The analysis of marsh cores from the tidal zones of the Siuslaw, Umpqua, and Coos River systems on the south-central Oregon coast provides supporting evidence of coseismic subsidence resulting from megathrust earthquakes and reveals the landward extent of the zero-isobase. The analysis is based on lithostratigraphy, paleotidal indicators, microfossil paleotidal indicators, and radiocarbon age. Coseismic activity is further supported by the presence of anomalous thin sand layers present in certain cores. The analysis of diatom assemblages provides evidence of relative sea-level displacement on the order of 1 to 2 m. The historic quiescence of local synclinal structures in the Coos Bay area together with the evidence of prehistoric episodic burial of wetland sequences suggests that the activity of these structures is linked to megathrust releases. The distribution of cores containing non-episodically buried marshes and cores that show episodically buried wetlands within this area suggests that the landward extent of the zero-isobase is between 100 km and 120 km from the trench. The zero-isobase has a minimum width of 10 to 15 km. Radiocarbon dating of selected buried peat sequences yields an estimated recurrence interval on the order of 400 years. The apparent overlapping of the landward margin of both the upperplate deformation zone (fold and/or thrust fault belt) and the landward extent of the zero-isobase is interpreted to represent the landward limit of the locked zone. The earthquake magnitude is estimated to be 8.5 based on an arbitrary rupture length of 200 km and a locked zone width of 105 km. The identification of the zero-isobase on the southcentral Oregon coast is crucial to the prediction of regional coseismic subsidence and tsunami hazards, the testing of megathrust dislocation models, and the estimation of megathrust rupture areas and corresponding earthquake magnitudes in the Cascadia Margin.

Th. doct.--Sci. de la terre et de l'eau--Montpellier 2, 2001. / Mention parallèle de titre ou de responsabilité : [@Contribution à l'étude de la terminaison ouest de la subduction des Ryukyus au niveau de la collision active arc-continent à Taiwan] : apports de la sismique réflexion et de la relocalisation hypocentrale. Bibliogr. p. 271-279. Résumés en français et en anglais.

Dissertation--Fachbereich Geowissenschaften--Mainz--Johannes Gutenberg Universität, 2004. Thèse de doctorat--Terre solide, géodynamique--Montpellier II, 2004. / Mention parallèle de titre ou de responsabilité : Contraindre l'évolution tectonique des systèmes de faille extensive dans les Cyclades (Grèce) en utilisant la thermochronologie de basse température. Thèse soutenue en co-tutelle. Bibliogr. Titre provenant de l'écran-titre.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 127-141).

The Franciscan Complex and Coast Range Ophiolite (CRO) are juxtaposed along the Coast Range Fault (CRF), which is steeply dipping to near vertical in the Stonyford area. The CRF has been interpreted as a thrust fault and a normal fault but no kinematic data has been presented for the Stonyford region. The CRO locally is internally disrupted and can be described as an ophiolitic mélange. Near Stonyford, serpentinites are in contact with Great Valley sediments to the east and with Franciscan rocks to the west. Mafic volcanics are only found at a few localities with some chert and gabbros. Massive serpentinites form most of the southernmost transect while foliated serpentinite mélange dominates the northern transects. Six structural geologic transects were made in the CRO along National Forest Service roads in the Mendocino National Forest near the Stonyford, California area. Data were collected from 21 road cuts totaling approximately 10 kilometers of CRO exposure. Exposures were typically two meters high with the main exception along Goat Mountain Road where the serpentinite was massive with outcrop heights of 10 to 20 meters. Fault plane orientations and sense of slip (where recognizable) were measured for all faults traceable for more than 10 cm. A total of 1,108 faults were measured, 414 contained lineations, and 326 had lineations with steps which determine sense of slip. Approximately two-thirds of the faults with full kinematics had evidence for normal offset. About 25% recorded reverse offset, mostly steeply dipping surfaces. Strike-slip faulting, both right and left-lateral, accounted for 10% of the data. The ascent of the Franciscan and CRO, and upturning of the Knoxville Formation (Great Valley Group) to near vertical attitude was mostly a result of normal faulting. The Great Valley Group strata, with little internal offset by faulting, indicates the disruption of the CRO near Stonyford predates most of the normal faulting. This is consistent with pre-subduction deformation of the CRO in an oceanic fracture zone. / text

Oxygen isotopes and major and trace element geochemistry have been used to evaluate the geochemical and tectonic history of a Franciscan hornblende-amphibolite and a eclogite block from Ring Mountain, Tiburon and three eclogite/blueschist blocks from Jenner Beach, California, all blocks have experienced varying amounts of retrogression. Relative to the presumed basaltic protolith, enrichments in large ion lithophile elements (LILEs) indicate interaction with sediment derived fluids in the retrograde eclogite and retrograde blueschist samples and high Mg, Cr, and Ni in actinolite rind indicate interaction with ultramafic rock. The [delta]¹⁸O values of chlorite from the Ring Mountain hornblende-amphibolite and the eclogite block have a narrow range of [delta]¹⁸O values (+7.7-8.2%₀, n=8) and actinolite from actinolite rind on the eclogite block from Ring Mountain and the blueschist/eclogite blocks from Jenner Beach are (+7.8-8.5%₀, n=5). Chlorite-actinolite geothermometry yields temperatures of 200-280°C for actinolite rind formation. Additionally, the [delta]¹⁸O values of both chlorite and actinolite at these temperatures indicates equilibrium with the measured value of Tiburon serpentinites, (7.6 to 8.1%₀, n = 3 Wenner and Taylor, 1974). Oxygen isotope analyses of garnet mineral separates from the eclogite and hornblende-amphibolite from Ring Mountain have [delta]¹⁸O values of +6.8±0.3%₀ (n=7), and +8.2±0.2%₀ (n=7), respectively. Garnets from the three eclogite/blueschist blocks at Jenner Beach have a [delta]¹⁸O value of +9.8±0.7%₀, (n=23). The difference in [delta]¹⁸O values of garnets between the high-grade blocks is likely due to in situ hydrothermal alteration of the seafloor basalt prior to subduction. The geochemical trends can be explained by a model in which during the early stages of subduction pieces of altered oceanic crust are detached from the downgoing slab and incorporated into the mantle wedge soon after reaching peak eclogite or amphibolite facies conditions. As subduction continues, the hanging wall cools and fluids released from subducted sediments infiltrate the overlying mantle wedge. As the blocks cool they develop a retrograde blueschist facies overprint under relatively static conditions. With cooling of the hanging wall and infiltration of sedimentary fluids, serpentinization induces reaction between the blocks and surrounding mantle wedge and Mg-rich actinolite rind is formed. The blocks are then plucked from the mantle wedge and incorporated into the subduction channel where they flow back to the surface via corner flow. / text

No description available.

This dissertation explores subduction zone-related deformation both on short time scales in the form of subduction zone earthquakes and over larger time and geographical scales in the form of subduction rollback or detachment of the subducting lithosphere. The study presented here is composed of two parts. First, we analyzed the source-rupture processes of the April 1, 2007 Solomon Islands Earthquake (Mw=8.1) using a body-wave inversion technique. Our analysis indicated that the earthquake ruptured approximately 240 km of the southeast Pacific subduction zone in two sub-events.In the second part of this study, we used shear-wave splitting analysis to investigate the effects of the subducting African lithosphere on the upper-mantle flow field beneath the Anatolian Plate in the Eastern Mediterranean region. Our shear-wave splitting results are consistent with relatively uniform southwest-directed flow towards the actively southwestward-retreating Aegean slab. Based on spatial variations in observed delay times we identified varying flow speeds beneath Anatolia and we attribute this variation to the differential retreat rates of the Aegean and the Cyprean trenches.Finally, we used teleseismic P-wave travel-time tomography to image the geometry of the subducting African lithosphere beneath the Anatolia region. Our tomograms show that the subducting African lithosphere is partitioned into at least two segments along the Cyprean and the Aegean trenches. We observed a gap between the two segments through which hot asthenosphere ascends beneath the volcanic fields of western Anatolia. Our results show that the Cyprean slab is steeper than the Aegean slab. We inferred that this steep geometry, in part, controls the flow regime of asthenosphere beneath Anatolia causing variations in flow speeds inferred from shear-wave splitting analysis.

This dissertation explores the uses for data collected at broadband seismic stations to investigate upper mantle structures. In the Barents Sea region, we use seismic waveform modeling on data collected from arrays in Norway and Finland to investigate the nature of the Hales discontinuity in this area. We find that the unusually high velocities required by the move-out of the diffracted first arrival requires a discontinuity below the Moho, which we believe is probably caused by a phase transition from spinel to garnet peridotite. In Chile and Argentina, we use data collected during the Chile Argentina Geophysical Experiment to perform a regional travel time tomography in order to investigate the nature of the mantle above this unusual subduction zone. The northern half of the study area (between 30° and 33°S) is characterized by the central Chilean flat slab segment, where the descending Nazca slab dives to 100 km depth and then flattens, traveling horizontally for hundreds of kilometers before resuming its descent into the mantle. The Nazca plate in the southern half of the study area has a relatively constant dip of about 30°. The southern half exhibits normal arc volcanism roughly above the 100 - 125 km contours of the downgoing slab. The northern half has had no active volcanism in the past 2 Ma, and underwent an eastward displacement of arc volcanism beginning ~10 Ma. The northern half is also remarkable for the basement-cored uplifts of the Sierras Pampeanas. Our study of the upper mantle above the southern half indicates low P wave velocities, low S wave velocities, and high Vp/Vs ratios below the arc, consistent with partial melt. Above the flat slab segment we find low Vp, high Vs, and low Vp/Vs ratios. While the nature of the material responsible for these velocities cannot be uniquely determined, the velocities indicate it must be dry, cold, and depleted. In the transition from flat to normal subduction geometries, we find velocities consistent with frozen asthenosphere, which may have been displaced by the advancing flat slab during the Miocene.

Au travers de l'étude pétrologique, géochronologique et structurale du dôme éclogitique du Tso Morari (E-Ladakh, Himalaya), les processus d'exhumation des roches de HP-BT sont discutés. La découverte d'éclogites à glaucophane, de métasédiments à jadéite-chloritoïde et de métagranites éclogitisés, implique la subduction du dôme du Tso Morari à plus de 70 km de profondeur (20 ± 3 kbar ; 580 ± 50°C). Son exhumation s'accompagne d'une décompression quasi-isothermale jusqu'à 40-30 km. Puis elle est associée à une augmentation de température (630 ± 30°C), et s'achève dans le faciès des Schistes Verts. Les unités adjacentes au dôme du Tso Morari sont peu métarmorphiques (faciès Schistes Verts ) et de nature différente. La chimie des basaltes montre une origine d'avant arc pour l'ophiolite de Nidar et d'OIB pour les unités de Drakkarpo et Ribil ; les roches basiques du Tso Morari sont au contraire des tholéiites continentales. L'origine indienne du Tso Morari est confirmée par les âges des orthogneisses à 458-457 Ma en Sm/Nd et Rb/Sr. La subduction de la marge indienne, est datée à 60-55 Ma par U-Pb et Lu-Hf. L'exhumation débute rapidement (≥ 4mm.an-1) entre 55 ± 7 Ma (Sm-Nd sur Grt-Gln-RT) et 48-45 Ma (Rb/Sr et 39Ar/40Ar sur des métapélites rétromorphosées), en contexte de subduction. L'exhumation se poursuit plus lentement (≈ 2 mm.an-1) de 48-45 Ma à 30 ± 1 Ma (âges 39Ar/40Ar sur micas), en contexte de collisions. Les structures (D1-D2), liées à l'extrusion verticale du dôme, sont indépendantes de celles des unités adjacentes. La transition entre D1, témoin d'un raccourcissement horizontal, et de D3 associé à du raccourcissement vertical , passe par un régime de déformation en constriction (D2). Les changements pétrologiques, structuraux et géochronologiques sont corrélés à des changements de géométrie à l'échelle des plaques. L'exhumation du Tso Morari débute par extrusion verticale à travers le coin mantéllique serpentinisé, en contexte de subduction continentale oblique. Puis le dôme est exhumé plus lentement à travers la croûte, à la faveur du sous-plaquage du cristallin du Haut Himalaya sous le Tso Morari, provoquant un épaississement crustal important, en contexte de collision. A partir de cette évolution, defférents modèles d'exhymations sont discutés