Thesis (Ph. D.)--Oregon State University, 2006. / Printout. Includes bibliographical references. Also available online.

Thesis (Ph. D.)--Universität Potsdam, 2007. / Includes bibliographical references (p. 83-93). Also available on the Internet.

Dans les zones de subduction, de nombreux indices attestent la circulation de fluides au-dessus de la plaque plongeante et dans le coin de manteau. L'interaction de péridotites avec des fluides aqueux issus de la déshydratation de la plaque plongeante favorise la formation de serpentinites à antigorite. Les interactions fluides-roche se font sous plusieurs formes : diffusion à l'état solide, percolation aux joints de grains et pression-solution. Afin d'étudier ces différentes interactions dans les conditions du coin de manteau, de l'antigorite et de l'eau ont été placées à haute pression (1.5-3.0 GPa) et haute température (315-540°C) dans une presse Belt ou une cellule à enclumes de diamant. De l'eau D2O a permis de suivre les processus d'inter-diffusion D/H dans l'antigorite et d'identifier les chemins de circulation de fluides et des traceurs de nickel ont été utilisés pour imager les recristallisations. L'analyse de monocristaux par spectroscopie Raman et nano-SIMS a permis de déterminer une loi d'inter-diffusion D/H pour l'antigorite : DD/H (m2/s) = 7.09 x 10-3 x exp(-202(-33/+70) (kJ/mol) /RT). La déformation de l'échantillon est localisées dans des zones de cisaillement ; elle augmente la porosité (jusqu'à 10 fractures/µm) et favorise les interactions fluides-roche. Des textures d'alignement de pores ont été identifiées comme des chemins actifs de circulation de fluides par la comparaison des volumes d'interaction fluides-roche et d'images MEB à haute résolution. Les recristallisations riches en nickel ont été étudiées par analyse EDX et imagerie en électrons rétrodiffusés. Les vitesses de cristallisation augmentent avec la température et la pression / In subduction zones many evidences confirm the circulation of fluids above the subducting slab and in the mantle wedge. The interaction of peridotites and water coming from the dehydration of the subducting slab favors the formation of antigorite serpentinites. Fluid-rock interactions include several processes: solid-state diffusion, percolation at grain boundaries and pressure-solution. In order to study the various interaction processes at the mantle wedge conditions, antigorite and water were interacted at high pressure (1.5-3.0 GPa) and high temperature (315-540°C) in a belt apparatus or in a diamond anvil cell. D2O-water was used as a tracer of D/H inter-diffusion processes in antigorite and in order to image circulation paths for aqueous fluids, and nickel tracers were used to image the recrystallizations. The analyses of single-crystals with a Raman spectrometer and nano-SIMS lead to a D/H inter-diffusion law in antigorite: DD/H (m2/s) = 7.09 x 10-3 x exp(-202(-33/+70) (kJ/mol) /RT). The sample deformation, due to the non-hydrostatic pressure in the belt apparatus, is localized in shear zones; it raises the porosity (up to 10 fractures/µm) and enhances the fluid-rock interactions. Textures of pore alignments were identify as active circulation paths for fluids from the comparison of maps of fluid-rock interactions and high resolution SEM images. Nickel-rich recrystallizations were studied with EDX analyses and backscattered electron imaging. Crystallization velocities raises with increasing temperature and pressure

Numerous observations suggest that the elastic description of the subduction earthquake cycles is incomplete. Micro-seismicity is recorded in active margins that are believed to be locked, while peculiar extensional earthquakes occur in convergent plate boundaries following tsunami earthquakes. The morphology of active margins, which evolves on time scales of 100s of kyr, shows similarities to ongoing deformation documented over 10–100 yrs and the coastal domains of Cascadia, Chile, and other subduction zones record long-term uplift. Lastly, the very threshold where faults break and earthquake nucleate has been vigorously debated for years. In this thesis, I combine various geophysical tools to study short- and long-term processes and learn how their interplay can shape the deformation field imparted by earthquake cycles, mainly in the upper plate of subduction zones. In the first chapter, I analyze surface heat flow measurements taken in the proximity of the southern Dead Sea fault to demonstrate its friction is 0.27±0.17. In the second chapter, I compute an updated horizontal and vertical GNSS velocity field for Bangladesh, Myanmar, and adjacent regions. I show that the Kabaw fault, which lies east of the primary thrust system, is accommodating shortening that was initially attributed to the main thrust and demonstrate that the Indo-Burma subduction is locked, converging, and capable of hosting great megathrust events. In the third chapter, I use thermomechanical models to show that reducing the dip angle of a subducting slab, on a timescale of millions of years, can result in extensional fault failure above a megathrust earthquake on timescales of seconds to months. In the fourth chapter, I demonstrate how the buildup of interseismic elastic stresses brings sections of the forearc into compressional failure, which yields irreversible uplift of the coastal domain per evidence from Chile. Finally, I argue that combining short- and long-term processes into subduction zone models can better mitigate tsunami and earthquake hazards. I show how long-term reduction of slab dip angle could culminate in devastating tsunamis. I argue that the collection of long-term uplift records of upper plates or volcanic arc migration can constrain slab dip changes and so may identify areas with increased tsunami potential. In addition, upper plate irreversible deformation should be introduced to earthquake rupture models as these may hold significant implications for understanding and mitigating earthquake hazards.

No description available.

No description available.

Thesis advisor: Ethan Baxter / Thesis advisor: Mark Behn / Subduction zones are areas of significant mass transfer between Earth’s crust and mantle. The dehydration of water-rich minerals such as serpentinite and lawsonite introduces water and volatiles into the subduction interface, that then travel to the mantle wedge above. The chemical composition, speciation, and redox effect of these fluids carry important implications for arc volcanism and the mobility of economically significant elements. This study uses Fe and Zn isotopic variation in eclogite-breccias from the Monviso ophiolite, combined with Sm-Nd garnet geochronology, to study the composition and redox effects of fluids that were produced during eclogite and blueschist facies metamorphism, and the timescales over which the brecciation and fluid flux events happened. Fe and Zn isotopic measurements were made on a series of four breccia matrix generations (M1-M4), generated during the progressive brecciation of the original Fe-Ti gabbros and the influx of both internally and externally derived fluids. The ∂56Fe and ∂66Zn data display a bi-modal distribution, with early matrix crystallization (M1-3) imparting progressively lighter ∂66Zn values while the ∂56Fe remains relatively unchanged. The last stage of metasomatic rind formation (M4) is associated with a decrease in both Fe and Zn isotopic values and a particularly significant shift in the Fe isotopes. This distribution suggests that early brecciation (M1-3) resulted from small-scale internal fluid flow that did not have a measurable effect on the isotopic composition and redox state of the system. By contrast, late metasomatic rind formation (M4) was facilitated by the flow of large amounts of external fluids with a strongly negative Fe and Zn isotope signature that affected the redox state of the mafic slab and may be responsible for transferring oxidized material into the mantle wedge. Dating of the M4 matrix generation yielded an age of 41.31± 0.60 Ma. A compilation of age data from Monviso suggests peak metamorphism and initial brecciation (M1 formation) likely occurred at ~45 Ma, the formation of the M4 matrix representing the end of eclogite-facies retrogression and brecciation at ~41 Ma, and final blueschist and greenschist retrogression at ~38-35Ma, yielding timescale of ~4Ma for the entire history of brecciation and fluid flux associated with the Monviso eclogite breccias. / Thesis (MS) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

No description available.

The Clarno Formation is a series of volcanic, volcaniclastic, and related intrusive rocks located in central Oregon. It is the westernmost extent of a broader Eocene magmatic belt that covers much the western United States. The magmatic belt stretches eastward from Oregon to western South Dakota, and from the Canadian Yukon to northern Nevada. While once attributed to subduction of the Farallon Plate under North America, more recent work suggests that a more complex tectonic regime involving extension was in place during the early Cenozoic. In the vicinity of Mitchell, Oregon, the Clarno Formation is well represented along with Mesozoic metamorphic and sedimentary units, and younger Tertiary volcanic and volcaniclastic units. In this area, Clarno volcanic activity occurred from ~52-42 Ma, producing mostly andesites and related volcaniclastic rocks. The Mitchell area is also underlain by related intrusive bodies ranging from basalt to rhyolite in composition. The Clarno was most active at ~49 Ma, and is dominantly calcalkaline. In addition, there are several coeval alkaline and peraluminous intrusives also scattered throughout the Clamo Formation. While these suites are less voluminous than the calc-alkaline magmatism, they offer insight into the tectonic and magmatic processes at work in this area during the Eocene. Whereas silicic intrusions are common in the Clarno, the high-silica rhyolite dike on the south face of Scott Butte is unusual due to its large garnet phenocrysts. The existence of primary garnet in rhyolitic magmas precludes middle to upper crustal genesis, a common source for silicic magmas. ⁴⁰Ar/³⁹Ar age determinations of the biotite indicate an age of ~51 Ma. This is after andesitic volcanism had commenced, but prior to the most active period of extrusion. The presence of the almandine garnet indicates that the dike represents partial melting of lower crustal (18-25 km) material. The presence of a high field strength element (HFSE) depletion commonly associated with subduction are magmatism indicates that either the source material had previously been metasomatised, or that some subduction melts/fluids (heat source) mixed with the crustal melt. Two alkaline suites, a high-K calc-alkaline basanite (Marshall and Corporate Buttes) and alkaline minette/kersantite lamprophyres (near Black Butte and Mud Creek), were emplaced ~49 Ma, during the height of calc-alkaline activity. The basanite lacks the HFSE depletion common in the other Clarno rocks. Instead it has a HIMU-type (eg. St Helena) ocean island basalt affinity, resulting from partial melting of enriched asthenospheric mantle. In contrast, the lamprophyres represent hydrous partial melts of metasomatized litho spheric mantle veins and bodies. Alkaline magmatism was not limited to the most active periods of calc-alkaline activity. The emplacement of an alkali basalt (Hudspeth Mill intrusion) at ~45 Ma occurred four million years after the largest pulse of volcanism, but still during calcalkaline activity. This alkali basalt represents partial melting of metasomatized lithospheric mantle. The occurrence of these alkaline suites coeval with the calc-alkaline activity is significant in that it disputes prior subduction theories for the broader Eocene magmatism that are based on spatial and temporal variations from calc-alkaline to alkaline magmatism. These suites also give further insight into the complex tectonic regime that existed in Oregon during the Eocene. The occurrence of asthenospheric melts not caused by fluid fluxing, along with lower lithospheric alkaline melts, are normally associated with extension. Extension provides these magmas with both the mechanism for melting, and the ability to reach shallow crust with little or no contamination. Extension is in agreement with both White and Robinson's (1992) interpretation that most Clarno Formation deposition occurred in extensional basins, and with other provinces in the broader Eocene magmatic belt. / Graduation date: 2002

The largest earthquakes on Earth take place on the megathrusts of subduction zones, but the slip behavior of megathrusts is variable. This thesis considers why by conducting local, regional and global studies of the interrelationships between the structure and seismogenic behavior of subduction zones. New marine geophysical data collected from the collision zone between the Louisville Ridge seamount chain with the Tonga-Kermadec trench constrain overthrusting and subducting plate structure. Mo'unga seamount is identified beneath the outer-forearc, which calibrates the association of residual bathymetric anomalies and subducting relief, implies an E-W geometry for the subducted ridge and suggests the 200 km wide Louisville seismic gap is modulated by the sediment filled flexural moat. Spectral averaging is then applied along the Tonga-Kermadec margin and along strike variations in overthrusting plate structure are verified by wide-angle seismic transects. The remnant Tonga-Ridge occupies the inner fore-arc and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110±20 km) and strike (~005° south of 25°S). Plate tectonic reconstructions suggest the Lau Ridge is unmodified by subduction related erosion, <200 km of the Tonga Ridge has been eroded, and neither ridge ever occupied the southern Kermadec arc. Crustal thickness variations are thus inherited, reflecting the Cenozoic tectonic evolution of the Tonga-Kermadec-Hikurangi margin. Spectral averaging is finally applied to all subduction zones on Earth. Part one develops a global catalogue of subducting relief, which is compared with seismological and geodetic inferences of fault-slip behavior. Most seamounts are aseismic, relatively undeformed and observations are not consistent with mechanical models proposing full-decapitation. Aseismic ridges are also associated with megathrust complexity, but are of a larger wavelength and contrasting mode of isostatic compensation. Part two shows almost all intra-margin along-strike transitions in seismogenic behavior are related to pre-existing crustal structure. A paired forearc anomaly is interpreted consisting of a trench-parallel ridge landward of the deep-sea-terrace basin. The ridge crest correlates with the down-dip limit of coseismic slip and strong interplate coupling, the up- dip limit of tremor epicentres, and is interpreted as defining the boundary between the velocity-weakening and seismogenic portion of the subduction interface and the down-dip frictional transition zone. Paired anomalies may be attributed to unrecovered interseismic elastic strain, the preferential subduction erosion of the outer-forearc and/or underplating beneath the inner forearc.