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Hemin Koyi - One of the best experts on this subject based on the ideXlab platform.
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influence of Decollement friction on anisotropy of magnetic susceptibility in a fold and thrust belt model
Journal of Structural Geology, 2021Co-Authors: Thorben Schofisch, Hemin Koyi, Bjarne AlmqvistAbstract:Abstract Anisotropy of magnetic susceptibility can provide insights into strain distribution in models simulating fold-and-thrust belts. Models with layers of sand and magnetite mixture shortened above adjacent Decollements with high and low friction, are used to study the effect of Decollement friction on the magnetic fabric. Above high-friction Decollement, an imbricate stack produced a ‘tectonic’ fabric with magnetic foliation parallel to thrusts. In contrast, above the low-friction Decollement deformation propagated farther into the foreland, and deformation intensity is gradual from the foreland to the hinterland by defining a transition zone in between. In this zone, magnetic lineation rotated parallel to the deformation front, whereas in the hinterland the principal axes do not show a preferred orientation due to different deformation mechanisms between “thrust-affected” and “penetrative-strain affected” area. Above both Decollement types, the principal axes of susceptibility developed tighter clustering with depth. Along the boundary between the two Decollements, a deflection zone formed where rotation of surface markers and magnetic fabric reflect the transition between structures formed above the different Decollements. Through quantifying magnetic fabric, this study reemphasises the clear link between Decollement friction, strain distribution and magnitude in fold-and-thrust belts.
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intermediate Decollement activation in response to the basal friction variation and its effect on folding style in the zagros fold thrust belt an analogue modeling approach
Tectonophysics, 2016Co-Authors: Ali Farzipoursaein, Hemin KoyiAbstract:Although the role of various basal and intermediate Decollement levels on structural style is well documented individually in many folded terrains, the interaction between basal and intermediate de ...
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effect of lateral thickness variation of an intermediate Decollement on the propagation of deformation front in the lurestan and izeh zones of the zagros fold thrust belt insights from analogue modeling
Journal of Structural Geology, 2014Co-Authors: Ali Farzipoursaein, Hemin KoyiAbstract:Although the role of various basal Decollement levels on structural style and deformation propagation is well documented in many folded belts, the effect of lateral variation in intermediate Decollements is poorly constrained. This work uses results of three scaled sand-box models shortened from one end to study the variation in structural development between areas with a ductile intermediate Decollement and areas without (or with a thinner) intermediate Decollement. Combined results of scaled models with field observations are used to argue that the presence of mechanically different intermediate Decollement horizons within the Zagros stratigraphy has resulted in deformation partitioning between the Lurestan and Izeh zones. A thick intermediate Decollement facilitates a faster propagation of deformation front and a lower taper in comparison with a thinner (or non-existing) intermediate Decollement during compression. However, the effect of lateral thickness variation in the intermediate Decollement on propagation of deformation is less profound than the effect of mechanical differences in basal Decollements.
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formation of orogen perpendicular thrusts due to mechanical contrasts in the basal Decollement in the central external sierras southern pyrenees spain
Journal of Structural Geology, 2009Co-Authors: Oskar Vidalroyo, Hemin Koyi, Josep Anton MunozAbstract:Two series of analogue models are used to explore the effect of ductile-frictional contrasts of the basal Decollement on the development of oblique and transverse structures during thin-skinned shortening. These models simulate the evolution of the Central External Sierras (CES; Southern Pyrenees, Spain), which constitute the frontal emerging part of the southernmost Pyrenean thrust. The CES are characterised by the presence of N-S to NW-SE anticlines, perpendicular to the Pyrenean structural trend and developed in the hanging-wall of the thrust system. They detach on unevenly distributed Triassic materials (evaporitic-dolomitic interfingering). The models simulated the effect of adjacent ductile versus frictional Decollements during shortening. Model Series A tests the thickness ratio between overburden and the ductile layer, whereas model Series B tests the width (perpendicular to the shortening direction) of frictional Decollement. Model results confirms that deformation reaches further in areas detached on a ductile layer whereas above frictional Decollement areas, shortening is accommodated by additional uplift and penetrative strain. This replicates the structural style of the CES: higher structural relief of N-S anticlines with regard to orogen-parallel structures, absence of a representative ductile Decollement in the core, plunging towards the hinterland and foreland-side closure not thrusted by the South Pyrenean thrust.
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effect of basal friction on surface and volumetric strain in models of convergent settings measured by laser scanner
Journal of Structural Geology, 2008Co-Authors: Faramarz Nilforoushan, Hemin Koyi, Jan Swantesson, Christopher J TalbotAbstract:Abstract This paper uses measurements by a high-accuracy laser scanner to investigate the role of basal friction on surface and volumetric strains in sandbox models simulating fold-thrust belts and accretionary wedges. We monitor progressive deformation, wedge growth, and strain distribution in three models with similar initial boundary conditions but with different basal frictions. Our analyses show that, in addition to influencing the kinematics and geometry of model wedges, basal friction also governs both the surface and volumetric strains of the wedge. After 16.3% bulk shortening, the volume decreased 5 ± 0.5%, 9.5 ± 0.5% and 12.5 ± 0.5% in the models shortened above low, intermediate and high friction Decollements, respectively. Applied to nature, our model results suggest that more compaction and penetrative strain is expected in convergent settings with a high-friction Decollement than those shortened above a low-friction Decollement or a weak basal bed (like the salt formation under parts of the Zagros fold-thrust belt). This volume decrease probably reduces the porosity in the deformed lithologies.
Nathan L Bangs - One of the best experts on this subject based on the ideXlab platform.
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along strike variations in the nankai shallow Decollement properties and their implications for tsunami earthquake generation
Geophysical Research Letters, 2014Co-Authors: Jinoh Park, Hajime Naruse, Nathan L BangsAbstract:Rupture of slow tsunami earthquakes at subduction zones propagates along a shallow plate-boundary fault (i.e., Decollement) nearly all the way to the trench. Seismic reflection profiles reveal that the shallow Decollements have variable reflection characteristics in the Nankai subduction zone, allowing us to divide the subduction zone into impedance-decreasing (inferred to be fluid-rich) and impedance-increasing (fluid-poor) Decollement regions. The fluid-rich Decollement regions with reverse-polarity reflections may play a role as conditionally stable patches because of elevated fluid pressures. In contrast, the fluid-poor Decollement regions with normal-polarity reflections could be unstable seismogenic patches with no unusual fluid pressures. We propose that when megathrust earthquakes nucleate at shallow depth, the small unstable fluid-poor patches are prone to slip. They may also accelerate (velocity-weakening) adjacent large, conditionally stable patches, generating large shallow slip and large tsunamis. As a result, along-strike contrast of fault properties can involve large tsunami earthquakes along the Nankai shallow megathrust fault.
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control of internal structure and fluid migration pathways within the barbados ridge Decollement zone by strike slip faulting evidence from coherence and three dimensional seismic amplitude imaging
Geological Society of America Bulletin, 2006Co-Authors: Christopher G Dileonardo, Casey J Moore, Susan Nissen, Nathan L BangsAbstract:The application of three-dimensional seismic reflection and coherence imaging to the study of the Decollement zone of the Barbados Ridge accretionary complex has provided new insights into the relationships among internal structure, fluid flow, and previously unrecognized strike-slip faulting. Combined coherence and seismic amplitude imaging of the Decollement zone reveal anomalous northeast-trending lineaments parallel to and abutting zones of high- amplitude, negative-polarity reflections. Analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip-slip offset. Isopach mapping of the accretionary wedge indicates significant right-lateral displacement across these structures. These faults apparently channel fluid flow within the Decollement zone, and the prominent northeast- trending conduits so formed are readily visible as high-amplitude, negative-polarity reflections. Additionally, north-northeast– trending zones of variable coherence and high positive amplitude are inferred barriers to up-structure fluid-migration pathways. Movement along strike-slip structures probably alternates with displacement along the Decollement zone. Northeast- trending strike-slip faults extend for >13 km, crossing the length of the survey area and into the incoming oceanic plate. Active arc-oblique strike-slip faulting of the Decollement zone beneath the Barbados Ridge accretionary wedge implies a stress regime in that σ 1 is fixed and σ 2 and σ 3 either transpose with time or are nearly equal. This state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many, as yet unrecognized, arc-oblique strike-slip faults at convergent margins.
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evolution of the nankai trough Decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging
Geology, 2004Co-Authors: Nathan L Bangs, Thomas H Shipley, Gregory F Moore, Sean P S Gulick, Shinichi Kuromoto, Yasuyuki NakamuraAbstract: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 Decollement 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 Decollement-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 Decollement 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 Decollement 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.
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12 physical properties along the developing Decollement in the nankai trough inferences from 3 d seismic reflection data inversion and leg 190 and 196 drilling data
2004Co-Authors: Developing Decollement, Nathan L Bangs, Nankai Trough, Drilling Data, Sean P S GulickAbstract:In the Nankai Trough, the Decollement megathrust initiates within the incoming sedimentary sequence along an interface within the lower Shikoku Basin facies. Scientists on Ocean Drilling Program Legs 190 and 196 penetrated the stratigraphic horizon that develops into the Decollement at a reference site 12 km seaward of the deformation front, Site 1173, and in the Decollement zone, at Site 1174. The core and logging-while-drilling data collected at these sites examine the lateral variation in physical properties prior to and just after initiation of Decollement thrusting. We used a prestack time migrated three-dimensional (3-D) seismic reflection data volume that overlaps with the Leg 190 and 196 drill sites, along with Site 1173 and Site 1174 physical property data, to invert a seismic reflection transect (Line 215) for seismic impedance across the trench from Site 1173 to the deformation front. We used porosity estimated from seismic inversion to infer compaction and dewatering activity within the upper and lower Shikoku Basin facies as the trench wedge overburden rapidly accumulates. A compacted layer (with porosity = ~40%) develops at the upper/lower Shikoku Basin facies boundary and the stratigraphic equivalent of the Decollement zone in the trench landward of Site 1173. The compacted layer may develop because of localized dewatering, diagenetic, or cementation effects within the compacted layer or the overand under1Bangs, N.L.B., and Gulick, S.P.S., 2005. Physical properties along the developing Decollement in the Nankai Trough: inferences from 3-D seismic reflection data inversion and Leg 190 and 196 drilling data. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196, 1–18 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Institute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Road, Austin TX 78759, USA. Correspondence author: nathan@ig.utexas.edu Initial receipt: 1 December 2003 Acceptance: 8 October 2004 Web publication: 21 March 2005 Ms 196SR-354 N.L.B. BANGS AND S.P.S. GULICK PHYSICAL PROPERTIES ALONG THE DEVELOPING Decollement 2 lying layers. We speculate that the compacted layer serves as an aquitard to delay consolidation beneath the horizon that becomes the Decollement zone and contributes to the Decollement development.
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fluid accumulation and channeling along the northern barbados ridge Decollement thrust
Journal of Geophysical Research, 1999Co-Authors: Nathan L Bangs, Casey J Moore, Thomas H Shipley, Gregory F MooreAbstract:A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the Decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 x 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protoDecollement and Decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the Decollement. We construct a regional map of density, and inferred porosity, within the Decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protoDecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the Decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1- to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protoDecollement. This zone can be explained as an area of the Decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the Decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the Decollement.
Casey J Moore - One of the best experts on this subject based on the ideXlab platform.
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control of internal structure and fluid migration pathways within the barbados ridge Decollement zone by strike slip faulting evidence from coherence and three dimensional seismic amplitude imaging
Geological Society of America Bulletin, 2006Co-Authors: Christopher G Dileonardo, Casey J Moore, Susan Nissen, Nathan L BangsAbstract:The application of three-dimensional seismic reflection and coherence imaging to the study of the Decollement zone of the Barbados Ridge accretionary complex has provided new insights into the relationships among internal structure, fluid flow, and previously unrecognized strike-slip faulting. Combined coherence and seismic amplitude imaging of the Decollement zone reveal anomalous northeast-trending lineaments parallel to and abutting zones of high- amplitude, negative-polarity reflections. Analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip-slip offset. Isopach mapping of the accretionary wedge indicates significant right-lateral displacement across these structures. These faults apparently channel fluid flow within the Decollement zone, and the prominent northeast- trending conduits so formed are readily visible as high-amplitude, negative-polarity reflections. Additionally, north-northeast– trending zones of variable coherence and high positive amplitude are inferred barriers to up-structure fluid-migration pathways. Movement along strike-slip structures probably alternates with displacement along the Decollement zone. Northeast- trending strike-slip faults extend for >13 km, crossing the length of the survey area and into the incoming oceanic plate. Active arc-oblique strike-slip faulting of the Decollement zone beneath the Barbados Ridge accretionary wedge implies a stress regime in that σ 1 is fixed and σ 2 and σ 3 either transpose with time or are nearly equal. This state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many, as yet unrecognized, arc-oblique strike-slip faults at convergent margins.
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fracture porosity in the Decollement zone of nankai accretionary wedge using logging while drilling resistivity data
Earth and Planetary Science Letters, 2003Co-Authors: Sylvain Bourlange, Casey J Moore, Pierre Henry, Hitoshi Mikada, A KlausAbstract:Abstract Fracture porosity in the Decollement zone of Nankai accretionary wedge is estimated by comparison of porosity measured on cores during Ocean Drilling Program Leg 131 and porosity calculated from resistivity logs acquired during Leg 196 using Logging While Drilling. Resistivity is converted to formation factor considering both pore fluid conductivity and surface conductivity of clay particles. Pore fluid conductivity is calculated from temperature and ion concentration in interstitial water, whereas surface conductivity is calculated from cationic exchange capacity data and exchangeable cation concentrations. Finally the formation factor is converted to porosity using the generalized Archie’s law. The Decollement appears as a zone of compacted rock where dilatant fractures have developed. The contrast between resistivity–porosity and core porosity is used to estimate fracture porosity in the Decollement, assuming that the total conductivity is the result of fracture network and rock fragment conductivities, behaving as resistors in parallel, in the direction of the fracture network. Fracture porosity increases downward in the Decollement zone from 1.8% to 8.5%. This suggests pore pressure in the Decollement zone is higher than the pore pressure estimated from compaction curves (excess pore pressure ratio of 0.47). A possible explanation is that dilatancy is associated with a high pressure transient. The migration of a pressure wave along the Decollement could occur at a velocity of 500 m/yr if the permeability of the dilated zone is higher than 10−12 m2. The characteristic time for transient dissipation by diffusion in the footwall and hanging wall of the Decollement is estimated to be 100–1000 years. Coexistence of dilatant and compactive shear localization structures is observed within the wedge and in the main fault zones. However, only the Decollement is currently dilated by fluids. We propose that fluids are injected into the Decollement zone during or after fracturing and that initial shear localization is always compactive and occurs ahead of the fluid injections. This sequence of events could occur during each fluid migration and slip event, constituting an increment of Decollement propagation.
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Decollement depth versus accretionary prism dimension in the apennines and the barbados
Tectonics, 2003Co-Authors: Sabina Bigi, Casey J Moore, Federica Lenci, Carlo Doglioni, Eugenio Carminati, Davide ScroccaAbstract:(1) Along representative cross sections of the Apennines and the Northern Barbados accretionary prisms, we measured the area, the Decollement depth, the angle a of the upper envelope and the angle b of the dip of the regional monocline. The continental sections of the Apennines accretionary prism have a deeper Decollement than the oceanic sections of the Northern Barbados, 6-10 km depth and <1 km depth, respectively, because the sediment pile is thinner on the incoming Barbados plate and its denser oceanic structure is more easily subducted. Considering the frontal 50 km, the Apennines have an average cross- sectional area of 500 km 2 and the Northern Barbados Ridge of 100 km 2 . The total area is a function of the depth of the Decollement plane. Therefore, at a given amount of subduction, the deeper the Decollement depth is, the bigger the area of the wedge will result, assuming negligible compaction and erosion. As a consequence, the larger area/volume and higher elevation of the Apennines with respect to the Barbados is determined by the Apenninic deeper Decollement. Despite these differences, the geometry of both Decollements is, in some cases, comparable, in particular, close to the boundary between the crystalline crust and the sediment pile, where the main density and strength contrasts are concentrated. Variations in depth of the Decollement occur moving along strike in both accretionary prisms. The geometry of the prisms is further controlled by the different values of a and b, their sum, and the distance of the accretionary prism relative to the subduction hinge. INDEX TERMS: 8099 Structural Geology: General or miscellaneous; 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); 8122 Tectonophysics: Dynamics, gravity and tectonics; KEYWORDS: Decollement depth, accretionary prism, Apennines, Barbados. Citation: Bigi, S., F. Lenci, C. Doglioni, J. C. Moore, E. Carminati, and D. Scrocca, Decollement depth versus
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fluid accumulation and channeling along the northern barbados ridge Decollement thrust
Journal of Geophysical Research, 1999Co-Authors: Nathan L Bangs, Casey J Moore, Thomas H Shipley, Gregory F MooreAbstract:A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the Decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 x 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protoDecollement and Decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the Decollement. We construct a regional map of density, and inferred porosity, within the Decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protoDecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the Decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1- to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protoDecollement. This zone can be explained as an area of the Decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the Decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the Decollement.
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consolidation patterns during initiation and evolution of a plate boundary Decollement zone northern barbados accretionary prism
Geology, 1998Co-Authors: Casey J Moore, Warner Brückmann, A Klaus, Nathan L Bangs, Barbara A Bekins, Christian J Bucker, Stephanie N Erickson, Olav Hansen, Thomas Horton, Peter IrelandAbstract:Borehole logs from the northern Barbados accretionary prism show that the plate-boundary Decollement initiates in a low-density radiolarian claystone. With continued thrusting, the Decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the Decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the Decollement zone connects to a fluid-rich conduit extending down the Decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.
Gregory F Moore - One of the best experts on this subject based on the ideXlab platform.
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evolution of the nankai trough Decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging
Geology, 2004Co-Authors: Nathan L Bangs, Thomas H Shipley, Gregory F Moore, Sean P S Gulick, Shinichi Kuromoto, Yasuyuki NakamuraAbstract: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 Decollement 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 Decollement-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 Decollement 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 Decollement 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.
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fluid accumulation and channeling along the northern barbados ridge Decollement thrust
Journal of Geophysical Research, 1999Co-Authors: Nathan L Bangs, Casey J Moore, Thomas H Shipley, Gregory F MooreAbstract:A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the Decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 x 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protoDecollement and Decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the Decollement. We construct a regional map of density, and inferred porosity, within the Decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protoDecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the Decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1- to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protoDecollement. This zone can be explained as an area of the Decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the Decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the Decollement.
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elevated fluid pressure and fault zone dilation inferred from seismic models of the northern barbados ridge Decollement
Journal of Geophysical Research, 1996Co-Authors: Nathan L Bangs, Thomas H Shipley, Gregory F MooreAbstract:In 1992, a large volume of three-dimensional seismic reflection data were acquired in a 5 × 25 km area across the toe of the Barbados accretionary complex that covers the Deep Sea Drilling Project leg 78A and Ocean Drilling Program legs 110 and 156 drilling sites. These data are used to examine the acoustic character of the Decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone. Seismic models have been constructed across a 6-km region of the Decollement, where it has been mapped as a moderate to bright polarity-reversed reflection. The models show that this segment of the Decollement reflection is caused by a low-velocity interval, usually 12–16 m thick. The top of the low-velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1.8 km/s to between 1.7 and 1.65 km/s, with some localized bright reflections with an even lower velocity of 1.6 km/s. The base of the low-velocity layer is less certain from modeling. The base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer, or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10-m-thick interval. Comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the Decollement, fluid pressure is at or near lithostatic. Furthermore, the reflection coefficients of the Decollement are sufficiently large that some dilation of the fault zone is required. The dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent Decollement reflection along a 5-km segment of the Decollement. It is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength.
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abnormal fluid pressures and fault zone dilation in the barbados accretionary prism evidence from logging while drilling
Geology, 1995Co-Authors: J C Moore, Thomas H Shipley, Gregory F Moore, David Goldberg, A T Fisher, Yujiro Ogawa, F Filice, Maria Jose Jurado, Alain Rabaute, H YinAbstract:Logs collected while drilling measured density in situ, through the accretionary prism and Decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to >90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal Decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the Decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.
Kohtaro Ujiie - One of the best experts on this subject based on the ideXlab platform.
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strain decoupling across the Decollement in the region of large slip during the 2011 tohoku oki earthquake from anisotropy of magnetic susceptibility
Earth and Planetary Science Letters, 2013Co-Authors: Tao Yang, Kohtaro Ujiie, Toshiaki Mishima, F M Chester, Jim Mori, Nobuhisa Eguchi, Sean ToczkoAbstract:Abstract The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary Decollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake ( M w 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the Decollement. In the frontal prism above the Decollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the Decollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the Decollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary Decollement that is mainly decoupled over the long-term.
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the role of compaction contrasts in sediments in Decollement initiation in an accretionary prism
Marine Geology, 2011Co-Authors: Saneatsu Saito, Kohtaro Ujiie, Toshio Hisamitsu, Hugues Raimbourg, Achim J Kopf, Yozo Hamano, Masataka KinoshitaAbstract:Abstract To understand how Decollements develop into the pristine sedimentary succession entering subduction zones, we have performed mechanical tests on samples from the sediment column entering the Nankai accretionary prism, Japan (ODP site 1173). Both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a ~ 100 m-thick domain in the upper section of the Lower Shikoku Basin unit, i.e. in a domain stratigraphically close to the actual location of the Decollement near the toe of the prism. These property contrasts provide a potential explanation for the outward migration of the Decollement into the incoming sediments. When approaching the deformation front, a given material particle is affected by an increase in stress, which has a component of vertical loading due to the deposition of overburden trench sediment, and also a component of lateral compression transmitted from the accretionary wedge. Depending on its initial mechanical state, the amount of lateral shortening in the incoming Nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone (upper section of the Lower Shikoku Basin at appx. 450–550 m depth) into which the Decollement eventually propagates. Future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure, both at Nankai and along other active margins.
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deformation and fluid pressure variation during initiation and evolution of the plate boundary Decollement zone in the nankai accretionary prism
Journal of Geophysical Research, 2003Co-Authors: Kohtaro Ujiie, Toshio Hisamitsu, Asahiko TairaAbstract:[1] The plate boundary Decollement zone in the Muroto region of the Nankai accretionary prism records deformation and consolidation histories that have been affected by temporal changes in fluid pressure. Microstructural observations and chemical analysis demonstrate that the Decollement zone initiated in an interval of porous clayey sediments characterized by cementation due to intergranular bonding of authigenic clays. Crosscutting relations of microstructures indicate that the Decollement zone records two compactive deformations. The early compactive deformation involved destruction of porous cemented structure, probably caused by fluid pressure fluctuation. The late compactive deformation was characterized by clay-particle rotation and porosity collapse along the sets of slip surfaces, resulting in zones of preferred orientation of clay particles. These compactive deformations led to significantly higher bulk densities within the Decollement zone compared to the compaction trend of the overlying prism sediments. Elevated fluid pressure following compactive deformations induced an overconsolidated state within the Decollement zone, with fluid-filled dilatant fractures. Bulk density abruptly decreases at the top of the underthrust sediments, but there is no microstructural evidence for cementation. Fluids in the dilated fractures and underconsolidated underthrust sediments are potential sources for the elevated fluid pressure in and below the Decollement zone, resulting in mechanical decoupling of the accretionary prism from underthrust sediments. The fault-fluid interactions in the Muroto region may be applicable to other convergent plate margins where high temperature associated with the subduction of a spreading ridge or hot, young oceanic crust enhances diagenesis and cementation.
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evolution and kinematics of an ancient Decollement zone melange in the shimanto accretionary complex of okinawa island ryukyu arc
Journal of Structural Geology, 2002Co-Authors: Kohtaro UjiieAbstract:Detailed structural analysis of the melange in the Shimanto accretionary complex of Okinawa Island in the Ryukyu Arc elucidates the spatial distribution and temporal progression of Decollement-related deformation. Early deformation took place in the footwall of the Decollement and is characterized by subhorizontal layer-parallel extension and subsequent heterogeneous shear of partially lithified sediments, resulting in alternation of asymmetric (e.g. S–C fabrics) and layer-parallel extensional (boudined layers) fabrics. Mud intrusions suggest that high fluid pressure was locally generated in the footwall of the Decollement. Late deformation is marked by the partitioning of deformation in the melange: the lower structural level of the melange was pervasively sheared along microfaults whereas the upper structural level was flattened with development of planar pressure solution cleavage. Because shear is concentrated in the lower structural level, late deformation is considered to have occurred in the hanging wall of the Decollement. Kinematic indicators suggest that shear directions during Decollement-related deformation reflect the relative plate motion during early Tertiary. After the melange was incorporated into the accretionary prism, crenulation cleavage and cylindrical upright folds were formed under subhorizontal shortening.
