian and Argentine subspecies showed concordant species-level differences (ΦST mtDNA = 0.82; ΦST autosomal = 0.30; ΦST Z chromosome = 0.45), including no shared mtDNA haplotypes. Demographic parameters estimated for mtDNA using IM and IMa2 analyses, and for autosomal markers using ∂a∂i (isolation-with-migration model), supported an old divergence (mtDNA = 600,000 years before present (ybp), 95% HPD range = 1.2 Mya to 200,000 ybp; and autosomal ∂a∂i = 782,490 ybp), between the two subspecies, characteristic of deeply diverged lineages. The populations were well-differentiated in Argentina but moderately differentiated in Peru, with low unidirectional gene flow in each country.

MAIN CONCLUSIONS

We suggest that the South American Arid Diagonal was preexisting and remains a current phylogeographic barrier between the ranges of the two torrent duck subspecies, and the adult territoriality and breeding site fidelity to the rivers define their population structure.

Early exhumation of the Frontal Cordillera (Southern Central Andes) and implications for Andean mountain-building at ~33.5°S

The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The onset of uplift and deformation of the Frontal Cordillera basement culmination of the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. To solve for this, new (U-Th)/He ages on apatite and zircon from granitoids of the Frontal Cordillera at ~33.5°S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12–14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample. The age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera invalidate the classically proposed east-vergent models of the Andes at this latitude. Additionally, they provide further support to recent views on Andean mountain-building proposing that the Andes at ~33.5°S grew firstly over west-vergent basement structures.

Andean mountain building and magmatic arc migration driven by subduction-induced whole mantle flow

Subduction along the western margin of South America has been active since the Jurassic, but Andean orogeny started in the middle Cretaceous and was preceded by backarc extension in the Jurassic-Early Cretaceous. The timing and sequence of these events has remained unexplained. Here I present a four-dimensional buoyancy-driven whole-mantle subduction model implying that the ~200 Myr geological evolution can be attributed to sinking of a wide slab into a layered mantle, where upper-mantle wide-slab subduction causes backarc extension, while whole-mantle (upper+lower) wide-slab subduction drives Andean orogeny. The model reproduces the maximum shortening and crustal thickness observed in the Central Andes and their progressive northward and southward decrease. The subduction evolution coincides with a 29° decrease in slab dip angle, explaining ~200 km of Jurassic-present eastward migration of the Central Andean magmatic arc. Such arc migration negates proposed long-term subduction erosion and continental destruction, but is consistent with long-term crustal growth.

The Andean orogeny commenced in the Cretaceous, but was preceded by backarc extension starting in the Jurassic. Here, the author presents a 4D geodynamic subduction model reproducing the evolution of overriding plate deformation along with a progressive decrease in slab dip, explaining Jurassic-present arc migration.

Radiation of the Drosophila nannoptera species group in Mexico

The Drosophila nannoptera species group, a taxon of Mexican cactophilic flies, is an excellent model system to study the influence of abiotic and biotic factors on speciation, the genetic causes of ecological specialization and the evolution of unusual reproductive characters. However, the phylogenetic relationships in the nannoptera species group and its position within the virilis-repleta phylogeny have not been thoroughly investigated. Using a multilocus data set of gene coding regions of eight nuclear and three mitochondrial genes, we found that the four described nannoptera group species diverged rapidly, with very short internodes between divergence events. Phylogenetic analysis of repleta group lineages revealed that D. inca and D. canalinea are sister to all other repleta group species, whereas the annulimana species D. aracataca and D. pseudotalamancana are sister to the nannoptera and bromeliae species groups. Our divergence time estimates suggest that the nannoptera species group radiated following important geological events in Central America. Our results indicate that a single evolutionary transition to asymmetric genitalia and to unusual sperm storage may have occurred during evolution of the nannoptera group.

Reconstructing plate-motion changes in the presence of finite-rotations noise

Understanding lithospheric plate motions is of paramount importance to geodynamicists. Much effort is going into kinematic reconstructions featuring progressively finer temporal resolution. However, the challenge of precisely identifying ocean-floor magnetic lineations, and uncertainties in geomagnetic reversal timescales result in substantial finite-rotations noise. Unless some type of temporal smoothing is applied, the scenario arising at the native temporal resolution is puzzling, as plate motions vary erratically and significantly over short periods (<1 Myr). This undermines our ability to make geodynamic inferences, as the rates at which forces need to be built upon plates to explain these kinematics far exceed the most optimistic estimates. Here we show that the largest kinematic changes reconstructed across the Atlantic, Indian and South Pacific ridges arise from data noise. We overcome this limitation using a trans-dimensional hierarchical Bayesian framework. We find that plate-motion changes occur on timescales no shorter than a few million years, yielding simpler kinematic patterns and more plausible dynamics.