The west coast of North America faces the risk of tsunamis generated by seismic rupture in three regions, namely, the Cascadia subduction zone extending from southwestern British Columbia to northern California, the southern Queen Charlotte margin in the Haida Gwaii area, and the Winona Basin just northeast of Vancouver Island. In this thesis, I construct tsunamigenic rupture models with a 3-D elastic half-space dislocation model for these three regions. The tsunami risk is the highest along the Cascadia coast, and many tsunami source models have been developed and used in the past. In efforts to improve the Cascadia tsunami hazard assessment, I use an updated Cascadia fault geometry to create 9 tsunami source models which include buried, splay-faulting, and trench-breaching rupture. Incorporated in these scenarios is a newly-proposed splay fault based on minor evidence found in seismic reflection images off Vancouver Island. To better understand potential rupture boundaries of the Cascadia megathrust rupture, I also model deformation caused by the 1700 C.E. great Cascadia earthquake that fit updated microfossil-based paleoseismic coastal subsidence estimates. These estimates validate the well-accepted along-strike heterogenic rupture of the 1700 earthquake but suggest greater variations in subsidence along the coast. It is recognized that the Winona Basin area just north of the Cascadia subduction zone may have the potential to host a tsunamigenic thrust earthquake, but it has not been formally included in tsunami hazard assessments. There is a high degree of uncertainty in the tectonics of the area, the presence of a subduction “megathrust”, fault geometry, and rupture boundaries. Assuming worst-case scenarios and considering the uncertainties, I construct a fault geometry using seismic images and generate six tsunami sources with buried and trench-breaching rupture in which downdip rupture extent is varied. The Mw 7.8 2012 Haida Gwaii earthquake and its large tsunami demonstrated the presence of a subduction megathrust and its capacity of hosting tsunamigenic rupture, but little has been done to include future potential thrust earthquakes in the Haida Gwaii region in tsunami hazard assessment. To fill this knowledge gap, I construct a new megathrust geometry using seismic reflection images and receiver-function results and produce nine tsunami sources for Haida Gwaii, which include buried and trench-breaching ruptures. In the strike direction, the scenarios include long ruptures from mid-way between Haida Gwaii and Vancouver Island to mid-way between Haida Gwaii and the southern tip of Alaskan Panhandle, and shorter rupture scenarios north and south of the main rupture of the 2012 earthquake. For all the tsunami source and paleoseismic scenarios, I also calculate stress drop along the fault. Comparison of the stress drop results with those of real megathrust earthquakes worldwide indicates that these models are mechanically realistic. / Graduate

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18802号 / 理博第4060号 / 新制||理||1584(附属図書館) / 31753 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 竹村 惠二, 教授 平原 和朗, 教授 大沢 信二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

No description available.

No description available.

Northeast Japan experienced an approximately constant, compressional deformation during the last 5 million years resulting from the steady subduction of the Pacific plate. Because the direction of the maximum compression axis is approximately perpendicular to the strike of the island arc, 2-D finite-element modeling can be used to examine the deformation over time of the island-arc lithosphere. The model geometry is based on geophysical and geological data, and each model run requires an assumed rheology and interplate coupling. Novel to our modeling is the ability to include erosion/deposition loading and the creation of strike-slip faults, based on a dynamically-applied fracture criterion. The criterion for acceptability is how well a model matches observed present-day topography, gravity, and seismicity patterns. Results given below are for models that satisfy this criterion. The long-term effective elastic thickness is 10 km in the inner arc, increasing to about 50 km near the trench. The effective elastic thickness in the inner arc is therefore much smaller than the about 30 km short-term elastic thickness estimated from seismological data. The viscosity of the lower crust is on the order of 1022 Pa s or less. The strength of interplate coupling off Sanriku is about two to four times greater than off Miyagi, and there is about twice as strong a coupling at greater depths. The relative strength of coupling correlates well with the observed interplate seismicity. Hence the inferred weaker coupling off Miyagi indicates a lack of seismogenic potential -- a low probability for large earthquakes in that region, not just a long return cycle. The same modeling procedure was also applied to southwest Japan. The viscosity of the lower crust is not more than 1021 Pa s, and the elas tic thickness is about 10 km. The calculated strength of interplate coupling for southwest Japan is about 1.5 times greater than for the off-Sanriku region in northeast Japan, which correlates well with the fact that there have been great (M>8) earthquakes in the Nankai Trough region, but none that large in the off-Sanriku region. / Ph. D.

2-D numerical computer models based on thermodynamic and kinematic principles have become invaluable tools for simulating geodynamic processes at these systems. Numerical models have proven effective for allowing the examination and computation of multiple factors simultaneously, providing scientists with an important resource with which to study complex systems. Previously, for instance, numerical models have been used for examining different factors involved in magma production at subduction zones and mid ocean ridges by modelling the influence and interplay of factors such as the effect of hydration and the influence of the depth of the fault between the two plates on the melting (van Keken, 2003; van Keken 2008). Additional models have explored the thermal structure of subduction zones and its relationship to the processes involved at convergent boundaries, including magma production (van Keken, 2023a). Syracuse et al. (2010) used numerical models for subduction zones, creating thermal models that examined dehydration and melting in subduction zones with a variety of slab geometries, convergence velocities, ages and structures. Still others have shown that thermal structure affects melt production, formation of arc volcanoes, dehydration, and seismicity, modelling the effects of varying slab dip, plate convergence velocity, plate age, etc. (Syracuse et al., 2010; Hayes et al, 2018). However, none have yet utilized models to systematically investigate magma production at either subduction zones or mid-ocean ridges to specifically examine both batch and fractional melting with the combination of multiple controlling factors including slab dip, convergence rate, hydration, minerology, and slab age. This project investigated the processes surrounding magma production at subduction and mid-ocean ridge systems through the creation of a numerical model and utilization of the developed model to explore the effects of a multitude of parameters on fractional and batch melting, as well as investigated the incorporation of incompatible elements, and other processes of interest in subduction and mid ocean ridge systems.

Thesis advisor: Ethan F. Baxter / Samarium/Neodymium (Sm-Nd) garnet geochronology of eclogites from Syros, Greece provides constraints on timing of peak metamorphism while thermodynamic modeling of the same samples allows a comparison of pressure-temperature (P-T) paths. Sm-Nd geochronology of four eclogite samples give ages of 48.8 ± 3.2 Ma (high 147Sm/144Nd = 0.49, n = 6, MSWD = 0.67), 48.1 ± 2.3 Ma (high 147Sm/144Nd = 1.22, n = 4, MSWD = 2.4), 44.7 ± 1.0 Ma (high 147Sm/144Nd = 3.9, n = 6, MSWD = 1.4), and 43.6 ± 1.6 Ma (high 147Sm/144Nd = 1.39, n = 6,MSWD = 2). These garnet growth ages span several million years and are younger than the only other published garnet eclogite ages from the island which use Lutetium/ Hafnium (Lu-Hf) garnet geochronology to place peak metamorphism at ~52 Ma (Lagos et al, 2007). Another eclogite sample dated less precisely yielded an age of 57.7 ± 6.3 Ma (high 147Sm/144Nd = 0.40, n = 10, MSWD = 1.9), significantly older than the other garnets dated in this study. The garnet ages from eclogites presented here suggest that high pressure-low temperature metamorphism, and related garnet growth and dehydration, on Syros lasted ~9 myr, similar to what has been reported for nearby Sifnos Island (Dragovic et al., 2015). Thermodynamic modeling of three samples reveals similar prograde P-T paths despite differences in tectonic setting and chemistry between samples. Water loss from mineral breakdown during the span of subduction zone garnet growth varies between samples from 1.09 to 5.13 weight percent but is greatest for the most ultramafic sample due to chlorite stability permitting greater capacity for water to be carried to depth. P-T paths reach greater maximum pressures (up to 2.42 GPa) than what is reported for Sifnos island (Dragovic et al., 2015) and greater than most previously published pressure estimates for Syros (ie. Okrusch and Bröcker, 1990; Putlitz et al., 2005). / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

O Ofiolito Arroio Grande, localizado no sudeste do Cinturão Dom Feliciano, próximo à fronteira Brasil/Uruguai, entre Arroio Grande e Jaguarão (RS), é uma associação metaultramáfica-máfica-sedimentar que representa fragmentos de uma mélange ofiolítica, relacionada à amalgamação do paleocontinente Gondwana Ocidental durante os estágios finais do ciclo orogênico Brasiliano-Panafricano. As rochas do Ofiolito Arroio Grande se encontram circundadas por rochas metassiliciclásticas do Complexo Arroio Grande, do qual o ofiolito faz parte, e também como xenólitos em meio a granitoides da Suíte Pinheiro Machado e do Granito Três Figueiras (os quais integram o Batólito Pelotas-Aiguá). A unidade metaultramáfica do ofiolito compreende serpentinitos e xistos magnesianos cromíferos. Sua unidade metamáfica é constituída por anfibolitos, metagabros e metadioritos. A unidade metassedimentar compreende mármores calcíticos, intrudidos por enxame de diques máficos. O Ofiolito Arroio Grande está posicionado ao longo da Zona de Cisalhamento Ayrosa Galvão-Arroio Grande (transcorrente, dúctil, alto ângulo), responsável pela milonitização da maioria das rochas dessa associação. As investigações desenvolvidas no ofiolito tiveram o objetivo de identificar as fontes magmáticas dos protólitos e os processos que ocorreram desde sua geração no manto/crosta oceânica até sua incorporação no continente, além de obter idades (absolutas e relativas) referentes a esses processos. Para os metaultramafitos, a geoquímica de rocha total (e.g. Ni >1000 ppm; Cr > 1500 ppm), em conjunto com a química mineral de cromitas (e.g. Cr# 0,6-0,8; TiO2 0,01-0,20 %peso; Fe2+/Fe3+ ± 0,9), sugeriu protólitos harzburgíticos mantélicos, cuja fonte é um manto depletado sob uma região de espalhamento oceânico de retroarco, que experimentou altas taxas de fusão parcial. Esses harzburgitos foram posteriormente serpentinizados em ambiente oceânico, sugerido pelas razões 87Sr/86Sr630 de um serpentinito (ca. 0,707). Para os metamafitos, a geoquímica de rocha total e isotópica sugeriram protólitos toleíticos oceânicos, gerados em um contexto de suprassubducção em ambiente de retroarco (e.g. Cr 260-600 ppm; Nb/Y 0,1-0,5; Ti/Y ± 500; La/Nb 2-5; Th/Yb 0,1-5 e Nb/Yb 1-5; padrões de REE; razões 87Sr/86Sr630 variando de MORB – 0,703 – a IAT – 0,705-0,707), cuja fonte magmática foi enriquecida por material crustal e fluidos relacionados à subducção. A idade mínima para a obducção e metamorfismo das unidades ofiolíticas foi estimada em 640 Ma, a partir da datação (U-Pb SHRIMP) de um quartzo sienito. Esse último é o resultado de fusões relacionadas a intrusões diorítico-tonalíticas, atribuídas ao magmatismo de arco continental da Suíte Pinheiro Machado. Essas intrusões afetaram os mármores e os anfibolitos (fragmentos dos enxames de diques máficos), de maneira que, em pelo menos 640 Ma, rochas da mélange ofiolítica (já metamorfizadas) estavam alojadas em ambiente continental. Um evento metassomático posterior (relacionado à intrusão do Granito Três Figueiras, sincinemática à zona de cisalhamento acima referida) afetou os serpentinitos, gerando zonas de talcificação, tremolitização e cloritização, essa última representando um blackwall que também envolveu unidades metassiliciclásticas do Complexo Arroio Grande. O Ofiolito Arroio Grande foi inserido no contexto geotectônico da bacia de retroarco Marmora, cujos fragmentos são encontrados na Namíbia (Terreno Marmora) e no Uruguai (Complexo Paso del Dragón e Bacia Rocha – Terreno Punta del Este). / The Arroio Grande Ophiolite, located in the southeastern region of the Dom Feliciano Belt, near the Brazil/Uruguay border, is a metaultramafic-mafic-sedimentary association which represents slices of an ophiolitic mélange, related to the Western Gondwana amalgamation during the late stages of the Brasiliano-Panafrican orogenic cycle. The Arroio Grande Ophiolite rocks are enveloped by metasiliciclastic units of the Arroio Grande Complex and occur as xenolyths within granitoids of the Pinheiro Machado Suite and within the Três Figueiras Granite (units of the Pelotas-Aiguá Batholith). The metaultramafites of the ophiolite comprise serpentinites and Cr-rich magnesian schists. The metamafites comprise amphibolites, metagabbros and metadiorites. The metasedimentary unit comprises calcitic marbles, which are intruded by mafic dykes. The ophiolite is found along the Ayrosa Galvão- Arroio Grande Shear Zone (transcurrent, ductile, high angle), responsible for the mylonitization of this association. The investigations developed in this ophiolite had the objective of identify the magmatic sources of the protoliths and the processes that occurred since their generation within the mantle/oceanic crust until their incorporation into the continental crust, including their absolute and relative ages. The bulk-rock chemistry of the metaultramafites (e.g. Ni >1000 ppm; Cr > 1500 ppm), together with the mineral chemistry of the chromites (e.g. Cr# 0.6-0.8; TiO2 0.01-0.20 wt%; Fe2+/Fe3+ ± 0.9), suggested harzburgitic protoliths, attributed to a depleted mantle source under a back-arc spreading region, which experienced high degrees of partial melting. These harzburgites were serpentinized in an oceanic setting, as suggested by the 87Sr/86Sr630 ratio of a serpentinite (ca. 0.707). The bulkrock chemistry of the metamafites suggested oceanic tholeiitic protoliths, generated in a supra-subduction setting in a back-arc environment (e.g. Cr 260-600 ppm; Nb/Y 0.1-0.5; Ti/Y ± 500; La/Nb 2-5; Th/Yb 0.1-5 and Nb/Yb 1-5; REE patterns; 87Sr/86Sr630 ratios ranging from MORB – 0.703 – to IAT – 0.705-0.707), whose magmatic source was contaminated by crustal material and subduction-related fluids. The minimum age for the obduction and metamorphism of the Arroio Grande Ophiolite rocks was estimated around 640 Ma from the U-Pb age of a quartz-syenite. The latter is the result of melting, related to dioritic-tonalitc intrusions, attributed to the continental magmatism of the Pinheiro Machado Suite. These intrusions affected both the marbles and the amphibolites (fragments of the mafic dykes), in order that, at least around 640 Ma, rocks of the ophiolitic mélange (already metamorphosed) were emplaced on the continent. A late metasomatic event (related to the emplacement of the Três Figueiras Granite, syn-kinematic to the abovementioned shear zone) affected the serpentinites, generating zones of talcification, tremolitization and chloritization, the latter representing a blackwall which also involved metasiliciclastic rocks of the Arroio Grande Complex. The Arroio Grande Ophiolite was inserted in the geotectonic context of the Marmora back-arc basin, whose fragments are found in Namibia (Marmora Terrane) and Uruguay (Paso del Dragón Complex and Rocha Basin – Punta del Este Terrane).

Several laterally extensive candidate tsunami deposits are preserved along coastlines facing the eastern Java Trench, indicating it has experienced mega-thrust earthquakes in the past. We investigated 37 coastal sites in Bali, Lombok, Sumba and Timor islands, many of which preserve course sand and pebble layers that overlie sharp basal contacts with scour marks into the mud, fine upward in grain size, and have bimodal grain size distributions. Other unique features are the common occurrence of marine fossils and concentrations of heavy minerals. The occurrence of these high-energy deposits interlayered with clay-rich units indicates the coarse clastics are anomalous because they were deposited in what is normally a very low-energy depositional environment. The lateral extent and paucity of thin, coarse clastic layers with marine organisms are inconsistent with local stream flood event, and the proximity to the equator of the sites diminishes the possibility of marine flood events from cyclones. The sparse, but consistent, the occurrence of at least two candidate tsunami deposits at depths of 1 and 2 meters over 950 km along the strike of the Java Trench may reveal that mega-thrust earthquakes have occurred there and generated giant tsunamis in the recent past.Five widely scattered imbricated boulder deposits are also found on Bali, Lombok, and Sumba. The boulders consist of slabs of hardpan up to 2.5 m in length and 80 cm thick that was torn from a near-shore seabed and stacked on top of one another. Some of the boulders were carried over the erosional coastal bank and deposited up to 100 meters inland. Comparisons with imbricated boulder ridges formed during the 1994 tsunami in east Java indicate that these deposits are from one or multiple tsunamis sourced by the Java Trench.Experiments in effective ways to communicate and implement tsunami disaster mitigation strategies have led us to train local communities about the 20-20-20 rule. If coastal communities experience more than 20 seconds of shaking from an earthquake, even if it is not intense, they should evacuate the coast. The time delay between the earthquake and arrival of tsunami waves is around 20 minutes, which is the time window for evacuation. Some tsunami waves may be as high as 20 meters, which is the target elevation for evacuation. Adopting the 20-20-20 rule could save thousands of lives throughout the region, especially in Bali where nearly 1 million people inhabit likely tsunami inundation zones.

Im Rahmen der Dissertation wurden an Wässern und freien Gasen aus Thermalquellen sowie an weniger als 5 Millionen Jahre alten basischen Vulkaniten des zentralandinen Puna-Hochplateaus (NE-Argentinien) umfangreiche element- und isotopengeochemische Untersuchungen durchgeführt und die Edelgasgehalte und -isotopensignaturen in diesen Medien bestimmt. Damit soll ein Beitrag zum besseren Verständnis der jüngeren Subduktionsgeschichte im Bereich der südlichen Zentralanden geleistet, die Wechselwirkungen zwischen ozeanischer Unter- und kontinentaler Oberplatte sichtbar gemacht und die Edelgassystematik verbessert werden. Wie die Ergebnisse der Untersuchungen an Gasen aus den Thermalquellen der Puna-Region zeigen, ist der Anteil an Mantel-Helium in den Thermalquellen dieser Region mit bis zu 67 % wesentlich höher als in der westlich gelegenen vulkanisch aktiven Westkordillere und den anderen angrenzenden Gebieten. In einigen Quellen konnten sogar Anteile an Mantel-Neon nachgewiesen werden, was aufgrund von Überlagerungen mit Neon atmosphärischen und krustalen Ursprungs weltweit bisher nur vereinzelt gelungen ist. Für kontinentale Bereiche mit großer Krustendicke ist ein solch starker Mantelgasfluss äußerst ungewöhnlich und bedeutet, dass Mantelschmelzen bis in die Kruste aufgedrungen sind und tief reichende Wegsamkeiten existieren, so dass die Mantelgase aufsteigen können, ohne stark krustal beeinflusst zu werden. Dass im Bereich der Puna rezent Mantelmaterial in die Kruste aufsteigt, zu diesem Ergebnis kommen auch aktuelle seismologische Untersuchungen. Zudem wurden junge, vorwiegend monogenetische Basalte bis basaltische Andesite geochemisch auf ihre Haupt-, Neben- und Spurenbestandteile sowie ihre Gehalte an Seltenenerdenelementen hin untersucht. Auch wurden die Isotopenverhältnisse von Sr, Nd und Pb in den Gesteinen bestimmt und petrographisch-mineralogische Analysen der darin enthaltenen Olivine und Pyroxene durchgeführt. Wie die Resultate belegen, haben die Magmen bei ihrem Aufstieg durch die Erdkruste insbesondere Material aus der Oberkruste assimiliert und sind zudem durch Fluide aus der abtauchenden Platte beeinflusst worden. Damit konnte gezeigt werden, dass einfache geochemische Methoden allein nicht ausreichen, um die Mantelquelle der Magmen ermitteln oder Aussagen über die Asthenosphärendynamik in der Region machen zu können. Im Gegensatz dazu zeigen die Messungen der Edelgasisotopenverhältnisse in den Fluideinschlüssen der Olivine und Pyroxene, dass deren Edelgaszusammensetzung nicht durch Krustenkontamination beeinflusst wurde, weil die Magmen erst nach der Olivin- bzw. Pyroxen-Kristallisation Schmelzen aus der Oberkruste assimiliert haben. Darüber hinaus konnten durch die Edelgasisotopenmessungen die bisher höchsten magmatischen He- und Ne-Isotopenverhältnisse von ganz Südamerika nachgewiesen werden. Aus der unterschiedlichen Höhe der Messwerte ist zu schließen, dass die im Osten der Puna vorkommenden älteren Laven aus einem nichtkonvektiven (lithosphärischen) Mantel stammen, während die am vulkanischen Bogen und Westrand der Puna gelegenen jüngeren Laven, ihren Ursprung in einer konvektiven (asthenosphärischen) Mantelquelle haben. Zudem konnte gezeigt werden, dass der Mantelgasfluss in der Region in den letzten 5 Millionen Jahren stark zunahm und sich die Eruption von mantelstämmigen basischen Laven in dieser Zeit kontinuierlich in westliche Richtung zum aktiven Vulkanbogen hin verlagerte. Im daraus abgeleiteten Modell beruht dieser Prozess (1) auf einer an die Kontinentalverschiebung gekoppelten W-Drift des Kontinents und (2) auf einem mit der Versteilung der Unterplatte verbundenen Vordringen des subkontinentalen asthenosphärischen Mantels nach W, nach dem Ende der Subduktion des unterseeischen aseismischen Juan Fernández-Rückens in der Region. Zudem gibt es starke Argumente dafür, dass die asthenosphärischen Magmen aus einer fluidreichen Zone in 500 – 600 km Tiefe parallel zur subduzierten Platte aufsteigen und nicht, wie bisher angenommen, durch Schmelzbildung in Bereichen unter 200 km Tiefe, allein durch Entwässerung der abtauchenden Platte erzeugt werden. Zu diesem Resultat führt vor allem die Kombination der He-Isotopenverhältnisse mit Ergebnissen seismologischer Untersuchungen. / This study has determined the concentrations and isotopic composition of noble gases in water and gas samples from thermal springs and in samples of post Miocene basic volcanic rocks from the central Andean Puna Plateau (NW Argentina). The aim of this study is to shed light on questions related to the Neogene subduction history, the geochemical relationship between the oceanic and continental plate and on the distribution of noble gases in mantle-derived rocks of the Central Andes. The results of the geothermal water study show that the Puna plateau has higher values of mantle-derived He between 22° and 26° S compared to the neighbouring Western and Eastern Cordilleras. The highest 3He/4He ratio (5,4 Ra) was obtained close to the Tuzgle volcano, and this is rather high for back-arc gases considering they have ascended through a relatively thick 65 km crust, enriched in crustal 4He. In some cases it was also possible to detect the presence of mantle-derived Ne, which has so far only been demonstrated in a few locations around the world, because of the ubiquitous contamination by atmospheric- and crustal-Ne. Hence, this study clearly demonstrates a higher flux of noble gases from the mantle in the Puna Plateau region than in nearby regions of the Altiplano, the Salta-Rift and the Eastern and Western Cordilleras. In addition to the water study, a series of samples from post Miocene basic volcanic rocks in the Puna back-arc region were analyzed for major, minor and trace element composition as well as Sr, Nd and Pb isotopic ratios. Mineralogical analysis of olivine and pyroxenes from the lavas show that the rocks compositions have signatures that vary depending on the distance from the volcanic arc. Accordingly, magma compositions reflect processes that took place in the subduction-modified mantle wedge and the overlying continental plate. During their ascent, most of the magmas were contaminated with acid crustal melts that mask the geochemical signature of their mantle sources. This makes it difficult to accurately reconstruct the specific geotectonic setting for the magmas and their related mantle sources from the whole rock compositions. However, it is possible to put limits on the origin and amount of contamination from the Pb, Nd and Sr isotopic data. The results show that contaminants are mainly from the upper mantle. Mixing models suggest degrees of crustal assimilation on the order of 10 % and less. Given the problems of interpreting conventional geochemical studies on the origin of the back-arc magmas as just described, the combination of whole rock geochemical results with the corresponding noble gas data is potentially very important. For this purpose, olivine and pyroxene mineral separates mechanically and thermally degassed in order to measure their noble gases content and isotopic ratios. As the results show, the He isotopic signatures in the rocks are quite variable (4,5 - 8,1 Ra), depending on the age and distance to the volcanic arc. Whereas the samples nearest to the arc have MORB-type 3He/4He > 7 Ra, those farthest from the arc have 3He/4He < 7 Ra similar to the typical signature of a subcontinental lithospheric mantle (SCLM). The youngest Puna lavas have the highest primordial 3He/4He ratios and therefore the highest values for mantle-derived He yet found in the whole Andes chain. This implies that the mantle gas flux in the Puna region has increased since the Pliocene while during the same time interval, the focus of back-arc volcanic activity migrated progressively west towards the position of the present volcanic arc. This migration can be correlated with steepening of the subducted slab and advance of the asthenospheric mantle wedge to the west as a consequence of the drift of the Juan Fernández ridge towards the south. A consequence of the westward drift of the subduction zone at ca. 26 km/Ma is the ascent of asthenosphere into the mantle wedge. Geophysical studies suggest that this material ascends parallel to the slab (return flow). The He isotopic signatures from this study show that the asthenospheric ascent was not spacious but took place along certain channels or branches that may relate to deep-reaching weak lithospheric zones but not to a widespread delamination in the SCLM. The isotopic He, Ne and Xe relations of the lava samples collected in the back-arc region far from the volcanic arc indicate the presence of SCLM during back-arc volcanism in this region, which is a grave argument against a crustal delamination. Crustal contamination could not have been responsible for these values as the assimilation of crustal melts essentially took place after the olivine crystallization, as indicated by the variations in Sr- and He-isotope data and the coexistence of quartz and olivine in the samples.