Chronology of cenozoic igneous and tectonic events in the central Chilean Andes — latitudes 35° 30′ to 36°S

Sixty-six K---Ar dates from igneous rocks in the central Chilean Andes between 33° and 38°S are reported in this study. From these results and observed field relations, major Cenozoic volcanic and intrusive rock units are divided into chronologic groups representing igneous events.Volcanic units of Oligocene (33.3–27.9 m.y.) and Early Miocene (20.2 m.y.) age have been dated west of the present range at 33°S but neither the magnitude nor extent of these volcanic events has yet been established. Extensive Middle to Late Miocene volcanism (15.3–6.4 m.y.) followed by regional folding is recognized in the map area between 35° 20′ and 36°S. Partly contemporaneous Middle Miocene volcanism (18.4–13.7 m.y.) also followed by regional folding is recorded in the Andes between 37° 30′ and 38°S. General volcanic quiescence from 6.4 to 2.5 m.y. is observed in the map area but whether this volcanic hiatus is of regional significance is not known.The majority of the K---Ar dates document a history of nearly continuous volcanism throughout the last 2.5 m.y. in the map area. The abundant and diverse sequences of volcanic strata formed during this time, have been divided into four successive age groups which as map units show the evolution and distribution of latest volcanic activity.Landforms preserved by this volcanic series show that topographic relief similar to the present has prevailed during this time. Deep incision of rivers into young volcanic terrain, estimated to be on the order of 1–2 m/1000 years, has produced a complex volcanic and morphologic record.Four plutons dated in this study give ages of 62.0, 41.3, 19.5, and 7.0 m.y. No spatial pattern of emplacement is observed in the map area where three of these plutons are represented.Similarities in structural style, orientation and degree of deformation of Miocene and Mesozoic strata suggest that Late Miocene regional folding may have accounted for a significant part of the observed deformation in older basement strata previously ascribed to earlier orogenies.A regional comparison of ages of recognized igneous and tectonic event at different latitudes in the central and southern Andes shows the gross chronology of Cenozoic events which can be correlated with sea-floor spreading and subduction events.     

Andean evolution at the Guañacos and Chos Malal fold and thrust belts (36°30′–37°s)

The Andes between 36°30′ and 37°S represent a Cretaceous fold and thrust belt strongly reactivated in the late Miocene. Most of the features that absorbed Neogene shortening were already uplifted in the late Cretaceous, as revealed by field mapping and confirmed by previous fission track analysis. This Andean section is formed by two sectors: a western-inner sector generated by the closure of the upper Oligocene-lower Miocene intra-arc Cura Mallín basin between the middle and late Miocene (Guañacos fold and thrust belt), and an eastern-outer sector, where late Triassic-early Jurassic extensional depocenters were exhumed in two discrete phases of contraction, in the latest early Cretaceous and late Miocene to the Present, respectively (Chos Malal fold and thrust belt). Late Miocene deformation has not homogeneously reactivated Cretaceous compressive structures, being minimal south of 37°30′S through the eastern-outer sector (southern continuation of the Chos Malal fold and thrust belt). The reason for such an inhomogeneous deformational evolution seems to be related to the development of a late Miocene shallow subduction regime between 34°30′ and 37°45′S, as it was proposed in previous studies. This shallow subduction zone is evidenced by the eastward expansion of the arc that was accompanied by the eastern displacement of the orogenic front at these latitudes. As a result, the Cretaceous fold and thrust belt were strongly reactivated north of 37°30′S producing the major topographic break along the Southern Central Andes.     

The Mid-Atlantic Ridge between 29°N and 31°30′N in the last 10 Ma

The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a⁻¹ to 24 mm a⁻¹, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.     

Potassium-argon dating of igneous activity in the central Chilean Andes — latitude 33°S

Potassium-argon dating of volcanic and plutonic rocks in the Andean region of central Chile has revealed previously unrecognized episodes of igneous activity during Cretaceous and Cenozoic time. These results indicate the need to re-evaluate the classic stratigraphic subdivisions that have evolved on lithologic rather than time-stratigraphic criteria.Four radiometric age groups have been identified in the coast range volcanic belt:

1. (1) Las Chilcas Formation — Early Cretaceous continental volcanic strata (120-110 m.y.).

2. (2) Lo Valle Formation — Late Cretaceous continental volcanic strata (78-65 m.y.).

3. (3) Late Oligocene extrusive volcanics (31-28 m.y.).

4. (4) Early Miocene intrusive volcanics (20.6–19.5 m.y.).

Two radiometric age groups have also been identified in the adjacent Andean Cordillera:

1. (1) Farellones Formation — continental volcanic strata (18.5–17.3 m.y.).

2. (2) Early Pliocene extrusive volcanics (5-4 m.y.).

An older group of continental volcanic strata in the Andes represented by the Abanico Formation remains undated but is intruded by plutons dated at 19.5 and 24 m.y.Available chronologic evidence indicates that volcanic activity moved eastward from the coast range volcanic belt to the Andean Cordillera between 20 and 18 m.y. ago and remained there to the present time.     

The topographic imprint of a transient climate episode: the western Andean flank between 15·5° and 41·5°S

Mountain‐range topography is determined by the complex interplay between tectonics and climate. However, often it is not clear to what extent climate forces topographic evolution and how past climatic episodes are reflected in present‐day relief. The Andes are a tectonically active mountain belt encompassing various climatic zones with pronounced differences in rainfall, erosion, and glacier extent under similar plate‐boundary conditions. In the central to south‐western Andes, climatic zones range from hyperarid desert with mean annual rainfall of 5 mm/a (22·5°S) to year‐round humidity with 2500 mm/a (40°S). The Andes thus provide a unique setting for investigating the relationship between tectonics, climate, and topography. We present an analysis of 120 catchments along the western Andean watersheds between 15·5° and 41·5°S, which is based on SRTMV3‐90m data and new medium‐resolution rainfall, tropical rainfall measurement mission (TRMM) dataset. For each basin, we extracted geometry, relief, and climate parameters to test whether Andean topography shows a climatic imprint and to analyze how climate influences relief. Our data document that elevation and relief decrease with increasing rainfall and descending snowline elevation. Furthermore, we show that local relief reaches high values of 750 m in a zone between 28°S to 35°S. During Pleistocene glacial stages this region was affected by the northward shifting southern hemisphere Westerlies, which provided moisture for valley‐glacier formation and extended glacial coverage as well as glacial erosion. In contrast, the southern regions between 35°S to 40°S receive higher rainfall and have a lower local relief of 200 m, probably related to an increased drainage density. We distinguish two different, climatically‐controlled mechanisms shaping topography: (1) fluvial erosion by prolonged channel‐hillslope coupling, which smoothes relief, and (2) erosion by valley glaciers that generates relief. Finally, Our results suggests that the catchment‐scale relief of the Andes between 28°S to 35°S is characterized by a pronounced transient component reflecting past climatic conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced scavenging ofPb andPo by processes associated with the East Pacific Rise near 8°45′N

In 1977 and 1981 a hydrothermal plume was detected at the East Pacific Rise (EPR) near 8°45′N and at MANOP Site M, 25 km east of the EPR, by anomalous²²²Rn and Mn concentrations. In 1981, samples were also taken for²¹⁰Pb,²¹⁰Po and²²⁶Ra analyses to determine if enhanced scavenging of these elements occurred in the plume. At both the ridge crest and at Site M, the ²¹⁰Pb/²²⁶Ra ratios range from 0.09 to 0.35, which are among the lowest values ever measured. It appears that removal of the²¹⁰Pb is occurring by processes operating at or near the seafloor. There is also significant ²¹⁰Po/²¹⁰Pb disequilibrium at both locations, which appears to increase away from the seafloor towards the plume (as indicated by elevated Mn concentrations). An in-situ water column scavenging process is suggested by correlation of [²¹⁰Pb-²¹⁰Po] and Mn concentrations in the plume. The residence time of the²¹⁰Po is only about 1 year, which is close to its residence time in surface waters but quite short compared to typical deep sea values. Thus rapid scavenging of the polonium in the plume and relatively rapid settling of particles from the plume is suggested.     

Lithological and hydrological controls on water composition: evaporite dissolution and glacial weathering in the south central Andes of Argentina (33°–34°S)

Lithological and hydrological influence on fluvial physical and chemical erosion was studied in a glacierized sedimentary basin with high evaporite presence. Suspended particulate matter (SPM), total dissolved solids (TDS) and major ion concentrations were analysed for 2 years of different hydrologic condition: (i) 2009–2010, Q = 100% average; and (ii) 2010–2011, Q = 60% average. Annual hydrograph was simple regime‐type with one peak in summer related to snow melting. The intra‐annual SPM and TDS variations were directly and inversely associated to Q, respectively. Snow chemistry showed continental influence (Na⁺/Ca²⁺ = 0.17), and atmospheric input of TDS was <1% of the total exported flux. River water was highly concentrated in Ca²⁺ and SO₄²⁻ (~4 mmol l⁻¹) and in Na⁺ and Cl⁻ (~3 mmol l⁻¹). Ca²⁺/SO₄²⁻ and Na⁺/Cl⁻ molar ratios were ~1 and related to Q, directly and inversely, respectively. Major ion relationships suggest that river chemistry is controlled by evaporite (gypsum and halite) dissolution having a summer input from sulfide oxidation and carbonate dissolution, and a winter input from subsurface flow loaded with silicate weathering products. This variation pattern resulted in nearly chemostatic behaviour for Ca⁺, Mg²⁺ and SO₄²⁻, whereas Na⁺, Cl⁻ and SiO₂ concentrations showed to be controlled by dilution/concentration processes. During the 2009–2010 hydrological year, the fluxes of water, SPM and TDS registered in the snow melting–high Q season were, respectively, 71%, 92% and 67% of the annual total, whereas for equal period in 2010–2011, 56% of water, 86% of SPM and 54% of TDS annual fluxes were registered. The SPM fluxes for 2009–2010 and 2010–2011 were 1.19 × 10⁶ and 0.79 × 10⁶ t year⁻¹, whereas TDS fluxes were 0.68 × 10⁶ and 0.55 × 10⁶ t year⁻¹, respectively. Export rates for 2009–2010 were 484 t km² year⁻¹ for SPM and 275 t km² year⁻¹ for TDS. These rates are higher than those observed in glacierized granite basins and in non‐glacierized evaporite basins, suggesting a synergistic effect of lithology and glaciers on physical and chemical erosion. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km² area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.     

Flux and dispersion of gases from the “Drachenschlund” hydrothermal vent at 8°18' S, 13°30' W on the Mid-Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH₄ to ³He in the neutrally buoyant plume is quite high, 4 × 10⁸. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s^− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH₄ calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of ³He is estimated to be between 0.14 and 1.2 nmol s^− 1. In either case, the ³He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr^− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m²m² s^− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.     

Structure of hydrothermal plumes at the Logatchev vent field, 14°45′N, Mid-Atlantic Ridge: evidence from geochemical and geophysical data

In the Seventh cruise of R/V “Professor Logatchev” anomalies of natural electric field (EF), Eh and pS were discovered using a towed instrument package (RIFT) at 14°45′N on the MAR (Logatchev hydrothermal field). The anomalous zone (AZ) is situated close (10–35 m) to two low-temperature venting areas of degrading sulphides and a black smoker (Irina-Microsmoke) forming a distinct buoyant plume. Over or close to the main area of high-temperature venting situated to the south-east from the AZ, no EF or Eh anomalies were observed. According to the results of Mir dives the highly mineralised solutions from smoking craters at the main mound mostly form non-buoyant plumes (reverse-plumes). The buoyant plume structure shows the differentiation of the electrical and Eh fields within the plume. Maxima of the EF, Eh and E_H2S anomalies were revealed in the lower part (15 m) of the plume. The negative redox potential plume coupled with a sulphide anomaly is more localized in comparison with the EF. This observation indicates a distinct change in the composition of buoyant plume water, which may be due to the formation and fallout of early formed Fe sulphide particles soon after venting.     

Spatial and temporal variations of subsidence of the East Pacific Rise (0–23°S)

The distributions of crustal depths as a function of age have been analysed for the southeast Pacific region, along the East Pacific Rise, between the Equator and the Easter microplate (23°S). Using age data and a new compilation of bathymetric data, subsidence rates (for both eastern and western flanks), asymmetry of subsidence and zero-age depths, are computed within flow-line corridors on the Nazca and Pacific plates. Variations of subsidence rates, axial depths and subsidence asymmetry are examined both in space (within corridors) and time (within several age intervals). The variability in these parameters along the strike of the East Pacific Rise is systematic and serves to define several orders of ridge segmentation. The largest variations of these parameters are correlated with the large-scale segmentation of the ridge axis (i.e. transform faults and very large overlapping spreading centres) and are interpreted as related to variations in mantle heterogeneities mainly dependent upon temperature. Smaller variations of subsidence parameters are correlated with second- (and sometimes third-) order segmentation of the ridge axis, which could be related to variations in axial magmatic supply. Across-strike variations of subsidence suggest the existence of small lateral temperature and density variations in the mantle. When analysing the slope of the distribution of depth versus square root of age within corridors, we have observed the existence of changes in the slope which occur at specific age limits. We have estimated the subsidence over different age ranges in order to determine the temporal evolution of subsidence parameters (rates and asymmetry). Such an analysis may inform on the past axial segmentation and on the persistence of axial discontinuities in time. A linear relationship between subsidence rates and axial depths is determined for each age range and suggests that shallower segments subside faster than deeper segments. Although a similar, statistically defined linear relationship exists for any mid-ocean spreading ridge (both for intermediate or fast–ultrafast spreading), the resultant slopes of this relationship vary from ocean to ocean and show that this relationship is not universal over all oceans.     

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia,South America

Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO₂; 4.3–5.9 wt.% Na₂O+K₂O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO₂; 4.0–4.4 wt.% Na₂O+K₂O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.     

Deep sea explosive activity on the Mid-Atlantic Ridge near 34°50′N: Magma composition, vesicularity and volatile content

Submersible observations and sampling were carried out in the rift valley of the Mid-Atlantic Ridge (MAR) near 34°40′N–35°N. The 4-km-wide rift valley consists of a Neo Volcanic Zone (NVZ) (<1 km wide) bounded at the west by a Median Ridge (MR) (5 km wide and 20 km long) and at the east by the first scarps of the eastern wall. The MR and the eastern wall are characterized by volcanic cones about 200–300 m height culminating at depths of 1500–1900 m which are made up of volcaniclastic deposits (pyroclasts and hyaloclasts) suggestive of explosive volcanism. Based on their surface morphology, degree of vesicularity, and composition, the erupted deposits are classified into four groups: (1) poorly vesicular (<15% vesicles) N- and T-MORBs (K/Ti <0.25, Na₂O+K₂O<2.9%) consisting of sheet flows and pillows formed during fissure eruptions in the NVZ at 2000–2300 m depths; (2) vesicular (15–30% vesicles) E-MORBs (K/Ti=0.25−0.45,Na₂O+K₂O>2.8−3.2%) and alkali basalts (K/Ti=0.45−0.70,Na₂O+K₂O>3.3−4) made up mainly of pillows; (3) highly vesicular (>35% vesicles) pillow lava and pyroclastic (scoria-like) alkali basalts (K/Ti>0.45−0.80,Na₂O+K₂O>3−4%); and (4) hyaloclastites consisting of glassy shards of alkali basalt composition. The total water and carbon contents of the deposits increase with the incompatible element concentrations. The estimated initial H₂O content for the N- and T-MORBs is less than 3500 ppm, whereas for the E-MORBs and alkali basalts the H₂O content is near 4000 and 7000 ppm, respectively. While the H₂O is mainly in the melt, the carbon is in the form of CO₂ filling vesicles. The vesicles are formed from magma with an initial carbon content of 1000–3000 for the N- and T-MORBs, 3000–6500 ppm for the E-MORBs and higher than 1 wt% for the alkali basalts.The various lava types were derived from a heterogeneous mantle source composed of enriched and depleted components during sequential eruptions of N-, T- and E-MORBs and alkali basalts (K/Ti>0.7). The amount of CO₂ and H₂O in equilibrium with the dissolved species present in the vesicles indicates that CO₂ (XCO₂=1−0.84) was the main exsolved compound responsible for bubble nucleation. The increase in the degree of vesicularity and pressure of the volatile phases is mainly due to the early exsolution of CO₂ from an alkali melt. The exsolution of significant amounts of dissolved water occurred only for the alkali basalt a few hundred meters beneath the seafloor and contributed to late bubble expansion. This subsequent addition of magmatic water to the vesicles increased the gas pressure and triggered explosions. An alternative hypothesis for the explosive volcanism is based on field observations. During crater collapsed, seawater could have been trapped in fractured volcanic conduits and later sealed by hydrothermal fluid circulation and precipitation. In such an environment, this seawater will be heated and vaporized during renewed magmatic upwelling. Both scenarios give rise to fragmented debris (hyaloclasts and pyroclasts) and the explosive events create turbulent flows followed by differential gravity settling of the particles (shards versus lapilli) through the seawater.     

Sources of dissolved silica to the fjords of northern Patagonia (44–48°S): the importance of volcanic ash soil distribution and weathering

Dissolved silica (DSi) plays an important biogeochemical role in the fjords of northern Chilean Patagonia (44–48°S), where it drives high biogenic productivity and promotes carbon burial. It is generally believed that the DSi riverine input to lakes and coastal environments is controlled by a combination of factors including lithology, climate, topography, vegetation, and meltwater input. In northern Chilean Patagonia several authors have proposed that the postglacial volcanic ash soils (andosols) may play a significant role in the high supply of DSi to the regional fjords. To assess the influence of andosols on DSi concentrations in north Patagonian rivers, we mapped andosol thickness and compared our results with river chemistry. The mineralogical and geochemical composition of three representative andosol profiles was also examined to evaluate the efficiency of weathering processes. The andosol thickness map clearly demonstrates that volcanic ash was predominantly deposited on the eastern side of the regional volcanoes, reflecting the influence of the prevailing westerly winds on the distribution of pyroclastic material. Mineralogical and geochemical results show that the andosol parent material has the typical andesitic basaltic signature of the regional volcanoes, i.e. high amounts of amorphous material, plagioclase, K‐feldspar, and pyroxene. Down‐profile variations in soil mineralogy and geochemistry indicate increased leaching of silica with depth, resulting from weathering of the volcanic parent material. For the five studied watersheds, a highly positive correlation (R²=0.98) was found between average andosol thickness and DSi concentrations, suggesting that andosol thickness is the main parameter affecting DSi concentrations in north Patagonian river systems. On seasonal timescales, increased precipitation (winter) and glacial meltwater (summer) input can significantly reduce DSi concentrations. We argue that the weathering of andosols constitutes the most important source of DSi to the lakes and fjords of northern Chilean Patagonia, explaining the particularly high regional rates of biogenic silica production. Copyright © 2015 John Wiley & Sons, Ltd.     

Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: Geochemistry and magma genesis of ignimbrites from 25 to 27 °S, 67 to 72 °W

Silicic volcanism in the Andean Central Volcanic Zone (CVZ) produced one of the world's largest Neogene ignimbrite provinces. The largest and best-known CVZ ignimbrites are located on the Altiplano-Puna plateau north of 24 °S. Their compositions and huge erupted volumes suggest an origin by large-scale crustal melting, and present-day geophysical anomalies in this region suggest still active zones of partial melting in the middle crust. Farther south in the CVZ, the Cerro Galán complex erupted ignimbrites in the late Miocene and Pliocene that are quite similar in volume and composition to those from north of 24 °S and they have a similar origin. However, there are a great many other, smaller ignimbrites in the southern CVZ whose compositions and geodynamic significance are poorly known. These are the subject of this paper.     

Possible contribution of the asthenosphere, below the subducted oceanic lithosphere, to the genesis of arc magmas: Geochemical evidence from the andean southern volcanic zone (33–46°S)

The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.