Slow seismic velocity anomalies are commonly imaged beneath subducting slabs in tomographic studies, yet a unifying explanation for their distribution has not been agreed upon. In South America two such anomalies have been imaged associated with subduction of the Nazca Ridge in Peru and the Juan Fernandez Ridge in Chile. Here we present new seismic images of the subslab slow velocity anomaly beneath Chile, which give a unique view of the nature of such anomalies. Slow seismic velocities within a large hole in the subducted Nazca slab connect with a subslab slow anomaly that appears correlated with the extent of the subducted Juan Fernandez Ridge. The hole in the slab may allow the subslab material to rise into the mantle wedge, revealing the positive buoyancy of the slow material. We propose a new model for subslab slow velocity anomalies beneath the Nazca slab related to the entrainment of hot spot material.

With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic melange within the western Yarlung suture zone of southern Tibet (Lopu Range region, similar to 50 km northwest of Saga). The siliciclastic matrix melange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the melange exhibit lower greenschist fades mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the melange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous-Paleocene Sangdanlin section, which records the earliest appearance (at similar to 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix melange, yielded indistinguishable maximum depositional ages of similar to 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix melange 200-500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix melange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at similar to 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix melange formation to imbricate-style thrust belt formation. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Permian to Triassic igneous rocks cropping out in the Coastal and Frontal cordilleras in northern Chile between 28 degrees 00'S and 30 degrees 15'S have long been interpreted to represent products of magmatism related to an extensional tectonic setting, either as the result of crustal anatexis or asthenospheric mantle decompression melting, in a passive continental margin. Eighty-six samples of plutonic (61) and volcanic (25) rocks from this region are characterized petrographically and geochemically. They are Permian to Early Jurassic in age, but the majority of the studied rocks correspond to the Lower to Middle Triassic Chollay Plutonic Complex, the volumetrically most important unit in the area. The rock samples have features typical of magmas derived from flux-induced melting of a depleted mantle such as: broad range of petrographic composition with predominance of intermediate to acid members, highly porphyrytic volcanic rocks, magnetite as the Fe-Ti oxide mineral phase, enrichment in LILE over HFSE, marked depletion in Nb, Ta, Ti, and P and moderate to no negative Eu anomalies. Few of the studied rock samples (<10%) have alkaline signature and trace element contents representative of anorogenic magmatism. In this work, we propose that subduction of an oceanic plate beneath the South American continent is responsible for the evolution of the margin from the Permian to Early Jurassic, at the studied latitudes. A preliminary interpretation of the margin architecture of the Andean margin from the Permian to the Triassic would be that the Chanchoquin and Chollay plutonic complexes represent the roots of a magmatic arc developed from the Permian to the Middle or early Late Triassic, whereas the Guanaco Sonso and Pastos Blancos formations would be the shallower parts of such arc. The La Totora Formation and some volumetrically minor Upper Triassic intrusive units represent magmatic products with alkaline signatures, which developed immediately before the establishment of the magmatic arc in the present-day Coastal Cordillera (during the Rhaetian to Early Jurassic).

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No description available.

The motivation of this study is to understand the seismic anisotropy observations from various subduction regions of the world. In subduction zone backarcs both trench-parallel and trench-normal seismic anisotropy, or fast wave polarization direction of shear wave, are observed. In the mantle the general assumption is that seismic anisotropy is caused by Lattice Preferred Orientation (LPO) of olivine minerals and that the direction of anisotropy is an indicator of the direction of mantle flow. The complex pattern of seismic anisotropy observations suggests that the flow geometry in the vicinity of subduction zones differs at different subduction zones with some subduction zones having trench perpendicular flow, consistent with corner flow in the mantle wedge while other subduction zones have trench parallel flow, consistent with a mode of flow where material from the mantle wedge flows around the edges of the slab. It should be noted that the direction of LPO orientation can also be modified by the presence or absence of water, pressure, and temperature in the mantle and that it is possible that the difference in anisotropy observations reflects a difference in water content or thermal structure of back arcs. The aim of this study is to test whether the flow geometry of mantle in numerical subduction calculations can influence the direction of seismic anisotropy and if we parameters that control the pattern of flow can be identified. In this study we explicitly assume that seismic anisotropy occurs only due to plastic and dynamic re-crystallization of mantle mineral forming LPO. To approach the problem two different models are formulated. In one of the models the trench evolves self-consistently, with no prescribed artificial zones of weakness. The self-consistent model has a sticky-air layer at the top of the model domain that mimics a "free-surface." The other model has the same initial conditions but a trench-migration velocity boundary condition is imposed to the model. The mantle flow pattern for the self-consistent model is consistent with the 2D corner flow with no flow around the trench and no trench migration. However when the trench-migration velocity boundary condition is imposed, 3D flow around the mantle is observed. The stress field from these simulations are used to calculated instantaneous strain axis directions which correlate with LPO directions. The LPO orientations are measured from the models showing that the seismic-anisotropy direction is primarily trench-perpendicular for both models. Because the models have different flow patterns, the trench-perpendicular anisotropy alignment that is calculated for both the models is a bit puzzling. It could be that factors such as high temperature and non-linear rheology cause the LPO direction to align trench perpendicular in both the cases. It can also be possible that the 3D vertical flow is not strong enough to cause change in orientation of the LPO direction. From the present study it can be concluded that by looking at the LPO direction nature of mantle flow might not be predicted. This suggests that in addition to flow direction other factors such as the presence of water in mantle wedge, pressure, and high temperature due to viscous coupling modify the seismic anisotropy directions. / Master of Science

Subduction zones are an important recycling center at which material from the exterior of the Earth is transported to Earth's interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense subducting lithosphere experiences prograde metamorphism as it transitions from blueschist to eclogite facies resulting in the breakdown of volatile-bearing minerals and producing anhydrous minerals and a free fluid phase. Previous works attempting to understand the evolution of subducted lithologies have provided a firm foundation in which to apply field work, computational thermodynamic modeling, and geochronological techniques in order to better constraint the Pressure-Temperature-time (P-T-t) paths and dehydration of subducted lithologies. This dissertation; 1.) Explores novel approaches to modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Questions what the calculated P-T-t path from eclogite lithologies reveals about early exhumation of subducted terrains. The second chapter focuses on how externally-derived hydrous fluids can decarbonate subducted basalt, liberate carbon and transfer it to the overlying mantle wedge, where it can be incorporated into melt that forms volcanic arcs. Here, the thermodynamic response to the infiltration of external fluids assuming open system, pervasive fluid flow, is quantified. It was determined that while hotter subduction zones have more favorable P-T conditions in which to facilitate decarbonation than colder subduction, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of underlying serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of thermodynamic modeling, 147Sm/144Nd garnet geochronology, and quartz-in-garnet geobarometry. This provides insight into early exhumation of subducted lithologies, and allows for the exploration of assumptions made in thermodynamic modeling and in quartz-in-garnet geobarometry. Results suggest that garnet grew over a 4.31my period from 45.71±0.98Ma to 41.4±1.7Ma, during initial exhumation from maximum subducted depths. Calculated exhumation rates are a relatively rapid, 0.4-1.7 cm/yr. Because field relationships on Syros suggest the width of the subduction channel along the slab/mantle interface is not adequate to facilitate buoyancy-driven ascension of metabasic blocks, initiation of southward retreat of the Hellenic Subduction Zone and subsequent slab rollback is proposed to have played an important role in the exhumation of subducted lithologies. The final chapter investigates the compositional controls on the P-T conditions at which dehydration due to the breakdown of hydrous minerals occur during subduction (blueschist/eclogite boundary), and the implications they have on the rheology, seismicity, and densification of the down going slab. Total Alkali Silica (TAS) diagrams reveal that eclogites are more alkali rich, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt in many cases. / Ph. D.

Great (Mw >8.5) earthquakes occur over long temporal intervals that extend beyond current historical (written and oral) records along most subduction zone coastlines often leading to the underestimation of magnitude, recurrence, and spatial extent of these events. Paleoseismic studies target low energy depositional environments that record primary and secondary evidence of earthquake occurrence within the coastal stratigraphy over much longer temporal scale, thus improving our understanding of the behavior of subduction zone earthquakes. Diatoms preserved within coastal stratigraphic records are sensitive to earthquake-induced environmental change and are useful bioindicators in paloesiesmology studies. The two studies in this dissertation employ diatoms to create novel approaches to investigate behavior and recurrence of earthquakes along two subductions zones: Alaska-Aleutian subduction zone and the Cascadia subduction zone. In these chapters we use diatoms to explore 1) the potential for combined slip along the Patton Bay splay fault system and the eastern Alaska-Aleutian subduction zone within Prince William Sound, Alaska, and 2) lacustrine turbidite source mechanisms in Ozette Lake, Washington to potentially improve the spatial and temporal earthquake record for the northern Cascadia subduction zone. This work has implications for improving our earthquake chronologies along subduction zone coastlines and making important contributions to coastal hazards assessments. / Doctor of Philosophy / Subduction zones are capable of producing great (>Mw 8.5) earthquakes with accompanying tsunamis that can impact nearby coastlines with devastating force. Great earthquakes occur over long timescales (thousands of years) and are often not captured in short historical records, leaving questions about the recurrence, behavior, and range of potential future earthquakes along these boundaries. Paleoseismology, the study of earthquake history, employs methods that use the earthquake-induced environmental changes along subduction zone coastlines to provide long-term records of earthquake occurrence. Diatoms, a type of siliceous microalgae entrained in coastal sediments, react to changes in pH, salinity, water depth, and sediment type, and are important indicators of environmental change that can be used to expand our understanding of earthquake behavior. This dissertation uses diatoms in two projects that explore the earthquake history along the Alaska-Aleutian subduction zone and the Cascadia subduction zone. First, we determine that secondary faults, called splay faults, in Prince William Sound are likely triggered only by slip along the Alaska-Aleutian subduction zone, suggesting that combined slip has occurred during four of the eight total megathrust earthquakes in the last ~4,200 years. Second, we investigate the sediment origins of the youngest six deposits (turbidites) in Ozette Lake, linking them to diatoms located on the subaqueous delta and shallow lake surfaces, leading us to infer the source is likely earthquake-induced slope failure. Both projects help to expand our understanding of subduction zone earthquake behavior, and will help inform future hazards assessments for coastal communities.

Cette thèse présente les résultats et conclusions issues d’une série de modèles analogiques de systèmes de compressif à différentes échelles : Les expériences réalisées à l’échelle crustale montrent que la symétrie de structures compressives, de type plis et chevauchements avec 3 niveaux de décollement, est fortement dépendante de la vitesse de sédimentation. Les résultats ont été appliqués au Subandin Bolivien. Les expériences réalisées à l’échelle lithosphérique simulent la subduction et l’extension arrière-arc dans un système subissant une compression parallèle à la marge continent-océan (COB). Ces modèles démontrent que la différence de densité entre les plaques continentales et océaniques est le paramètre clé pour expliquer l'extension arrière-arc: plus petite est la différence de densité, plus faible est l'extension produite. Les résultats ont été appliqués al ‘Anatolie. Enfin, ces modèles ont été utilisés pour tester la reproductibilité et les limites de la modélisation analogique. / This thesis presents the results and conclusions from a series of analogue modelling of deformation in compressive and subduction zones (crustal scale and lithospheric scale) : The experiments carried out at the crustal scale show that the symmetry of compressive structures, folds and trust belts with 3 levels of décollement is strongly dependent on the rate of sedimentation. The results were applied to the Subandin Bolivian. The experiments carried out at the lithospheric scale simulate subduction and back-arc extension in a system under compression parallel to the continent - ocean margin (COB). These models show that the density ratio between the continental and oceanic plates is the key factor to explain the back-arc extension: as smaller the difference in density is, less extension occurred. The results were applied to Anatolia. Finally, these models were used to test the reproducibility and limits for analog modeling.

L’interprétation structurale de sections de Sismique Réflexion Multitrace-2D acquises pendant la campagne SISTEUR sur la marge de l’Équateur Central et migrées en profondeur avant sommation (PSDM) a été combinée avec la bathymétrie multifaisceaux, des modèles tomographiques de sismique grand-angle OBS, un modèle d’inversion GPS, et 13 années de sismicité relocalisée, afin de déchiffrer les causes de la variabilité de la sismicité et du Couplage Inter Sismique (CIS) le long de la subduction. La partie marine de cette marge est étroite et érosive. Elle chevauche vers l’Ouest, à 4.7 cm/an, la Ride de Carnégie. Le segment nord de la zone d’étude est bloqué, et aucun chenal de subduction n’est identifié. Ce segment révèle la présence d’un important (50 X 40+ km) massif océanique (MO) subduit, haut de ~2.5 km, et dont le flanc arrière plonge vers le continent de 2-4°, et coïncide avec la zone de CIS bloquée, et avec le socle océanique résistant (Vp= 5 km/s) de la marge. Le flanc avant du MO déduit de notre étude coïncide avec une zone de CIS partiel et des essaims de séismes chevauchant déformant le socle de la marge. A l’inverse, le segment sud est découplé, et affiche une pente sous-marine très perturbée avec des escarpements abrupts. Le contact interplaque plonge de 6-7° sous le continent et porte des monts sous-marins isolés séparés par un chenal de subduction de ~1km d’épaisseur qui agit comme lubrifiant. Un scénario en 3 étapes est proposé pour la subduction d’un MO de forme émoussée sous la marge résistante de l’Ile La Plata. Un modèle cinématique est proposé pour rendre compte de la surrection de l’île de La Plata en réponse au MO au cours des derniers 1.3-1.4 Ma. / The structural interpretation of 2D-Pre-stack Depth Migrated Multichannel Seismic Reflection sections collected during the SISTEUR cruise across the Ecuadorian margin was combined with multibeam bathymetry, OBS wide-angle tomographic models, a GPS inversion model, and 13 years of relocated seismicity to decipher the causes of the along-trench variability of the seismicity and Inter-Seismic Coupling (ISC). The margin submarine part is narrow and dominated by subduction erosion. It is underthrust eastward at 4.7 cm/yr by Carnegie Ridge, and figures a decoupled subduction centered over La Plata Island region. Our study shows that the Central Ecuador margin divides in two contrasting segments with dissimilar long-lived physical properties that may account for their specific ISC and seismicity patterns. The locked northern segment shows a smooth outer-wedge slope scalloped by a gentle re-entrant. No subduction channel is detected across this segment that reveals a broad 50 X 40+ km, ~2.5-km-high subducted Oceanic Massif (OM), which dips landward 2-4°, which coincides with the strong (Vp= 5 km/s) oceanic margin basement. In contrast, the decoupled southern segment shows a highly disrupted outer-wedge seafloor. The plate interface dips landward ~6-7°, and is spotted by isolated seamounts separated by a ~1 km-thick subduction channel that may act as a lubricant favoring inter-plate decoupling. A 3-step scenario is put forward for the subduction of a low-drag shaped OM beneath the resistant margin wedge of La Plata Island. Moreover, a kinematic model accounting for the uplift history of La Plata Island is proposed as a result of the OM subduction over the last 1.3-1.4 Myr.