Browsing by Subject "Accretionary wedge"
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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 Stress, porosity, and pore pressure in fold-and-thrust belt systems(2018-10-12) Gao, Baiyuan; Flemings, Peter Barry, 1960-; Nikolinakou, Maria; Saffer, Demian; Lavier, Luc; Cardenas, BayaniI use forward geomechanical modeling to study the mechanical and fluid flow behaviors in compressional regions such as fold-and-thrust belts and accretionary wedges. Under drained conditions, lateral tectonic loading increases the mean effective stress and deviatoric stress and drives the sediments to shear-failure as the sediment approaches the deformation front (or trench location). The shear-induced porosity-loss accounts for about one third of the total porosity-loss during tectonic loading under drained conditions. There are four characteristic stress regions in my model: far-field, transition, critical state wedge, and footwall. In the transition zone, the shear-stress ratio varies significantly and the stress state changes from uniaxial-strain compression state to critical state. Increasing the basal friction coefficient leads to a higher surface slope angle and more porosity loss in the footwall whereas increasing the sediment internal strength leads to a lower surface angle and more porosity loss in the hanging wall. Fluid flow has a great impact on stress and compression in fold-and-thrust belts. My models show that the hanging-wall overpressure is greater than the footwall near trench but lower than the footwall overpressure towards the inner wedge. The high hanging-wall overpressure near trench is cause by the rapid increase of total mean and deviatoric stress. A significant finding is that the high overpressure near trench reduces the vertical effective stress and causes the décollement to be weak near the frontal wedge. Low permeability and high convergence rate promote overpressure generation and enlarge the overpressure-weakened decollement region. This study has broad impacts on the earthquake studies, faults stability analysis, and topics associated fluid flow transport in compressional margins.