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Herve F.,University of Chile | Herve F.,Andres Bello University | Calderon M.,Servicio Nacional de Geologia y Mineria | Fanning C.M.,Australian National University | And 2 more authors.
Gondwana Research | Year: 2013

We present detrital zircon U. Pb SHRIMP age patterns for the central segment (34-42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34-42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3. Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34-38°30'S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39-42°S). The youngest igneous detrital zircons in both the northern Western (307. Ma) and Eastern Series (345. Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and Lu. Hf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253-295. Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times. © 2012. Source

Castruccio A.,University of Chile | Clavero J.,Servicio Nacional de Geologia y Mineria
Journal of Volcanology and Geothermal Research | Year: 2010

The Villarrica and Calbuco volcanoes, of the Andean Southern Volcanic Zone, are two of the most active volcanoes in Chile and have erupted several times in the XX century. The 1961 eruption at Calbuco volcano generated lahars on the North, East and Southern flanks, while the 1971 eruption at Villarrica volcano generated lahars in almost all the drainages towards the north, west and south of the volcano. The deposits from these eruptions in the Voipir and Chaillupén River (Villarrica) and the Tepú River (Calbuco) are studied. The 1971 lahar deposits on Villarrica volcano show a great number of internal structures such as lamination, lenses, grading of larger clasts and a great abundance of large floating blocks on top of the deposits. The granulometry can be unimodal or bimodal with less than 5% by weight of silt + clay material. SEM images reveal a great variety of forms and compositions of clasts. The 1961 lahar deposits on Calbuco volcano have a scarce number of internal structures, steeper margins and features of hot emplacement such as semi-carbonized vegetal rests, segregation pipes and a more consolidated matrix. The granulometry usually is bimodal with great quantities of silt + clay material (> 10% by weight). SEM images show a uniformity of composition and forms of clasts. Differences on deposits reveal different dynamics on both lahars. The Villarrica lahar was generated by sudden melt of ice and snow during the paroxysmal phase of the 1971 eruption, when a high fountain of lava was formed. The melted water flowed down on the flanks of the volcano and incorporated sediments to become transition flows, highly energetic and were emplaced incrementally. Dilution of the flows occurs when the lahars reached unconfined and flatter areas. In cases where the lahar flow found large water streams, dilution is enhanced. The Calbuco lahars were generated by the dilution of block and ash pyroclastic flows by flowing over the ice or snow or by entering active rivers, transforming to debris flows. The differences on dynamics of both flows show the importance to understand initiation processes of lahars in order to make better hazard assessment due to laharics flows. © 2009 Elsevier B.V. All rights reserved. Source

Watt S.F.L.,UK National Oceanography Center | Pyle D.M.,University of Oxford | Mather T.A.,University of Oxford | Naranjo J.A.,Servicio Nacional de Geologia y Mineria
Geophysical Research Letters | Year: 2013

Global arc magmatism is sustained by a continuous fluid flux that is returned to the mantle in subduction zones. Despite considerable advances in simulations of melting processes, models of arc magmatism remain incompletely tested against erupted products. Here, we show that a suite of primitive volcanic rocks from across the southern Chilean arc preserves the signature of a systematic down-slab gradient in fluid chemistry. The chemical gradient is consistent with predictions from modeling, geothermometry and experiments. We infer that increasing slab-surface temperatures cause the sub-arc slab flux to become less water-rich and increasingly dominated by hydrous melts over a distance of a few kilometers behind the arc front. This change exerts a first-order control on magma chemistry, and implies discrete melt-transport pathways through subduction zones. Our results replicate patterns in other arcs, implying common sub-arc slab-surface temperature ranges in thermally-diverse subduction zones. © 2013 American Geophysical Union. All Rights Reserved. Source

Pritchard M.E.,Cornell University | Jay J.A.,Cornell University | Aron F.,Cornell University | Henderson S.T.,Cornell University | Lara L.E.,Servicio Nacional de Geologia y Mineria
Nature Geoscience | Year: 2013

Large earthquakes provoke unrest in volcanic areas hundreds of kilometres away from their epicentre. For example, earthquakes can induce ground deformation, thermal anomalies, additional earthquakes, hydrological changes or eruptions in volcanic regions. Two earthquakes in the Chilean subduction zone, in 1906 and 1960, triggered eruptions in the Andean southern volcanic zone within one year, yet no significant eruptions in the past three years are clearly associated with the 2010 M w 8.8 Maule, Chile earthquake. We use satellite Interferometric Synthetic Aperture Radar (InSAR) and night-time thermal infrared data to analyse subtle changes in ground deformation and thermal activity at volcanoes in the southern volcanic zone since 2010. We document unprecedented subsidence of up to 15 cm in five volcanic areas within weeks of the earthquake, but no detectable thermal changes. We suggest that the deformation is related to coseismic release of fluids from hydrothermal systems documented at three of the five subsiding regions. The depth and shape of these hydrothermal reservoirs can also be constrained by our deformation data, implying that coseismic volcano subsidence could be used to prospect for geothermal resources. Similar subsidence observed at Japanese volcanoes following the 2011 Tohoku earthquake suggests this phenomenon is widespread. Source

Roperch P.,CNRS Geosciences Laboratory of Rennes | Chauvin A.,CNRS Geosciences Laboratory of Rennes | Le Pennec J.-L.,French National Center for Scientific Research | Lara L.E.,Servicio Nacional de Geologia y Mineria
Physics of the Earth and Planetary Interiors | Year: 2014

Additional paleomagnetic data are necessary to improve geomagnetic models of secular variation during the Holocene, especially from the southern hemisphere. In most of the Andean volcanoes from Ecuador to the Chilean central volcanic zone, very well dated lava flows are rare. In contrast, andesitic to basaltic pyroclastic density current (PDC) deposits commonly contain charcoal, facilitating their age determination with 14C. In this study we present the magnetic properties and the paleomagnetic results obtained from three PDC deposits of basaltic to andesitic composition. One is the 2006 PDC deposit from the Ecuadorian Tungurahua volcano and the two others are well-dated PDC deposits from Chilean volcanoes Osorno and Villarica. Although most paleomagnetic studies in pyroclastic rocks deal with the estimation of emplacement temperatures from bulk deposits or accessory and accidental (non-juvenile) clasts, we show that juvenile clasts embedded in PDC deposits provide well-grouped paleomagnetic directions indicating post-emplacement in situ cooling below Curie points. Moreover, the rapid cooling of the juvenile clasts yields an abundance of single domain titanomagnetite grains providing low unblocking temperatures and a reliable material for paleointensity determination. © 2013 Elsevier B.V. Source

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