Coastal Cordillera Research Articles

The southern extension of the Eocene Andean orogeny: New sedimentary record of the foreland basin in the southern Central Andes at 32° S

AbstractThe Eocene compressional phase is well known to have contributed to the construction of the Andes orogen at latitudes north of 30° S, but its extension to the south has not been fully studied. Moreover, synorogenic deposits of Eocene age across the foreland are scarce. The Cenozoic Manantiales Basin records the unroofing sequence of the Andes at 32° S. This study focuses on the basal infill of the Manantiales Basin, informally called the Areniscas Chocolate, which has received less attention than the upper infill until now. Sedimentological, geochronological and provenance studies were carried out on this unit. Here, we present the first ages for the Areniscas Chocolate sequence, of ca. 35–39 Ma (maximum depositional age, MDA). This MDA is interpreted as close to its depositional age, which together with their stratigraphic characteristics, allow us both to separate it from the overlying Miocene Chinches Formation and to propose it as an independent lithostratigraphic unit called Río de los Patos Formation (nov. den.). The provenance analysis of the Río delos Patos Formation indicates a sediment input from western sources located in the Coastal Cordillera and Western Principal Cordillera. Facies associations suggest that the Río de los Patos Formation represents the distal synorogenic deposits during the construction of an Eocene relief to the west. Therefore, the Manantiales Basin started during the late Eocene as a distal foreland basin, indicating that Eocene compression reached latitudes as far south as 32° S. Our results shed light into the characterization of the earliest infill of the Manantiales Basin, as well as into the tectonic evolution of the basin.

Trace Elements in Magnetite and Origin of the Mariela Iron Oxide-Apatite Deposit, Southern Peru

To better understand the origin of the Andean iron oxide-apatite (IOA) deposits, we conducted a study on the geology and magnetite geochemistry of the Mariela IOA deposit in the Peruvian Iron Belt, central Andes. The Mariela deposit is hosted by gabbroic and dioritic intrusions. The major high-grade massive ores are primarily composed of magnetite and contain variable amounts of apatite and actinolite. Based on textural and geochemical characteristics, three different types of magnetite are recognized: Type I magnetite occurs in the massive magnetite ore, subclassified as inclusion-rich (I-a), inclusion-free (I-b), and mosaic (I-c); Type II magnetite is associated with abundant actinolite and titanite; and Type III magnetite is disseminated in altered host rocks. However, the magnetite geochemistry data for the Mariela deposit plot shows different genetic areas in [Ti + V] vs. [Al + Mn], Ti vs. V, and Fe vs. V/Ti discrimination diagrams, indicating a paradox of magmatic and hydrothermal origins. Our interpretation is as follows: Type I-a magnetite had an initial magmatic or high-temperature magmatic-hydrothermal origin, with slight modifications during transportation and subsequent hydrothermal precipitation (Types I-b and I-c). Type II magnetite is formed from hydrothermal fluid due to the presence of abundant actinolite. Disseminated magnetite (Type III) and veinlet-type magnetite formed after fluid replacement of the host rock. We stress that elemental discrimination diagrams should be combined with field studies and textural observations to provide a reasonable geological interpretation. A clear cooling trend is evident among the three subtypes of Type I magnetite (I-a, I-b, and I-c), as well as Type II and Type III magnetite, with average formative temperatures of 737 °C, 707 °C, 666 °C, 566 °C, and 493 °C, respectively. The microanalytical data on magnetite presented here support the magmatic-hydrothermal flotation model to explain the origin of IOA deposits in the Coastal Cordillera of Southern Peru.

Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)

Abstract. Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the weathering of granitic rocks in two 6 m deep weathering profiles in Mediterranean and humid climate zones along the Chilean Coastal Cordillera. We used geochemical and mineralogical methods such as (micro-) X-ray fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron microprobe (EMP) mapping to elucidate the transformations involved during weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing (WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III) oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the Mediterranean climate zone is significantly controlled by WIF. It generates a surface–subsurface connectivity that allows fluid infiltration to great depth and hence promotes a deep weathering zone. Moreover, the water supply to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense formation of secondary minerals in the upper section of the weathering profile. This intense formation arises from pronounced dissolution of primary minerals due to the high water infiltration (high precipitation rate) into the subsurface. The secondary minerals, in turn, impede the infiltration of fluids to great depth and thus mitigate the intensity of primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary mineral weathering in the upper regolith are controlled by positive and negative feedbacks between the formation of secondary minerals and the infiltration of fluids.

Microbial Community and Enzyme Activity of Forest Plantation, Natural Forests, and Agricultural Land in Chilean Coastal Cordillera Soils

Despite the global expansion of forest plantations in Chile, their effect on biology properties of soil has still been only scarcely studied. Land use change in the Chilean Coastal Cordillera (36° to 40° S) is mainly attributed to the conversion of native forest to agriculture and forest plantations (Eucalyptus globulus and Pinus radiata de Don). The aim of this paper was to evaluate the changes in microbial composition (PCR-DGGE) and enzyme activity after the substitution of a native forest (e.g., Nothofagus spp.) by fast-growing exotic species and cropping. The most important factors that influence the abundance and diversity of bacteria and the fungi community were the soil organic matter (SOM) content, phosphorous (P-Olsen), calcium (Ca), boron (B), and water-holding capacity. These variables can better predict the microbial community composition and its enzymatic activity in the surface Ah horizon. Land use change also affected chemical soil properties of biogeochemical cycles. However, to deeply understand the connection between chemical and physical soil factors and microbial community composition, more research is needed. On the other hand, the expansion of forest plantations in Chile should be subject to legislation aimed to protect the biological legacy as a strategy for forest productivity as well as the soil microbial biodiversity.

The story of a summit nucleus: hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera

Abstract. While landscapes are broadly sculpted by tectonics and climate, on a catchment scale, sediment size can regulate hillslope denudation rates and thereby influence the location of topographic highs and valleys. In this work, we used in situ 10Be cosmogenic radionuclide analysis to measure the denudation rates of bedrock, boulders, and soil in three granitic landscapes with different climates in Chile. We hypothesize that bedrock and boulders affect differential denudation by denuding more slowly than the surrounding soil; the null hypothesis is that no difference exists between soil and boulder or bedrock denudation rates. To evaluate denudation rates, we present a simple model that assesses differential denudation of boulders and the surrounding soil by evaluating boulder protrusion height against a two-stage erosion model and measured 10Be concentrations of boulder tops. We found that hillslope bedrock and boulders consistently denude more slowly than soil in two out of three of our field sites, which have a humid and a semi-arid climate: denudation rates range from ∼5 to 15 m Myr−1 for bedrock and boulders and from ∼8 to 20 m Myr−1 for soil. Furthermore, across a bedrock ridge at the humid site, denudation rates increase with increasing fracture density. At our lower-sloping field sites, boulders and bedrock appear to be similarly immobile based on similar 10Be concentrations. However, in the site with a Mediterranean climate, steeper slopes allow for higher denudation rates for both soil and boulders (∼40–140 m Myr−1), while the bedrock denudation rate remains low (∼22 m Myr−1). Our findings suggest that unfractured bedrock patches and large hillslope boulders affect landscape morphology by inducing differential denudation in lower-sloping landscapes. When occurring long enough, such differential denudation should lead to topographic highs and lows controlled by bedrock exposure and hillslope sediment size, which are both a function of fracture density. We further examined our field sites for fracture control on landscape morphology by comparing fracture, fault, and stream orientations, with the hypothesis that bedrock fracturing leaves bedrock more susceptible to denudation. Similar orientations of fractures, faults, and streams further support the idea that tectonically induced bedrock fracturing guides fluvial incision and accelerates denudation by reducing hillslope sediment size.

Adjacent Iron Oxide-Apatite and Iron Oxide Copper-Gold Mineralization in the Acarí District, Peru: The Magmatic to Hydrothermal Transition

Abstract Hierro Acarí and Cobrepampa are adjacent, Lower Cretaceous iron oxide-apatite (IOA) and iron oxide copper-gold (IOCG) districts, respectively, emplaced in the Coastal Cordillera of southern Peru, separated by a regional NW-trending fault. Mineralization in both districts is hosted mainly by Lower Cretaceous plutons of the Coastal batholith that intrude sedimentary and volcanic sequences of the inverted Cañete basin. The deposits form part of a region that includes the much larger Marcona and Pampa del Pongo IOA/skarn and the Mina Justa IOCG deposits, although Marcona is 60 m.y. older than the others. Hierro Acarí contains fracture-controlled steeply dipping, shallow-plunging N- and NE-trending magnetite veins in a 101.7 ± 1.4 Ma dioritic intrusive, and hematite mantos in a quartzite roof pendant. The magnetite ore shoots show colloform and microporous textures and grade downward within 150 m to actinolite and carbonates, which replace the magnetite. The Cobrepampa district comprises numerous NW-striking chalcopyrite-pyrite ± hematite veins and oxidized equivalents in a 99.7 ± 0.4 Ma monzonitic intrusive. Fluid inclusions indicate that the copper mineralization was deposited at 166° to 266°C from a fluid with low to moderate salinity of 3.8 to 16.5 wt % NaCl equiv. Colloform and microporous textures in magnetite are interpreted in comparable Chilean deposits to indicate colloidal transport of magnetite microlites, crystallized from a magma, which attach to bubbles in the magmatic-hydrothermal fluid in a process resembling froth flotation. Magnetite textures at Hierro Acarí suggest that magnetite was transported by this late magmatic process and deposited in the roof zone of the host intrusive. The hydrothermal fluid may have evolved to deposit hematite as mantos in the overlying sediments under more oxidizing conditions and may then have deposited the Cobrepampa copper and hematite veins in distal NW-trending faults. Recent studies into the geochemistry of Cañete basin volcanic rocks suggest that a mid-ocean ridge was subducted at the latitude of the Raúl-Condestable IOCG deposit, 350 km northwest of the Acarí district, at approximately 130 Ma, predating current Andean subduction below southern Peru. That situation could result in a configuration resembling the Gulf of California, where a series of fault-terminated pull-apart basins focus hydrothermal fluids at intervals along the main trough. IOCG and IOA/skarn mineralization in the Acarí district occurred during the early stages of Late Cretaceous basin inversion and intrusion of the Coastal batholith. However, the Marcona IOA deposit in the same districts was emplaced during an earlier, unrelated Jurassic to Early Cretaceous mineralizing episode.

Recently fixed carbon fuels microbial activity several meters below the soil surface

Abstract. The deep soil, >1 m, harbors a substantial share of the global microbial biomass. Currently, it is not known whether microbial activity several meters below the surface is fueled by recently fixed carbon or by old carbon that persisted in soil for several hundred years. Understanding the carbon source of microbial activity in deep soil is important to identify the drivers of biotic processes in the critical zone. Therefore, we explored carbon cycling in soils in three climate zones (arid, mediterranean, and humid) of the Coastal Cordillera of Chile down to a depth of 6 m, using carbon isotopes. Specifically, we determined the 13C : 12C ratio (δ13C) of soil and roots and the 14C : 12C ratio (Δ14C) of soil organic carbon and CO2–C respired by microorganisms. We found that the Δ14C of the respired CO2–C was significantly higher than that of the soil organic carbon in all soils. Further, we found that the δ13C of the soil organic carbon changed only in the upper decimeters (by less than 6 ‰). Our results show that microbial activity several meters below the soil surface is mostly fueled by recently fixed carbon that is on average much younger than the total soil organic carbon present in the respective soil depth increments, in all three climate zones. Further, our results indicate that most decomposition that leads to enrichment of 13C occurs in the upper decimeters of the soils, which is possibly due to stabilization of organic carbon in the deep soil. In conclusion, our study demonstrates that microbial processes in the deep soil several meters below the surface are closely tied to input of recently fixed carbon.

Paleozoic Gondwanan structure along the Maule river valley between Toconey and Constitución (35.4°S, Chilean Pacific Coast)

During the late Carboniferous–middle Permian, the basement of the Chilean Coastal Cordillera was deformed by the Gondwanan Orogeny, developed by the subduction of the Paleo-Pacific plate under Gondwana from the early Carboniferous. In the Maule sector (35.4°S), the following metamorphic rock units are distinguished: (i) Western Series, formed by metapelites, metabasites, metacherts, metaconglomerates, and marbles, mixed in a subduction channel, with probably ages between the Cambrian and the Carboniferous; (ii) Eastern Series, consisting of metasediments of a fore-arc basin, Carboniferous in age. In the Eastern Series two units are recognized in terms of lithology and deformation style: the Eastern Pelitic Series and the Eastern Sandy Series. Three Gondwanan deformation episodes have been distinguished, affecting both series unequaly. In the Western Series, D1 structures consist of a foliation (S1) developed under high-pressure and low-temperature conditions, while in the Eastern Series it comprise tight east-verging folds, with the associated S1 developed under low PT metamorphic conditions (greenschist facies). D2 structures are only present in the Western Series and Eastern Pelitic Series and represent the exhumation of the accretionary prism and its emplacement to the east over the fore-arc basin, with the development of east-verging recumbent folds and a thick shear-zone. Difficulties in the propagation of the deformation by the presence of the Coastal Batholith to the east led to the development of the D3 deformation event, only well developed in the Eastern Sandy Series, and represented by the Los Tablones Back-thrust and associated folds, as well as N–S open right folds formed under low PT metamorphic conditions.

Origin of Late Triassic Granitoids of the Coastal Cordillera of Southern Central Chile (34°–37°S): Multi‐Isotopic Evidence of Slab Tearing Effects on Pre‐Andean Magmagenesis

AbstractThe Coastal Cordillera of Central Chile (34°–37°S) holds a series of Late Triassic granitoids classically interpreted as early Andean subduction‐related magmatism based on their arc‐like geochemical signature. Here, we present geochemical, isotopic, and geochronological data and a tomotectonic analysis that challenge this idea indicating a local interruption of the normal subduction process likely associated with a slab‐tearing event. Our results suggest that the source of the magmas is related to melting of asthenospheric mantle and crustal rocks of a metasedimentary Paleozoic complex. We suggest that partial melting of these sources was triggered by a slab tear‐related asthenosphere upwelling producing high‐silica S/I‐ and S‐Type granites of the Constitución and Hualpén areas, and anorogenic A‐Type granitoids in Cobquecura area. Also, partial melting of a metasomatized asthenospheric mantle plus continental crust that experienced previous high‐temperature hydrothermal alteration would have generated high‐silica magmas with low δ18O, high Pb, CHUR‐like 87Sr/86Sr, and 143Nd/144Nd ratios that originated La Estrella Granite. Our results offer an alternative explanation for the existence of subtle magmatic arc‐like geochemical signatures in the study area and support a segmentation of the active margin during the Late Triassic. The widespread upper‐plate magmatic record of slab‐tearing, spanning the Coastal Cordillera of Central Chile to the intraplate Neuquén basin in Argentina, and the lower mantle record of a slab gap, detected in ours and previous tomotectonic analyses, make the Late Triassic slab‐tearing event in southwestern Pangea the most robustly constrained pre‐Cenozoic slab tear process so far.

Geological and hypogene mineralization characteristics of the hematite‐rich Pelusa IOCG prospect, Antofagasta Region, Northern Chile

AbstractThe Japón, Tailandia and China Sur deposits in the Pelusa mineralized area correspond to a hematite‐dominant IOCG clan within the coastal Cordillera in the Antofagasta Region, Northern Chile, and their mineralization occurs in basaltic to andesitic volcanic rocks of the Jurassic La Negra Formation. In addition to geological and mineralogical descriptions, bulk chemistry of the host rocks, microthermometric and sulfur isotropic analyses were performed to elucidate ore‐forming characteristics of the three orebodies. Chemical data of the host rocks indicate a subalkaline calc‐alkaline affinity with enrichment of the incompatible LILEs, displaying a typical pattern of igneous rocks formed in subduction zones as a general characteristic of the Jurassic‐Lower Cretaceous magmatic arc. Chalcopyrite is the predominant hypogene copper mineral, that variously occurs as veinlets, disseminations and breccia matrix. Fluid inclusions data indicate formation temperatures of the hypogene mineralization ranging mostly from 186 to 377°C, with a high‐salinity range of 30.1 and 45.0 wt% NaCl. The sulfur isotopic ratios (δ34SCDT) of pyrite and chalcopyrite present a narrow range of values from −10.6 to −7.5‰. The negative values in the limited range imply that sulfide minerals of the Pelusa area were formed under a relatively oxidizing and/or neutral pH condition, compared with other Chilean IOCG deposits.

Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).

Source Parameters of the Mw 5.7 Pica Crustal Earthquake in Northern Chile

Abstract On 10 September, 2008, an Mw 5.7 earthquake occurred under the Central Valley of northern Chile near the town of Pica at a depth of ∼33 km within the continental crust of the South America plate. We find this earthquake to be a high stress-drop, reverse-oblique event that generated unusually high ground accelerations of up to 0.67g. Overall, its observed ground motion intensities are considerably larger than those predicted by ground motion models, particularly at short periods. The source properties inferred through waveform modeling indicate reverse-oblique fault motion on a ∼75 km2 plane dipping to the northeast, which is corroborated by the located aftershock distribution. Stress-drop values of the mainshock and larger aftershocks were estimated through S-wave spectrum modeling, with values up to ∼250 MPa for the mainshock. The event occurred in a cold section of the continental crust under the Central Valley, and its fault kinematics and orientation are consistent with the dominant style of faulting and stress field under the neighboring Coastal Cordillera. Although our recurrence analysis shows that crustal events in the region occur at a lower rate than interplate and inslab events, crustal events of similar or higher magnitude than the Pica earthquake have occurred, on average, approximately once every three years in northern Chile, which could pose an important hazard to nearby populations or critical infrastructure.

Gold Provenance in Placers from Pureo Area, Southern Chile Coastal Cordillera, and Their Relationship with Paleozoic Metamorphic Rocks

Southern Chile placer gold deposits have been known and exploited since Spanish colonial times. Despite this, precise knowledge about their origin is scarce. This work aims to identify possible primary sources of the gold in the Pureo placers by studying the morphological and chemical characteristics of gold particles according to their spatial distribution. The former was determined by measurements and classification under a binocular microscope, allowing us to acquire a set of parameters related to the amount of transport that had affected the samples. The microchemical characteristics were determined by studying gold particles using optical microscopy, scanning electron microscopy (SEM) and electron microprobe (EMPA), where the native gold composition (in terms of major and minor elements) and the suite of mineral inclusions were obtained. The results regarding morphological characteristics suggest a low amount of transport from a primary source (<15 km). Microchemical data from gold particles indicate two compositional sub-populations, distinguished in both native gold composition (<15 Ag wt% with up to 4 Hg wt% and >15 Ag wt% with Hg bellow 1 wt%) and mineral inclusions (pyrite-galena rich and arsenopyrite rich, respectively), indicating two different primary gold sources. These results suggest a local origin of gold in the Coastal Cordillera, where the possible primary sources are associated with (i) massive sulfide deposits present in Paleozoic–Triassic metamorphic rocks and (ii) hydrothermal deposits associated with more recent Cenozoic intrusive activity. These conclusions have implications for the exploration of new placer deposits and of gold-bearing hypogene deposits (e.g., metamorphosed VMS deposits) in unexplored zones of southern Chile Coastal Cordillera.

Loa-Geo1: A field regional transect to unravel the structure of the western Central Andes

The structure of the Central Andes forearc of northern Chile has been an enigma for many years. Along this region, the preservation of a long pediplain constituted by gravels and unconsolidated sediments has hidden the structure of the region, making it difficult to understand its tectonic evolution. The traditional and regional-scale structural models are mainly supported by outcrop-scale and local observations. To unravel the structure of this Andes region, we carried out a regional-scale study based on local and regional observations, geological mapping, and the elaboration of local and regional balanced cross-sections, thereby illustrating the geometry and distribution of the first-order structural styles. We examined four specific sectors along a regional transect: the Loa and San Salvador rivers (parallel to each other), Sierra de Limón Verde, and Cerros de Tuina. Three structural styles were identified in the hybrid thick- and thin-skinned and doubly verging fold-and-thrust belt. These styles comprise partially inverted Permian to Jurassic normal faults, thick-skinned reverse faults affecting Paleozoic crystalline basement blocks, and shallow and thin-skinned fault-related folds. The shallow and thin-skinned fault-related folds usually result from the propagation of large, thick-skinned thrust ramps into stratigraphic successions. The restoration of a regionally balanced cross section constrained by field data and the previously interpreted two-dimensional (2-D) seismic profiles suggest that much of the crustal shortenings are accommodated by the reverse reactivation of Triassic and Jurassic normal faults bounding half-graben structures. It is also accommodated by blind thrust faults, whose geometry, kinematic, and depth are debatable. However, we have proposed a large, thick-skinned thrust ramp as the structure responsible for the greater shortening in the region, along which a significant thick- to thin-skinned transition occurs. The age of the synorogenic deposits identified here is unknown, specially those exposed at the Loa and San Salvador rives, however, the basal successions exposed at the Tuina sector reported Upper Cretaceous ages, thus suggesting that the orogeny in the forearc could have started during this time spam. Regional correlations with similar synorogenic deposits recognized in neighboring regions (e.g., Salar de Atacama Basin and Coastal Cordillera) also affirm an Upper Cretaceous–Paleocene age for the initiation of the crustal contraction in the region.

Latitudinal and Altitudinal Gradients of Riverine Landscapes in Andean Rivers

Exact knowledge of the physical structures of different river sections that govern their ecological structure and function is essential for the efficient conservation and management of riverine ecosystems. Eleven Andean river basins (Maipo, Rapel, Mataquito, Maule, Itata, Biobío, Toltén, Valdivia, Bueno and Puelo) comprise large scale latitudinal and altitudinal gradients and accommodate 71% of the Chilean population that strongly depend on their ecosystem services. Here, based on 16 hydrogeomorphic variables (on basin, valley and channel scales), we assessed the riverine landscapes (Functional Process Zones; FPZs) of these river basins using a top-down multivariate statistical approach. Two steep valley and downstream slope FPZs, three sinuous FPZs and two braided FPZs emerged in 8906 river sections. The proportion of the occurrence of FPZs was characterised by a clear latitudinal pattern which is strongly related to the proportions of each river basin within the large morphostructural units of Chile. As such, the proportion of each river basin within the Andes Cordillera, Central Valley and Coastal Cordillera is a strong driver of the fluvial geomorphology and, thus, of the FPZs’ arrangement in each river network. FPZ classification captured geomorphic diversity that coincided with the latitudinal and altitudinal gradients of Chilean Andean river basins strongly related to the hydrological characteristics of the assessed river basins and large scale spatial distribution of fish fauna endemism. As such, the identified large geomorphic units (FPZs) that are strongly tied up with hydrology and ecology hierarchies of riverine landscape provide robust operational tools that can be instrumental for river ecosystem monitoring and management at a basin scale.

Seismic source analysis of two anomalous earthquakes in Northern Chile

Northern Chile is a seismically very active region driven by subduction of the Nazca plate beneath the South American plate. Anomalous focal mechanisms were reported by international agencies for two recent earthquakes with magnitudes larger than Mw 6. The September 11, 2020, Mw 6.2 Loa River earthquake occurred in the forearc under the Coastal Cordillera and had a strike-slip mechanism, while most common regional seismicity is characterised by NS-oriented, trench-parallel, thrust mechanisms, consistent with the subduction geometry. The June 3, 2020, Mw 6.8 San Pedro de Atacama earthquake occurred at intermediate depth with a normal faulting mechanism. In this case, the anomalous behavior involves the NW-SE striking of the focal mechanism, which deviate from the typical NS orientation in the region. In this study we reconstruct the rupture geometry for these two earthquakes by means of moment tensor inversion, rupture directivity analysis and aftershock distribution. Seismological results are discussed in the frame of the spatial and temporal distribution of local seismicity, which are analyzed using clustering techniques. Our results show that both earthquakes reveal the activation of seismogenic structures in the continental crust or within the subducting oceanic crust close to the subduction interface. The Loa River earthquake occurred at the base of the continental crust, at the contact with the slab. This finding highlights the role of seismicity associated with crustal faults in the South American plate, which poses a secondary seismic hazard for the region, alongside the seismicity directly associated with the subduction zone. The San Pedro de Atacama earthquake, in turn, occurred within the subducting slab. Its anomalous mechanism may indicate a local anomaly in the geometry of the subducting slab.

Geology of the Altamira and Las Luces deposits, Coastal Cordillera, northern Chile: implications for the origin of stratabound Cu–(Ag) deposits

Geology of the Altamira and Las Luces deposits, Coastal Cordillera, northern Chile: implications for the origin of stratabound Cu–(Ag) deposits

Vegetation strategies for nitrogen and potassium acquisition along a climate and vegetation gradient: From semi-desert to temperate rainforest

Nutrient acquisition strategies of plants regulate water flow and mass transport within ecosystems, shaping earth surface processes. Understanding plant strategies under current conditions is important to assess and predict responses of natural ecosystems to future climate and environmental changes. Nitrogen (N) and potassium (K) (re-)utilization from topsoil and their acquisition from subsoil and saprolite were evaluated in a continental transect, encompassing three study sites – an arid shrubland, a mediterranean woodland, and a temperate rainforest – on similar granitoid parent material in the Chilean Coastal Cordillera. The short-term (<1 year) plant N and K acquisition was traced with 15N and the K analogs rubidium and cesium. To do so, the tracers were either injected into topsoil, subsoil, or saprolite, in the immediate vicinity of eight individual plants per study site and injection depth. The long-term (>decades) K uplift by plants was investigated by the vertical distribution of exchangeable K+ and Na+. Recoveries of 15N and K analogs by arid shrubland plants were similar from topsoil, subsoil, and saprolite. Mediterranean woodland shrubs recovered the tracers primarily from topsoil (i.e., 89 % of recovered 15N and 84 % of recovered K analogs). Forest plants recovered the tracers from topsoil (15N = 49 %, K analogs = 57 %) and partially from greater depth: 38 % of recovered 15N and 43 % of recovered K analogs were acquired from subsoil and saprolite, respectively. Low nutrient accessibility in the topsoil (e.g., because of frequent droughts) drives shrubland plants to expand their N and K uptake to deeper and moister soil and saprolite. Woodland and forest plants dominantly recycled nutrients from topsoil. In the forest, this strategy was complemented by short-term uplift of N and K from depth. The vertical distribution of exchangeable K indicated long-term uplift of K by roots in all three sites. This highlighted that long-term K uplift from depth complements the nutrient budget across the continental transect.

Diversity of Root-Associated Fungi of the Terrestrial Orchids Gavilea lutea and Chloraea collicensis in a Temperate Forest Soil of South-Central Chile.

The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in temperate forests has scarcely been examined. This study aimed to analyze the diversity of mycorrhizal and rhizosphere-associated fungal communities in the terrestrial orchids Gavilea lutea and Chloraea collicensis growing in high-orchid-population-density areas in the piedmont of the Andes Cordillera with native forest (Nothofagus-Araucaria) and Coastal Cordillera with an exotic plantation (Pinus-Eucalyptus) in south-central Chile. We focused on rhizosphere-inhabiting and peloton-associated OMF in a native forest (Andes Cordillera) and a mixed forest (Coastal Cordillera). The native terrestrial orchids G. lutea and C. collicensis were localized, mycorrhizal root segments were taken to isolate peloton-associated OMF, and rhizosphere soil was taken to perform the metabarcoding approach. The results revealed that Basidiomycota and Ascomycota were the main rhizosphere-inhabiting fungal phyla, showing significant differences in the composition of fungal communities in both sites. Sebacina was the most-abundant OMF genera in the rhizosphere of G. lutea growing in the native forest soil. In contrast, Thanatephorus was the most abundant mycorrhizal taxa growing in the rhizosphere of orchids from the Coastal Cordillera. Besides, other OMF genera such as Inocybe, Tomentella, and Mycena were detected. The diversity of OMF in pelotons differed, being mainly related to Ceratobasidium sp. and Tulasnella sp. These results provide evidence of differences in OMF from pelotons and the rhizosphere soil in G. lutea growing in the Andes Cordillera and a selection of microbial communities in the rhizosphere of C. collicensis in the Coastal Cordillera. This raises questions about the efficiency of propagation strategies based only on mycorrhizal fungi obtained by culture-dependent methods, especially in orchids that depend on non-culturable taxa for seed germination and plantlet development.

Simulation of cooling in a magma chamber: Implications for geothermal fields of southern Peru

Numerical simulations of a geothermal reservoir often assume that the primary heat source is a magmatic system; however, heat from the host rock and the coupled complex transport phenomena between magma chamber and host rock also need to be considered. This research numerically simulated the cooling history of an enclosed magma chamber and the thermal effects on the host rock around it from which geothermal energy could be extracted. Modeling of the magma body included natural convection, the effect of latent heat of phase change when the crystals are being formed between the liquidus temperature and the solidus temperature, and heat conduction when the temperature is below the solidus temperature. This study takes as an example a geothermal reservoir in southern Peru (Western Cordillera) whose heat source is a rhyolitic magma chamber like those that gave rise to the intrusive rocks of the Peru coastal cordillera. This analysis varies the chamber shape and uses three solidification temperature ranges for convection and conduction models above the rock formation temperature (solidus) to study the implications for heating of the host rock. This is the first study of its kind in this area. The center of an average-sized magma chambers takes approximately 500 ka to cool from 800 °C to 300 °C. Simulated cooling times between intrusion and solidus temperatures decreased 3 ka when convection was modeled along with conduction cooling. Cooling times decreased by up to 6 ka when the solidification temperature range was increased. The host rock temperature pattern depends strongly on the stage at which the magma chamber is modeled to begin cooling. The temperatures results near the surface of the host rock obtained in this work match well with measurements at hot springs founded in several places in the Western Cordillera. Application of the methodology proposed in this study can reduce uncertainties in planning geothermal energy extraction wells. The accuracy of the numerical model described here could be improved by including more ground data from exploration wells, e.g., soil stratigraphy and temperatures variation with depth.