Origin and Evolution of the Decollement Zone in the Nankai Trough Accretionary Prism
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This paper presents results of an interdisciplinary investigation of the relation between fluids, fluid flow, and deformation in the toe region of the Nankai accretionary prism.The techniques include thin-section petrography, SEM, TEM and microprobe analyses, and X-ray computed tomography as well as laboratory experiments.Together, the data suggest three structural/hydrologic regimes within the prism.These are: (1) the accreting sediments above the décollement zone, (2) the décollement zone, and (3) the underthrust sediments.The regime above the décollement is characterized by sediments that are progressively dewatered through both a penetrative fabric and a pervasive, but apparently poorly interconnected, set of core-scale deformation structures.The décollement is characterized by a relatively high density of structures/meter and is considered to be a regime of low stress but frequent failure.Hydrologically the décollement retards the vertical flow of fluids and enhances the potential for overpressuring in the footwall.Finally, the footwall regime contains very few tectonic structures and is structurally isolated from the stresses related to plate convergence.This regime provides an important component to the tectonics of the Nankai prism, however, because it supplies the overpressured fluids that cause the décollement to fail at relatively low shear stresses.
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Research Article| April 01, 2004 Evolution of the Nankai Trough décollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging Nathan L. Bangs; Nathan L. Bangs 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Search for other works by this author on: GSW Google Scholar Thomas H. Shipley; Thomas H. Shipley 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Search for other works by this author on: GSW Google Scholar Sean P.S. Gulick; Sean P.S. Gulick 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Search for other works by this author on: GSW Google Scholar Gregory F. Moore; Gregory F. Moore 2Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA Search for other works by this author on: GSW Google Scholar Shinichi Kuromoto; Shinichi Kuromoto 3Center for Deep Earth Exploration, Japan Marine Science and Technology Center, 2-15, Natshusima-cho, Yokohama, Kanagawa 237-0061, Japan Search for other works by this author on: GSW Google Scholar Yasuyuki Nakamura Yasuyuki Nakamura 4Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Nathan L. Bangs 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Thomas H. Shipley 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Sean P.S. Gulick 1Institute for Geophysics, University of Texas, 4412 Spicewood Springs Road, Austin, Texas 78759, USA Gregory F. Moore 2Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA Shinichi Kuromoto 3Center for Deep Earth Exploration, Japan Marine Science and Technology Center, 2-15, Natshusima-cho, Yokohama, Kanagawa 237-0061, Japan Yasuyuki Nakamura 4Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan Publisher: Geological Society of America Received: 01 Oct 2003 Revision Received: 26 Dec 2003 Accepted: 06 Jan 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (4): 273–276. https://doi.org/10.1130/G20211.2 Article history Received: 01 Oct 2003 Revision Received: 26 Dec 2003 Accepted: 06 Jan 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Nathan L. Bangs, Thomas H. Shipley, Sean P.S. Gulick, Gregory F. Moore, Shinichi Kuromoto, Yasuyuki Nakamura; Evolution of the Nankai Trough décollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging. Geology 2004;; 32 (4): 273–276. doi: https://doi.org/10.1130/G20211.2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We mapped the amplitude of the Nankai Trough subduction thrust seismic reflection from the trench into the seismogenic zone with three-dimensional seismic reflection data. The décollement thrust forms within the lithologically homogeneous Lower Shikoku Basin facies along an initially nonreflective interface. The reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section. A décollement-amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25–30 km landward. Three other observations coincide with the amplitude decline: (1) the décollement initially steps down to deeper stratigraphic levels, (2) the wedge taper increases dramatically, and (3) the thrust becomes seismogenic. The amplitude decline and the coincident décollement and accretionary- wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Echelon formation
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The structural stress field of the Okinawa Trough and the accretionary prism of the Eurasian Plate is simulated by means of the finite element method.The fault development of the finite element models is determined according to the Mohr-Coulomb criterion.The influence of the extensional process of the continental lithosphere and the convergence process of the Philippine Sea Plate to the stress field and fault development within the continental plate verge is studied quantificationally.The spreading diaplacement along the crustal bottom results in normal faults in the trough,and finally leads to the formation of the Okinawa Trough.The subduction of the PHS plate causes the thrust development within the accretionary prism of the Eurasia plate.The experiment is in agreement with the earthquake focal mechanism in the study area.
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Seismic profiles across the eastern Nankai accretionary prism show evidence for diffuse deformation through stratal thickening and uplift of the accreting sediment package, thought to reflect the combination of small-scale ductile and brittle strains evident within drill cores. Using a kinematic solution based on changes in stratal thickness and porosities, diffuse strains are estimated for a transect across the eastern Nankai accretionary prism toe, in the vicinity of ODP Site 808. Calculated element displacements are used to reconstruct the undeformed configuration of the prism toe, providing a new method for balancing and restoring deformation in accretionary prisms. The results of this analysis demonstrate a heterogeneous distribution of strain within the prism toe, which appears to correlate with the distribution of brittle deformation structures in drill cores. The greatest vertical tectonic thickening and horizontal shortening estimates are obtained within the deepest sediments, which also display abundant brittle shears. Shallower sediments exhibit high volume loss and lower horizontal shortening, and in drill cores display very few deformation structures. This spatially variable strain distribution may result from inferred high pore pressures near the frontal thrust and décollement inducing a brittle overprint of previous ductile strains.
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Abstract The Nankai Trough in Southwest Japan exhibits a wide spectrum of fault slip, with long-term and short-term slow-slip events, slow and fast earthquakes, all associated with different segments down the plate interface. Frictional and viscous properties vary depending on rock type, temperature, and pressure. However, what controls the down-dip segmentation of the Nankai subduction zone megathrust and how the different domains of the subduction zone interact during the seismic cycle remains unclear. Here, we model a representative cross-section of the Nankai subduction zone offshore Shikoku Island where the frictional behavior is dictated by the structure and composition of the overriding plate. The intersections of the megathrust with the accretionary prism, arc crust, metamorphic belt, and upper mantle down to the asthenosphere constitute important domain boundaries that shape the characteristics of the seismic cycle. The mechanical interactions between neighboring fault segments and the impact from the long-term viscoelastic flow strongly modulate the recurrence pattern of earthquakes and slow-slip events. Afterslip penetrates down-dip and up-dip into slow-slip regions, leading to accelerated slow-slip cycles at depth and long-lasting creep waves in the accretionary prism. The trench-ward migrating locking boundary near the bottom of the seismogenic zone progressively increases the size of long-term slow-slip events during the interseismic period. Fault dynamics is complex and potentially tsunami-genic in the accretionary region due to low friction, off-fault deformation, and coupling with the seismogenic zone.
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A series of finite element models are used to simulate the evolving paleostress field within an accretionary prism during Late Cretaceous accretion in the eastern Aleutian arc-trench system,Alaska.The Late Cretaceous accretion is modeled with two types of accretionary wedges of different length and width. The rheology of the accretionary prism is assumed to be elasticity.The stress field is calculated by the finite element program.Fault development is predicted according to Mohr-Coulomb criterion.The subduction of the Pacific plate beneath the North American plate and associated underplating of oceanic crust marks the boundary condition of finite element model.The finite element modeling suggests that the paelostress field within the accretionary prism evolved during Late Cretaceous accretion.During the initial stage of accretion,paleostress σ1 is compressive and nearly horizontal and thrust faults develop within the accretionary prism.Underplating of the oceanic crust causes a reorientation of the paleostress axes within the accretionary prism.Normal faults develop at the base of the accretionary wedge,and strike-slip faults develop in the deeper portion of the oceanic crust.The underplating process possibly results in lateral accretion and thickening of the accretionary wedge.During the final stage of accretion,paleostress σ1 is compressive and nearly vertical at the toe of the wedge,as the Late Cretaceous sediments are poorly consolidated.However,σ1 is compressive and subhorizontal and thrust faults develop at the toe of the wedge for the well consolidated sediments.The degree of lithification of Late Cretaceous sediments controlled the paleostress distribution and fault development at the toe of the accretionary wedge.
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