Browsing by Subject "Subduction zone"
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Item An analysis of subduction related tectonics offshore southern and eastern Taiwan(2014-12) Eakin, Daniel Hoyt, Jr.; Lavier, Luc Louis; McIntosh, Kirk D.Arc-continent collision is associated with vigorous mountain building and terrane accretion on relatively short (<10 Ma) geologic timescales. It is believed to be an important mechanism for the growth of continents. Taiwan represents one of the few active examples of this process. As such, is the perfect natural laboratory to investigate the nature of the continent ocean boundary and the uncertain behavior of the accretionary prism and extended, transitional rifted margin crust during the collision process. Taiwan also provides a unique opportunity to investigate structures in the backarc, yielding key insights into the still controversial tectonic conditions that were responsible for the unique subduction-collision system observed today. The obliquity of the collision between the North Luzon Arc and the Chinese rifted margin allows for examination of different temporal stages of collision at different locations. Recently acquired seismic reflection and wide-angle seismic refraction data, offshore Taiwan, document the crustal structure of the incipient mountain belt and of the Philippine Sea Plate in the backarc domain to the east. Geophysical profiles offshore southern Taiwan show evidence for a transition from the subduction of ocean crust to highly extended, transitional continental crust of the northern South China Sea distal margin. During oceanic subduction, accretion and underplating of thick sedimentary cover sequences create a large 13-15 km thick accretionary prism. Prior to the encroachment of the continental shelf, there is evidence for further underplating of transitional distal margin crust to the base of the prism. These findings support a multi-phase collisional model in which early growth of the mountain belt is driven by structural underplating of the previously sedimentary-only accretionary prism with blocks of transitional crust from the distal rifted margin. Geophysical profiles offshore eastern Taiwan show evidence for asymmetric crustal thickening, from 12-18 km, along the entire length of the Gagua Ridge suggesting the West Philippine Basin oceanic crust is underthust beneath that of the Huatung Basin. In this interpretation, the Gagua Ridge was the result of a failed subduction initiation event during the early Miocene that may have existed simultaneously and, for a short time, competed with the Manila subduction zone in accommodating convergence between the Eurasia and Philippine Sea plates.Item Geophysical investigations in the Nankai Trough and Sumatran subduction zones(2011-12) Martin, Kylara Margaret; Gulick, Sean P. S.The 2004 Sumatra-Andaman and the 2011 Tohoku-Oki earthquakes demonstrate the importance of understanding subduction zone earthquakes and the faults that produce them. Faults that produce earthquakes and/or tsunamis in these systems include plate boundary megathrusts, splay faults (out of sequence thrusts), and strike-slip faults from strain partitioning. Offshore Japan, IODP Exp. 314 collected logging while drilling (LWD) data across several seismically-imaged fault splays in the Nankai Trough accretionary prism. I combine LWD resistivity data with a model of fluid invasion to compare the permeabilities of sands. My results indicate that sands within faulted zones are 2-3 orders of magnitude more permeable than similar undisturbed sands. Therefore fault zones are likely to be fluid conduits within the accretionary wedge. Fluids can affect the physical and chemical properties of the faulted material, increasing pore pressures and effectively lubricating the faults. Fluids play an important role in fault slip, but hazard analysis also requires an understanding of fault geometry and slip direction. Both Japan and Sumatra exhibit strain partitioning, where oblique convergence between tectonic plates is partitioned between the megathrust and strike-slip faults proximal to the arc. Offshore Sumatra, I combine profiles from a 2D seismic survey (SUMUT) with previous bathymetry and active seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Along this fault I interpret transpressional flower structures that cut older thrust faults. These flower structures indicate that the modern West Andaman Fault is a right lateral strike-slip fault and thus helps to accommodate the translational component of strain in this highly oblique subduction zone. Offshore the Kii Peninsula, Japan, I analyze a trench-parallel depression that forms a notch in the seafloor just landward of the megasplay fault system, along the seaward edge of the forearc Kumano Basin. Using a 12 km wide, 3D seismic volume, I observe vertical faults and faults which dip toward the central axis of the depression, forming apparent flower structures. The along-strike geometry of the vertical faults makes predominantly normal or thrust motion unlikely. I conclude, therefore, that this linear depression is the bathymetric expression of a transtensional fault system. While the obliquity of convergence in the Nankai Trough is small (~15 degrees), this Kumano Basin Edge Fault Zone could be due to partitioning of the plate convergent strain. The location of the West Andaman Fault and KBEFZ within the forearc may be controlled by the rheology contrast between active accretionary wedges and the more stable crust beneath forearc basins.Item Heat flow variability at the Costa Rica subduction zone as modeled by bottom-simulating reflector depths imaged in the CRISP 3D seismic survey(2012-08) Cavanaugh, Shannon Lynn; Bangs, Nathan Lawrence Bailey; McIntosh, Kirk D.; Barnes, Jaime; Tatham, Robert3D seismic reflection data were acquired by the R/V Langseth and used to extract heat flow information using bottom-simulating reflector (BSR) depths across the southern Costa Rica convergent margin. These data are part of the CRISP Project, which will seismically image the Middle America subduction zone in 3D. The survey was conducted in an area approximately 55x11 km, northwest of the Osa Peninsula, Costa Rica. For the analysis presented here, seismic data were processed using a post-stack time migration. The BSR—a reverse polarity seismic reflection indicating the base of the gas hydrate phase boundary—is imaged clearly within the slope-cover sediments of the margin wedge. If pressure is taken into account, in deep water environments the BSR acts as a temperature gauge revealing subsurface temperatures across the margin. Two heat flow models were used in this analysis. In the Hornbach model BSR depth is predicted using a true 3D diffusive heat flow model combined with Integrated Ocean Drilling Program (IODP) thermal conductivity data and results are compared with actual BSR depth observations to constrain where heat flow anomalies exist. In the second model heat flow values are estimated using the heat flow equation. Uniform heat flow in the region should result in a deeper BSR downslope toward the trench due to higher pressure; however results indicate the BSR is deepest at over 325 meters below the seafloor (mbsf) further landward and shoals near the trench to less than 100 mbsf, suggesting elevated heat flow towards the toe of the accretionary prism. Heat flow values also reflect this relation. In addition to this survey-wide trend, local heat flow anomalies appear in the form of both circular patterns and linear trends extending across the survey, which can be related to mounds, thrust faults, folds, double BSRs, and seafloor erosion imaged in the seismic data. I suggest that these areas of higher local heat flow represent sites where advection of heat from deep, upward-migrating, thermogenically-sourced fluids and/or gases may be taking place. These heat flow trends have implications for not only earthquake nucleation, but also methane hydrate reserve stability.Item Morphology and structure of the accretionary prism offshore North Sumatra, Indonesia and offshore Kodiak Island, USA : a comparison to seek a link between prism formation and hazard potential(2016-08) Frederik, Marina Claudia Geraldina; Gulick, Sean P. S.; Austin, James Albert; Bangs, Nathan L. B.; Lavier, Luc L; Barnes, Jaime D; McNeill, Lisa CSumatra and Kodiak Islands experienced recent megathrust earthquakes with devastating tsunami; recurrence of large earthquakes is predicted. Studies of the accretionary prism offshore of northern Sumatra, 1-7°N and 92-97°E, reveal a steep outer slope (5-12°), a plateau ~100-120 km wide, and a steep inner slope adjacent to the Aceh Basin. Three primary structural zones are consistent along strike where a predominantly landward vergence zone exists from the deformation front for a distance ~70 km landward. An extended landward vergence zone is not common; for northern Sumatra, a seaward dipping rigid backstop may be the reason, which assists subsequent younger accreted sediment to form the observed zone. The prism toe region shows prominent mass failures presumably related to activation of thrust faults and/or the shaking in response to the 2004 Sumatra-Andaman earthquake (Mw 9.1). These seafloor changes suggest that the 2004 rupture energy reached near the accretionary prism toe. The rigid backstop in the inner wedge together with the suggested dynamic backstop within the outer wedge, and the consolidated sediment on the outer slope form a rigid block dynamically, which together allows earthquake rupture to propagate under it and farther seaward toward the Sunda Trench, resulting in enhanced tsunami potential. Along the Aleutian Trench offshore of Kodiak Island, 145-155° W and 55-58° N, exist a distinct horizon, associated with the onset of the Surveyor Fan sedimentation along which the preferred zone forms. Most if not all of the sediment beneath this horizon seemed subducted, smoothing the high relief of the subducting plate. Subduction of large-buried seamounts begins with creation of a proto-thrust zone seaward of the existing deformation front. As a seamount reaches the deformation front, steepening of the prism toe occurs by formation of out-of-sequence thrusts. Upon further subduction, a deformation front jump occurs where the outer limit of proto-thrust zone becomes the new deformation front. This study contributes insights to other subduction zones with similar characteristics such as thick incoming sediment, subducting seamounts, and/or recent megathrust events. This study also underlines the need to establish fundamental time series data sets for mitigation efforts in hazard-prone areas.Item Oxygen isotope evidence for interaction of Franciscan high-grade blocks in the mantle wedge with sediment derived fluids, Ring Mountain (Tiburon) and Jenner Beach, California(2012-08) Errico, Jessica Cori; Barnes, Jaime D.; Cloos, Mark; Catlos, ElizabethOxygen 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.Item Tectonics of the poietic self : the formation of Juan Larrea and Lorenzo García Vega(2015-05) Manning, Sean O'Malley; Salgado, César Augusto; Fierro, Enrique; Wolitz, Seth; Sutherland-Meier, Madeline; Reed, Cory; Jato, MonicaThis dissertation examines the literary discipleship and poetic formation of Juan Larrea (Spain, 1895-1980) and Lorenzo García Vega (Cuba, 1926-2012) through a multidisciplinary model of poietic tectonics that brings together geological, psychoanalytical, and artistic structures of creation. As writers of complex prose works, Juan Larrea’s cultural hermeneutics and Lorenzo García Vega’s fragmentary testimonies appear to have little in common. However, when their trajectories are placed in parallel, a shared evolutive sequence becomes evident. This study concentrates on the first of a proposed two-cycle transformation process, tracing each poet’s psychic and writerly growth from pre-literary adolescence to the publication of their first works. The four chapters correspond to four tectonic stages in magma production as they occur within the geological subduction zone. The first explores the oceanic plate period that I describe, using Harold Bloom’s terminology, as a framing darkness characterized by national crises, unsatisfactory religious educations, and psychological suffering. Compelled to seek a guide out of these environments, the next chapter considers the period of convergence and subduction between the oceanic and continental plates describing the encounters between Larrea and Vicente Huidobro, and between García Vega and José Lezama Lima, as well as the completion of their literary apprenticeships to these maestros. At a certain point during its descent the oceanic plate releases essential elements that initiate an ascent into a magma chamber, equated with Larrea and García Vega’s respective appearances in the literary magazines Favorables París Poema and Orígenes, and a reconciliation of influence with instinct through psychological individuation. The cycle concludes when the magma reaches at the Earth’s surface, and similarly, the publication of two small books of poetry, Larrea’s Oscuro dominio (1934) and García Vega’s Suite para la espera (1948), signaled the eruption of each poietic self, prepared to continue their growth into the writers that they would later become. The multiple points of contact between the two poets and the methodology for identifying such commonalities provide a unique approach to the formation of the artistic personality and offer dialoguing exegeses between the works of two significant, yet understudied authors.