Andean Southern Volcanic Zone Research Articles

Origin of the compositionally zoned Paso Puyehue Tephra, Antillanca Volcanic Complex, Chile

The origin of gaps or zoning in the composition of erupted products is critical to understanding how sub-volcanic reservoirs operate. We characterize the compositionally zoned magma that produced the 2053 ± 50 cal. yr BP Paso Puyehue Tephra from the Antillanca Volcanic Complex in the Andean Southern Volcanic Zone (SVZ). The 3.7 km3 Paso Puyehue Tephra is zoned from dacite (69 wt% SiO2) lapilli and ash comprising the lowermost 80% of the deposit that abruptly transitions upward into basaltic andesite scoria (54 wt% SiO2) making up the remaining ∼20%. Variations in whole-rock, matrix glass, and mineral compositions through the deposit allow us to estimate pre-eruptive magma storage conditions and to develop a model of how this magma body was generated.Our findings suggest that amphibole-bearing basaltic andesitic magma stored at ∼8.0 ± 1.3 km depth fractionally crystallized and cooled from 1048 ± 1.1 to 811 ± 28.6 °C under highly oxidizing conditions to produce silicic a melt that upon extraction and rise, pooled at ∼6.4 ± 1.2 km depth at temperatures as low as 810 °C before eruption. MELTS models suggest that crystallization of a basaltic andesite parent magma with 4 wt% dissolved H2O can produce the dacite under conditions predicted by mineral thermobarometers with phase compositions comparable to those measured in minerals. Pervasive normal zoning at the rims of plagioclase crystals—most pronounced at the transition between dacite and basaltic andesite, and compatible vs. incompatible trace element concentrations, suggest that magma mixing was limited and likely occurred at the interface between the dacitic and basaltic andesitic magmas during ascent within the conduit upon eruption. Compositionally bimodal tephras are increasingly recognized throughout the SVZ with several interpreted to reflect basaltic recharge and mixing into extant rhyolitic reservoirs. In further contrast to other SVZ rhyolitic products, e.g., from the nearby Cordón Callue and Mocho Choshuenco volcanoes, the Paso Puyehue magma was highly oxidized. This may reflect enhanced delivery of H2O from the subducting plate into the mantle wedge, which in turn may facilitate efficient extraction and separation of buoyant, low-viscosity rhyolitic melt from crystal-rich basaltic andesitic parent magmas and the co-eruption of both end members.

Decoding the state of stress and fluid pathways along the Andean Southern Volcanic Zone

Decoding means decrypting a hidden message. Here, the encrypted messages are the state of stress, fluid pathways, and volcano tectonic processes occurring in volcanoes of the Andean Southern Volcanic Zone (SVZ). To decode these messages, we use earthquake focal mechanisms, fault slip data, and a Monte Carlo simulation that predicts potential pathways for magmatic and hydrothermal fluids. From this analysis, we propose that SVZ volcanoes have three end-member stress patterns: (i) Stress-A, a strike-slip regime coupled with the regional far-field tectonic stress; (ii) Stress-B, an extensional regime that may be promoted by volcanic edifice loading and upward pressure due to magma inflation occurring within the upper brittle-crust; and (iii) Stress-C, a local and transient fluid-driven stress rotated ~90 degrees from Stress-A. Notoriously, Stress-C pattern was observed in most volcanoes with historical eruptions. We propose that volcanoes presenting Stress-B are attractive geothermal targets, while Stress-C could be used as a predicting signal for impending eruptions.

Fluid flow migration, rock stress and deformation due to a crustal fault slip in a geothermal system: A poro-elasto-plastic perspective

Geothermal systems are commonly genetically and spatially associated with volcanic complexes, which in turn, are located nearby crustal fault systems. Faults can alter fluid flow in their surroundings, potentially acting as barriers or conduits for fluids, depending on their architecture and slip-rate. However, this fundamental control on fluid migration is still poorly constrained. Most previous modeling efforts on volcanic and hydrothermal processes consider either only fluid flow in their formulations, or only a mechanical approach, and seldom a full, monolithic coupling between both. In this work, we present a poro-elasto-plastic Finite Element Method (FEM) to address the first-order, time-dependent control that a strike-slip crustal fault exerts on a nearby geothermal reservoir. For the model setting, we selected the Planchón-Peteroa geothermal system in the Southern Andes Volcanic Zone (SAVZ), for which the geometry and kinematics of a potentially seismogenic fault and fluid reservoir is constrained from previous geological and geophysical studies. We assess the emergence and diffusion of fluid pressure domains due to fault slip, as well as the development of tensile/dilational and compressive/contractional domains in the fault' surroundings. Mean stress and volumetric strain magnitudes in these domains range between ±1 [MPa] and ±10−4 [-], respectively. Our results show the appearance of negative and positive fluid pressure domains in these dilational and contractional regions, respectively. We also investigate the spatial and temporal evolution of such domains resulting from changes in fault permeability and shear modulus, fluid viscosity, and rock rheology. These variations in fluid pressure alter the trajectory of the reservoir fluids, increasing migration to the eastern half of the fault, reaching a maximum fluid flux of 8 to 70 times the stationary flux. Pressure-driven fluid diffusion over time causes fluid flow to return to the stationary state between weeks to months after fault slip. These results suggest that the mechanism that exerts a first-order control is similar to a suction pump, whose duration heavily depends on fault permeability and fluid viscosity. We also show how a von Mises plasticity criterion locally enhances fluid flow. The transient process analyzed in this work highlights the importance of addressing the solid-fluid coupling in numerical models for volcano-tectonic studies.

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).

Intercalibration of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile and WiscAr 40Ar/39Ar laboratories for Quaternary dating

Accurate and precise dating of Quaternary lavas and pyroclastic flow or fall deposits is essential for understanding the evolution of active volcanoes and providing context for future eruptions and hazard assessment. The 40 Ar/ 39 Ar method is commonly employed to date these volcanic materials, however, dating young (<150 ka) K 2 O-poor materials can be challenging owing to low radiogenic 40 Ar* contents that can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration exercise involving the University of Wisconsin-Madison WiscAr Laboratory and the 40 Ar/ 39 Ar Laboratory of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile was conducted on a common set of samples with the aim of refining our methods and optimizing precision and accuracy of age determinations. Groundmass and plagioclase samples were analyzed on a 5-collector Noblesse ion counting mass spectrometer in the WiscAr lab, whereas measurements in the SERNAGEOMIN lab were performed using an ARGUS VI spectrometer equipped with faraday detectors and one compact discrete dynode electron multiplier. Samples for the intercalibration were collected jointly from three Andean Southern Volcanic Zone volcanoes to evaluate the capability of each laboratory to date different materials. Samples from lava flows with 1.0–3.2 wt % K 2 O from Planchon-Peteroa volcanic complex and with <1.0 wt % K 2 O from Calbuco Volcano that are the focus of ongoing geological studies were measured in both laboratories. Single crystals of plagioclase (0.6–1.0 wt% K 2 O) were measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments were conducted including co-irradiation of samples at Oregon State University, as well as irradiations using the CCHEN reactor in Chile to investigate differences in neutron fluence parameters. As a result, SERNAGEOMIN has modified long-used protocols for the CCHEN reactor so that Quaternary samples may be irradiated for periods of time most appropriate for their age. Although less precise than plateau ages, the isochron ages generated in the two laboratories agree at 2σ for each sample. Six of six co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K 2 O lavas from Calbuco proved more challenging with only three out of five plateau ages in agreement between labs. SERNAGEOMIN blanks were higher and more variable in Calbuco experiments, thus, differences in the variability of the measured 36 Ar blanks between the two laboratories may explain the discrepancy in plateau ages. Analysis of single plagioclase crystals from the Diamante Ignimbrite show excellent agreement between labs for both weighted mean apparent ages and isochron ages. We favor an isochron age for the ignimbrite of 132.4 ± 2.2 ka, however, discrepancies in results between samples from three different outcrops present an interesting geochronologic problem that warrants further study. Overall, the consistency of the results between labs is promising. These new precise age determinations significantly improve our understanding of the temporal evolution of these active volcanoes. • Broad agreement of ages from tephras and lavas measured in two labs. • 40 Ar/ 39 Ar dating of low K 2 O lavas from Calbuco is challenging due to limited 40 Ar*. • We favor an isochron age of ∼132 ± 2 ka ka for the 270 km 3 Diamante Ignimbrite. • Care in sample preparation and irradiation, and control of blanks are essential.

Sulfur and chlorine budgets control the ore fertility of arc magmas

Continental arc magmas supply the ore-forming element budget of most globally important porphyry-type ore deposits. However, the processes enabling certain arc segments to preferentially generate giant porphyry deposits remain highly debated. Here we evaluate the large-scale covariation of key ore-forming constituents in this setting by studying silicate melt inclusions in volcanic rocks from a fertile-to-barren segment of the Andean Southern Volcanic Zone (33–40 °S). We show that the north-to-south, fertile-to-barren gradient is characterized by a northward increase in S and Cl concentrations and a simultaneous decrease in Cu. Consequently, we suggest that the concentration of S and Cl rather than the concentration of ore metals regulates magmatic-hydrothermal ore fertility, and that the loss of volatiles prior to arrival in the upper crust impacts ore-forming potential more than magmatic sulfide saturation-related ore metal scavenging.

Composition, age and origin of Pleistocene turbidite deposits at ODP site 1232, Nazca plate: Implications for volcanism and climate change in central south Chile

A 371 m sequence of Pleistocene age turbidites was recovered at ODP (Ocean Drilling Program) Leg 202, Site 1232 at 39°53.45'on the Nazca Plate, seaward of the Peru-Chile Trench. Coarsest sediments within the turbidites (<300 μm) contain large amounts of fresh, angular volcanic glass shards. We investigated the variation in proportions of coarsest (>149 μm) glass, minerals and polymineralic grains, and microfossils downcore with the goal of discerning temporal changes and their underlying causes. Between 214 and 160 mbsf, the dominant coarse particle type changes from rounded, clay-rich granules, interpreted as altered pumice, upward to fresh, angular volcanic glass. This change correlates with an upward increase in turbidite thickness noted by Leg 202 scientists. Major and trace element analyses of individual glass shards show a geochemical signature consistent with a source within the adjacent Andean Southern Volcanic Zone. New calcareous nannofossil biostratigraphy brackets the sediment ages of the upper 288 m of the section to between 12.47 kyr and 275 kyr, which is consistent with published ages for the upper 97 m of the section and estimated maximum ages.Based on our dating of the section, the observed change in dominant coarse sediment type occurs between 225 and 185 kyr and correlates with a climatic transition from interglacial to glacial conditions. A plausible connection with climate is that the altered pumice grains formed by chemical weathering and fluvial transport under warm surface conditions, whereas the glass shards were transported by aeolian and glacial processes, leading to a lack of weathering and an angular texture. However, as a climate-related temporal change, this scenario conflicts with the absence of a similar change in sediment type during the subsequent Valdivian and Llanquihue interglacial-glacial transition starting at about 125kyr. Therefore, our conclusion is that, although the two coarse sediment types formed and were transported under different climate conditions, the temporal change is related to a shift in sediment source, supply or transportation pathway during the 185–225 kyr interval. A related possibility is that the onset of angular glass involvement in turbidity flows, during a glacial stage, marks the eruption of a new volcanic source or sources. Resolving these possibilities could be addressed with more detailed geochemical analysis.

Insights for crystal mush storage utilizing mafic enclaves from the 2011–12 Cordón Caulle eruption

Two distinct types of rare crystal-rich mafic enclaves have been identified in the rhyolite lava flow from the 2011–12 Cordón Caulle eruption (Southern Andean Volcanic Zone, SVZ). The majority of mafic enclaves are coarsely crystalline with interlocking olivine-clinopyroxene-plagioclase textures and irregular shaped vesicles filling the crystal framework. These enclaves are interpreted as pieces of crystal-rich magma mush underlying a crystal-poor rhyolitic magma body that has fed recent silicic eruptions at Cordón Caulle. A second type of porphyritic enclaves, with restricted mineral chemistry and spherical vesicles, represents small-volume injections into the rhyolite magma. Both types of enclaves are basaltic end-members (up to 9.3 wt% MgO and 50–53 wt% SiO2) in comparison to enclaves erupted globally. The Cordón Caulle enclaves also have one of the largest compositional gaps on record between the basaltic enclaves and the rhyolite host at 17 wt% SiO2. Interstitial melt in the coarsely-crystalline enclaves is compositionally identical to their rhyolitic host, suggesting that the crystal-poor rhyolite magma was derived directly from the underlying basaltic magma mush through efficient melt extraction. We suggest the 2011–12 rhyolitic eruption was generated from a primitive basaltic crystal-rich mush that short-circuited the typical full range of magmatic differentiation in a single step.

Volcanic unrest at Nevados de Chillán (Southern Andean Volcanic Zone) from January 2019 to November 2020, imaged by DInSAR

The volcanic complex of Nevados de Chillán, located in the Southern Volcanic Zone (SVZ) of the Andes, has been active for the past 640 ± 20 ka. Its volcanic activity includes dome forming eruptions, explosive events, and lava flows. The most recent eruption cycle started in January 2016. We employ DInSAR time-series from Sentinel-1 data to investigate the unrest episode from January 2019 to November 2020. Two distinct periods of unrest are recognized in the time series. The first period (from January to October 2019) coincides with explosive events, dome growth inside the active crater, and a decrease in seismic activity but does not present a significant deformation. The second period (October 2019 to November 2020) is characterized by a displacement towards the sensor's line-of-sight of 100–120 mm. The observed surface deformation is compatible with an inflation source approximately 1.5 km south-southwest of the present active vent, at 5.5 ± 0.5 km depth from the surface, and with a volume change of 0.044 ± 0.014 km3. The most likely explanation for the observed inflation of Nevados de Chillan is the intrusion of magma in a reservoir feeding the current eruption cycle.

Mantle Xenoliths from Huanul Volcano (Central-West Argentina): A Poorly Depleted Mantle Source under Southern Payenia

Huanul is a shield volcano with several lava flows hosting mantle xenoliths erupted during the Pleistocene (0.84 ± 0.05 Ma). It is located in the southern part of the Payenia Volcanic Province, which is among the largest Neogene-Quaternary volcanic provinces of South America. The volcanism here has been ascribed as the northernmost expression of the back-arc volcanism of the Andean Southern Volcanic Zone. We present the first petrographic and mineral chemistry study of mantle xenoliths collected from Huanul lavas with the aim of reconstructing directly the mantle source of the Payenia Volcanic Province. Xenoliths are commonly small (&lt;5 cm in radius) but scarcely crossed by basaltic veins. All xenoliths have a fertile lherzolitic modal composition and are equilibrated in the spinel-facies. Most of them exhibit an almost primitive-mantle geochemical affinity, characterized by slightly depleted clinopyroxene REE patterns reproducible by partial melting degrees between 0 and 4% of a PM source. Geothermobarometric P-T estimates of clinopyroxene-orthopyroxene couples form a linear trend between 10 and 24 kbar with constant increase of T from 814 to 1170 °C along a 50–60 mW/m2 geotherm. Evidences of interaction with the host basalts occur as spongy textures in clinopyroxene and reacted spinel, which tend to became more restitic in composition and show chromatographic or complete overprinting of the trace element compositions. The presence of plagioclase and calculated P-T values constrain this melt/rock reaction process between 6 and 14 kbar, during magma ascent, and fit the mantle adiabat model. Calculated melts in equilibrium with the primary clinopyroxenes do not fit the composition of the host basalt and, together with the geothermobarometric estimations, point to an asthenospheric mantle source for the magmatism in southern Payenia. The PM geochemical affinity of the xenoliths of Huanul is an extremely rare finding in the South America lithospheric mantle, which is commonly extensively refertilized by subduction-derived melts.

Unravelling the hydrothermal system of Laguna del Maule restless volcanic field, in the Andean Southern Volcanic Zone (36° 10′S)

Laguna del Maule Volcanic Field (LdMVF, 36° 10′ S), located in the Southern Andean Volcanic Zone, concentrates an extraordinary postglacial (<25 ka) rhyolitic volcanism and records one of the highest world rates of surface uplift (>20 cm/yr since 2007). Different papers have been published concerning magmatism and geophysical survey but no research has been performed concerning the associated hydrothermal system present in the area. In this work, we analyze both past and present-day hydrothermal manifestations present in the LdMVF. Seven hydrothermal fossil alteration zones were described, with different mineral assemblages suggesting paleo-fluids temperatures ranging between 150 °C - 250 °C for hydrothermal alteration. Additionally, five active fluid discharge zones were also recognized. Based on their spatial distribution, and new data on surficial water/gas geochemistry including stable isotopic composition (δ18O-δD-H2O and δ13C-CO2), the Laguna del Maule Hydrothermal System is here defined. A conceptual two-layer model is proposed, which consists in a deep reservoir with estimated temperature of 178 °C -195 °C and over this a shallow aquifer system with temperature of 116 °C -143 °C. Water composition from outflow zones revealed fluid-rock interaction, due to carbonates and sulphates dissolution of Mesozoic sedimentary units. The upflow fluid discharges, which are spatially coincident with the ongoing surface uplift, provides geochemical evidence of deep origin, and consequently the arrival of fluids capable of increasing pressure in the upper crust. The spatial distribution of both past and present hydrothermal manifestations suggests a strong structural control promoting fluid circulation due to enhanced permeability, throughout the entire lifetime of the volcanic field.

The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

AbstractThe Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ18Oolivine values from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ18O values, providing a different explanation for the low‐δ18O signatures observed in continental arc settings.

Mondaca Volcano lahar of December 3, 1762, Maule Region (35°28’S): one of the largest volcanic disasters in Chilean history

The Mondaca Volcano comprises a thick rhyolitic lava-field and a dome of similar composition, located near the Lontué River Valley headwaters in the northern part of the Southern Andes Volcanic Zone. It reaches a total volume of ~0.85 km3, and it is formed by 4 subunits, named Mondaca 1, 2, 3 and 4, which correspond to successive rhyolitic blocky lava flows, emitted from a rounded dome structure. They present well-preserved flow structures and, in the surroundings, restricted to the south and east of the dome, pyroclastic fall, as well as block and ash deposits are also exhibited. Downstream, along the Lontué River, a laharic deposit is recognized. The lahar was produced after the collapse of an ephemeral ~0.44 km3 lake generated after the river obstruction by viscous lavas, during the 1762 first eruptive phase. Proximal lahar facies are well exposed between 5 and 30 km from their source. The profuse agricultural activity has completely obliterated the lahar’s medium facies deposits along the Central Depression, but are well identified at the mouth of the Mataquito River, 180 km downstream, as a beige-coloured layer, interbedded within dark coastal beach-sands. The identification of overflows and super-elevation deposits formed during the debris flow emplacement along the Lontué River valley, allows to determine a high flow mobility, with estimated velocities that locally reached up to 114 km/h. Petrographic characteristics in addition to chemical composition of lavas from the volcano, pyroclasts and juvenile blocks of the laharic deposit, indicate that all they correspond to high K calcoalkaline rhyolites with subalkaline affinity. These backgrounds, together with the geographical continuity between the lavas and debris deposits along the Lontué and Mataquito rivers, verify facies correlation and common origin as the result of the 1762 Mondaca Volcano eruption complex evolution. Although it was a mainly effusive eruption that could not be observed from Curicó, the collateral consequences would have been catastrophic over a vast area to the south of that city, and evidences one of the largest volcanic disasters in Chilean history. Probably because of the low density polulation at that time, the consequences could have been minor.

Volatile Content Implications of Increasing Explosivity of the Strombolian Eruptive Style along the Fracture Opening on the NE Villarrica Flank: Minor Eruptive Centers in the Los Nevados Group 2

Potential flank eruptions at the presently active Villarrica, Southern Andes Volcanic Zone (33.3–46 °S) require the drawing of a comprehensive scenario of eruptive style dynamics, which partially depends on the degassing process. The case we consider in this study is from the Los Nevados Subgroup 2 (LNG2) and constitutes post-glacial minor eruptive centers (MECs) of basaltic–andesitic and basaltic composition, associated with the northeastern Villarrica flank. Petrological studies of the melt inclusions volatile content in olivine determined the pre-eruptive conditions of the shallow magma feeding system (&lt;249 Mpa saturation pressure, 927–1201 °C). The volatile saturation model on “pressure-dependent” volatile species, measured by Fourier Transform Infrared Microspectrometry (FTIR) (H2O of 0.4–3.0 wt.% and CO2 of 114–1586 ppm) and electron microprobe (EMP), revealed that fast cooling pyroclasts like vesicular scoria preserve a ~1.5 times larger amount of CO2, S, Cl, and volatile species contained in melt inclusions from primitive olivine (Fo76–86). Evidence from geological mapping and drone surveys demonstrated the eruption chronology and spatial changes in eruption style from all the local vents along a N45° corridor. The mechanism by which LNG2 is degassed plays a critical role in increasing the explosivity uphill on the Villarrica flank from volcanic vents in the NE sector (&lt;9 km minimum saturation depth) to the SW sector (&lt;8.1 km), where many crystalline ballistic bombs were expulsed, rather than vesicular and spatter scoria.

The interplay of a fault zone and a volcanic reservoir from 3D elasto-plastic models: Rheological conditions for mutual trigger based on a field case from the Andean Southern Volcanic Zone

The Southern Andes margin hosts active and fossil volcanic, geothermal, and mineralized systems documenting intense geofluid migration through the crust. Fluid flow is also spatially associated with crustal faults that accommodate the bulk deformation arising from oblique plate convergence. Although recognized, the precise local mechanical interaction between faults and crustal reservoirs is yet to be better understood. Here we present 3D numerical models of a magmatic reservoir and a fault zone set about 4 km apart, inspired by the Tatara-San Pedro volcanic complex in the Southern Volcanic Zone (~36°S), which displays a geothermal field and a margin-parallel dextral active fault zone constrained by published magnetotelluric profiles and crustal seismicity respectively. We investigate elasto-plastic deformation and stress patterns in the intermediate bedrock space between the reservoir and the fault zone and test how shear stress, volumetric strain, and plastic strain develop. We also test the potential of enabling brittle failure of their counterpart by imposing either (1) a strike-slip displacement along the fault zone, or (2) a magmatic overpressure at the cavity walls. Parametric tests of Young's modulus and frictional strength provide the conditions for macro-scale brittle failure and show the development of diffuse domains of dilational strain of the order of 10−5 –10−3 in the intervening bedrock. This dilation is a proxy to the opening of voids or volumetric cracking in the bedrock, which tends to increase porosity and permeability allowing over-pressurized geofluids to migrate within these domains. Our results show that a minimum of 60 m of fault displacement is required to trigger brittle failure of an upper crustal cavity if the bedrock is stiff, whereas, for a more compliant bedrock, more than 100 m of localized slip motion is required. This implies that it is rather the accumulated effect of repeated crustal fault displacement that potentially favors fluid pathways upwards, rather than a single seismic event. On the other hand, a minimum of 7.5 MPa of fluid overpressure is required for a mid-crustal cavity (15 km depth) to trigger brittle failure of the fault zone. This threshold overpressure increases up to 50 MPa when the cavity is shallower (6 km depth). Our results show that in general, shallow reservoirs must be very close to fault zones (less than 1–2 km apart) to reactivate them. The models show that localized strike-slip tectonics and magma intrusions build a dilational stress field at the scale of several kilometers, that promotes fluid pathways to the surface. Further combining this interaction with the regional transpressional stress field may explain observations of transient fluid pathways on seemingly independent timescales along the Andean margin.

The petrology of a hazardous volcano: Calbuco (Central Southern Volcanic Zone, Chile)

The recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding touristic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al2O3-rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H2O content of the basalt (3–3.5 wt. % H2O at 50 wt. % SiO2) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210–460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.

Monitoring and forecasting hazards from a slow growing lava dome using aerial imagery, tri-stereo Pleiades-1A/B imagery and PDC numerical simulation

In December of 2017, a lava dome emerged at the Nevados de Chillan volcanic complex in the southern Andean volcanic zone, Chile, at the base of a summit crater excavated by explosions during two preceding years of unrest. This posed a number of potential hazards to the surrounding touristic region, so the eruption was carefully monitored. Structure from Motion techniques were used to generate DEMs from satellite and aerial images, from which several useful measurements could be made. Dome growth was characterised at an unprecedented resolution, allowing for the calculation of discharge rates and effusion rates in near real time. A simple model fit to the distance between the dome and crater rim predicted relatively accurately the arrival of the dome toe at the crater rim and the onset of dome collapse outside the crater. Simulations of the path and extent that potential pyroclastic density currents (PDC) generated by dome collapse would follow showed that PDC were not directly threatening populated areas. Over its life cycle as of August 2019, the dome growth was punctuated by frequent explosions, averaging around 30 per day, one of which generated a minor 600 m long PDC on 13 to 15 of July 2018. There appears to be a positive correlation between explosion frequency and lava dome growth rate suggesting that both explosive and effusive processes can coexist, operating at different timescales but responding to the same driving force. A positive correlation is apparent between dome growth rate and seismic activity such as the frequency of tremor and long-period earthquakes suggesting that these might be used as proxies to estimate effusion rate. Initial lava dome effusion rates of 1730 ± 110 m3/day in January 2018 declined to 100 ± 150 m3/day in June 2019. These growth rates are extremely slow when compared to other lava domes, about 300 to 600 times slower than the lava domes at Mt Unzen (1992) and Mt. St. Helens (1980).

The role of climate and disturbance regimes upon temperate rainforests during the Holocene: A stratigraphic perspective from Lago Fonk (∼40°S), northwestern Patagonia

Climate and disturbance regimes play key roles in shaping the structure, composition and functioning of terrestrial ecosystems. Despite this importance, very few stratigraphic studies in the temperate rainforests from northwestern Patagonia have explored this relationship in detail along a time continuum through the entire Holocene. Here we present a high-resolution fossil pollen and charcoal record from Lago Fonk (median resolution: 20 years), a small closed-basin lake in the lowlands of the Chilean Lake District (41°S), where wildfires and explosive volcanism have intermittently taken place during the Holocene, along with pronounced human-induced disturbance in post-colonial time. Our results show persistence of temperate rainforest throughout the Holocene, with changes in the composition and structure of Valdivian rainforests (VRF) at millennial timescales. We detect centennial-scale alternations in dominance between the VRF tree Eucryphia/Caldcluvia and generalist trees found in VRF and North Patagonian rainforests after ∼6.5 cal ka BP. Intervals dominated by VRF coincide with enhanced fire occurrence signaling negative hydroclimate anomalies with a mean duration of ∼150 years, which alternate with positive hydroclimate anomalies lasting ∼312 years on average.Our results suggest that the magnitude and rapidity of vegetation changes detected at 10.2–9.9, 4.0–3.0, ∼1.0, and ∼0.7 cal ka BP were amplified by disturbance regimes, and led to the establishment and maintenance of Eucryphia/Caldcluvia-dominated forests in the Longitudinal Valley of the Chilean Lake District. On several occasions the higher incidence of fire disturbance during warm/dry climate intervals coincided with episodes of heightened explosive volcanic activity from multiple eruptive centers within the Southern Andean Volcanic Zone.

Reactivation of Fault Systems by Compartmentalized Hydrothermal Fluids in the Southern Andes Revealed by Magnetotelluric and Seismic Data

AbstractIn active volcanic arcs such as the Andean volcanic mountain belt, magmatically sourced fluids are channeled through the brittle crust by faults and fracture networks. In the Andes, volcanoes, geothermal springs, and major mineral deposits have a spatial and genetic relationship with NNE trending, margin‐parallel faults and margin‐oblique, NW trending Andean Transverse Faults (ATF). The Tinguiririca and Planchón‐Peteroa volcanoes in the Andean Southern Volcanic Zone (SVZ) demonstrate this relationship, as their spatially associated thermal springs show strike alignment to the NNE oriented El Fierro Thrust Fault System. We constrain the fault system architecture and its interaction with volcanically sourced hydrothermal fluids using a combined magnetotelluric (MT) and seismic survey that was deployed for 20 months. High‐conductivity zones are located along the axis of the active volcanic chain, delineating fluids and/or melt. A distinct WNW trending cluster of seismicity correlates with resistivity contrasts, considered to be a reactivated ATF. Seismicity occurs below 4 km, suggesting activity is limited to basement rocks, and the cessation of seismicity at 9 km delineates the local brittle‐ductile transition. As seismicity is not seen west of the El Fierro fault, we hypothesize that this structure plays a key role in compartmentalizing magmatically derived hydrothermal fluids to the east, where the fault zone acts as a barrier to cross‐fault fluid migration and channels fault‐parallel fluid flow to the surface from depth. Increases in fluid pressure above hydrostatic may facilitate reactivation. This site‐specific case study provides the first three‐dimensional seismic and MT observations of the mechanics behind the reactivation of an ATF.

Evidence for primitive magma storage and eruption following prolonged equilibration in thickened crust

In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded from smaller, monogenetic cinder cones. Manantial Pelado (35.5° S) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45–50 km) that produces almost exclusively mafic material. As Manantial Pelado is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from Manantial Pelado to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. A geologic description of the volcano reveals a mostly monotonous eruptive history of basaltic andesites that are now accessible through glacially carved valleys. New 40Ar/39Ar dating constrains most of the volcano’s cone constructing phase to last from ~ 220 to 190 ka. At ~ 30 ka, small-volume activity and different petrography of more intermediate magmas were present reflecting a change in the volcano’s character. A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at Manantial Pelado are among the most primitive magmas in the thickened crust of the SVZ. Evidence for this primitive signature consists of textural and zonation patterns in olivine, the presence of Cr-spinel in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivines with uniform core compositions (Fo80–84) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~ 25–6000 years). Diffusive equilibration in those intermediate Fo olivines masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivines at intermediate Fo content may represent cryptic evidence for recharge with primitive magmas that experienced prolonged crustal storage.