Northern Barbados Research Articles

Taphonomic problems in reconstructing sea-level history from the late Quaternary marine terraces of Barbados

AbstractAlthough uranium series (U-series) ages of growth-position fossil corals are important to Quaternary sea-level history, coral clast reworking from storms can yield ages on a terrace dating to more than one high-sea stand, confounding interpretations of sea-level history. On northern Barbados, U-series ages corals from a thick storm deposit are not always younger with successively higher stratigraphic positions, but all date to the last interglacial period (~127 ka to ~112 ka), Marine Isotope Substage (MIS) 5.5. The storm deposit ages are consistent with the ages of growth-position corals found at the base of the section and at landward localities on this terrace. Thus, in this case, analysis of only a few corals would not have led to an error in interpreting sea-level history. In contrast, a notch cut into older Pleistocene limestone below the MIS 5.5 terrace contains corals that date to both MIS 5.5 (~125 ka) and MIS 5.3 (~108 ka). We infer that the notch formed during MIS 5.3 and the MIS 5.5 corals are reworked. Similar multiple ages of corals on terraces have been reported elsewhere on Barbados. Thus, care must be taken in interpreting U-series ages of corals that are reported without consideration of taphonomy.

Permeability–porosity relationships of subduction zone sediments

Permeability–porosity relationships for sediments from the northern Barbados, Costa Rica, Nankai, and Peru subduction zones were examined based on sediment type, grain size distribution, and general mechanical and chemical compaction history. Greater correlation was observed between permeability and porosity in siliciclastic sediments, diatom oozes, and nannofossil chalks than in nannofossil oozes. For siliciclastic sediments, grouping of sediments by percentage of clay-sized material yields relationships that are generally consistent with results from other marine settings and suggests decreasing permeability as percentage of clay-sized material increases. Correction of measured porosities for smectite content improved the correlation of permeability–porosity relationships for siliciclastic sediments and diatom oozes. The relationship between permeability and porosity for diatom oozes is very similar to the relationship in siliciclastic sediments, and permeabilities of both sediment types are related to the amount of clay-size particles. In contrast, nannofossil oozes have higher permeability values by 1.5 orders of magnitude than siliciclastic sediments of the same porosity and show poor correlation between permeability and porosity. More indurated calcareous sediments, nannofossil chalks, overlap siliciclastic permeabilities at the lower end of their measured permeability range, suggesting similar consolidation patterns at depth. Thus, the lack of correlation between permeability and porosity for nannofossil oozes is likely related to variations in mechanical and chemical compaction at shallow depths. This study provides the foundation for a much-needed global database with fundamental properties that relate to permeability in marine settings. Further progress in delineating controls on permeability requires additional carefully documented permeability measurements on well-characterized samples.

Reanalysis ofin situpermeability measurements in the Barbados décollement

A cased and sealed borehole in the Northern Barbados accretionary complex was the site of the first attempts to measure permeability in situ along a plate boundary decollement. Three separate efforts at Hole 949C yielded permeability estimates for the decollement spanning four orders of magnitude. An analysis of problems encountered during installation of the casing and seals provides insights into how the borehole conditions may have led to the wide range of results. During the installation, sediments from the surrounding formation repeatedly intruded into the borehole and casing. Stress analysis shows that the weak sediments were deforming plastically and the radial and tangential stresses around the borehole were significantly lower than lithostatic. This perturbed stress state may explain why the test pressure records showed indications of hydrofracture at pressures below lithostatic, and permeabilities rose rapidly as the estimated effective stress dropped below 0.8 MPa. Even after the borehole was sealed, the plastic deformation of the formation and relatively large gap of the wire wrapped screen allowed sediment to flow into the casing. Force equilibrium calculations predict sediment would have filled the borehole to 10 cm above the top of the screen by the time slug tests were conducted 1.5 years after the borehole was sealed. Reanalysis of the slug test results with these conditions yields several orders of magnitude higher permeability estimates than the original analysis which assumed an open casing. Overall the results based on only the tests with no sign of hydrofracture yield a permeability range of 10−14–10−15 m2 and a rate of increase in permeability with decreasing effective stress consistent with laboratory tests on samples from the decollement zone.

Northern Barbados accretionary prism: Structure, deformation, and fluid flow interpreted from 3D seismic and well‐log data

AbstractWe reanalyzed 3D seismic reflection and logging‐while‐drilling data from the toe of the northern Barbados accretionary prism to interpret structure, deformation, and fluid flow related to subduction processes. The seafloor amplitude and coherence reveal an abrupt change in the thrust orientation from NNE at the thrust front and north and NNW about 5 km west of the thrust front. These thrust sets are separated by a triangular‐shaped quiet area, which may represent a zone of low strength. The northeast‐trending band of strong negative amplitude and high coherence in the décollement, known to be an interval of arrested consolidation, overlaps the quiet area, suggesting that the arrested consolidation may be related to the lack of thrust imbrication, and thus, vertical drainage for fluid in the accretionary prism. Fractal analysis of the décollement and top of the subducting oceanic basement indicates that the relief of the décollement correlates with the topography of the oceanic basement. Differential compaction of the underthrust sediment overlying the rugged oceanic basement, together with the basement faults that penetrate into the décollement probably caused relief or even faulting in the décollement.

Three‐dimensional numerical simulation of fluid flow and heat transport within the Barbados Ridge accretionary complex

The thickness of the Barbados accretionary prism varies along strike. The increased thickness of the prism to the south is believed to affect the transport of heat by fluids, resulting in anomalously high heat flow at the deformation front. We utilized a coupled three‐dimensional numerical fluid flow and heat transport model to investigate the effect of variable accretionary prism thickness on fluid flow patterns and heat transport in the décollement zone. The variation in sediment thickness produced along‐strike fluid flow in the more permeable décollement. Although surface heat flow is higher for simulations where flow occurs along both the décollement and underthrust sediments, the resultant advective heat transport was not sufficient to appreciably raise steady state surface heat flow at the latitude of the Ocean Drilling Program transect. Simulated temperatures in the décollement were consequently much lower than in situ borehole measurements, suggesting an additional source of warmer fluid. We then investigated the transient hydrologic and thermal response of the system to enhanced fault permeability. Model results provide insight into the degree to which episodic faulting may drive fluid flow and heat transport within subduction complexes. A transient scenario in which décollement permeability was increased two orders of magnitude with fluid flow along both the décollement and underthrust sediments caused the temperature gradient at the location of the drill sites to increase by a factor of two within 7100 years. The results of the three‐dimensional numerical simulations indicate that while both along‐strike advective heat transport and transient fluid flow events should be considered in the heat flow distribution of the northern Barbados accretionary complex, the effects of episodic fluid flow are more significant.

Fluid flow at the toe of convergent margins: interpretation of sharp pore-water geochemical gradients

Fluid expulsion greatly impacts chemical and mass budgets at subduction zones, particularly if it is focused along faults and stratigraphic conduits. Geochemical anomalies centered at décollement zones, such as pore-water freshening and the presence of thermogenic hydrocarbons, indicate long-distance, focused flow of deeply sourced fluids. The sharp gradient of these anomalies below the décollement zone has been interpreted as evidence for recent pulses of fluid flow, initiating a few to tens of ka. However, this interpretation does not consider that underthrust sediments are moving arcward beneath the décollement zone. In addition, upward flow from the compacting underthrust sediments can modify chemical profiles. Here, we use a simple model that couples fluid flow and solute transport to evaluate these sharp chemical gradients. We find that observed geochemical anomalies at the Northern Barbados and Costa Rican subduction zones can be explained either by recent pulses of flow, or by sustained flow along the décollement zone coupled with modest vertical fluid expulsion from consolidating underthrust sediments. The latter interpretation is consistent with estimates of upward flow rate at Costa Rica based on estimated pore pressure gradients and measured permeabilities within the underthrust sediments. One important implication is that recent pulses of flow along fault conduits may not be required to explain the geochemical anomalies. Furthermore, mixing of locally derived fluids flowing upward from the underthrust sediments and deeply sourced fluids flowing along the décollement zone provides an explanation for the observed changes in pore-water freshening along the décollement at Costa Rica.

Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai

At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging‐while‐drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section ∼1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near‐surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (λ) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth.

Décollement depth versus accretionary prism dimension in the Apennines and the Barbados

Along representative cross sections of the Apennines and the Northern Barbados accretionary prisms, we measured the area, the décollement depth, the angle α of the upper envelope and the angle β of the dip of the regional monocline. The continental sections of the Apennines accretionary prism have a deeper décollement 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 km2 and the Northern Barbados Ridge of 100 km2. The total area is a function of the depth of the décollement plane. Therefore, at a given amount of subduction, the deeper the décollement 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 décollement. Despite these differences, the geometry of both décollements 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 décollement occur moving along strike in both accretionary prisms. The geometry of the prisms is further controlled by the different values of α and β, their sum, and the distance of the accretionary prism relative to the subduction hinge.

Controls on localization and densification of a modern décollement: Northern Barbados accretionary prism

Research Article| March 01, 2003 Controls on localization and densification of a modern décollement: Northern Barbados accretionary prism G. Wallace; G. Wallace 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar J.C. Moore; J.C. Moore 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar C.G. DiLeonardo C.G. DiLeonardo 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar Author and Article Information G. Wallace 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA J.C. Moore 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA C.G. DiLeonardo 1Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA Publisher: Geological Society of America Received: 20 Oct 2000 Revision Received: 05 Jun 2002 Accepted: 20 Aug 2002 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2003) 115 (3): 288–297. https://doi.org/10.1130/0016-7606(2003)115<0288:COLADO>2.0.CO;2 Article history Received: 20 Oct 2000 Revision Received: 05 Jun 2002 Accepted: 20 Aug 2002 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. Wallace, J.C. Moore, C.G. DiLeonardo; Controls on localization and densification of a modern décollement: Northern Barbados accretionary prism. GSA Bulletin 2003;; 115 (3): 288–297. doi: https://doi.org/10.1130/0016-7606(2003)115<0288:COLADO>2.0.CO;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 SocietyGSA Bulletin Search Advanced Search Abstract In the Northern Barbados Ridge area, the décollement zone beneath the accretionary prism lies in a lower Miocene radiolarian claystone. Seismic data allow recognition of this continuous stratigraphic horizon seaward of the accretionary prism. Logging-while-drilling (LWD) data show that the radiolarian claystone is of low density relative to surrounding sediments and indicate that the interval densified heterogeneously with progressive deformation. The low-density interval is relatively weak, and possibly overpressured, which together foster décollement zone propagation. Characterization of densification processes in this interval by image analysis shows that reorientation of clays by shear-enhanced consolidation and tectonic overburden is the dominant densification process. The correlation between densification and clay reorientation suggests that predeformational conditions play a deciding role in décollement propagation.3D seismic reflection data show that the topographic relief of the décollement zone arcward of the deformation front is greater than that in the protodécollement zone and is apparently controlled by basement structure. Thus, it is unlikely that the processes forming the décollement zone are truncating relief in the protodécollement surface, but rather sliding over it along a weak stratigraphic interval. Although localization of the décollement may be originally due to low density and weak sediments, strain softening in a smectite-rich lithology and, perhaps, development of a low permeability cap may contribute to the continued deformation in this stratigraphic interval. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Modeling consolidation and dewatering near the toe of the northern Barbados accretionary complex

At the toe of the northern Barbados accretionary complex, temperature and pore water chemistry data indicate that fluid flow is channeled along the décollement and other shallow thrust faults. We examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the décollement. High‐resolution bulk density data from five boreholes show that the décollement is well consolidated at some sites while other sites remain underconsolidated. Underconsolidated décollement behavior is associated with kilometer‐scale negative‐polarity seismic reflections from the décollement plane that have been interpreted to be fluid conduits. We use a coupled fluid flow/consolidation model to simulate the loading response of a 10‐km‐long by 680‐m‐thick slice of sediment as it enters the accretionary complex. The simulations capture 185 ka (5 km) of subduction, with a load function representing the estimated effective stress of the overriding accretionary prism (3.8° taper angle). Simulation results of bulk density in the décollement 3.2 km arcward of the deformation front are compared with observations. The results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior. The scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the décollement from deeper in the complex. Simulated seaward fluxes in the décollement (1–14 cm yr−1) lie between previous estimates from modeling studies of steady state (&lt;1 cm yr−1) and transient (&gt;1 m yr−1) flow. Maximum simulated instantaneous fluid sources (2.5×10−13 s−1) are comparable to previous estimates. The simulations show minor swelling of incoming sediments (fluid sources ∼−3×10−15 s−1) up to 3 km before subduction that may help to explain small‐scale shearing and normal faulting proximal to the protodécollement.

Abundance of smectite and the location of a plate-boundary fault, Barbados accretionary prism

Isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics. The frontal decollement zone of the northern Barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy, fluid flow, chemical interactions, and sediment physical properties affect strain localization. This plate-boundary fault is centered at a lithologic contact between Miocene claystone and fine-grained Oligocene turbidites. The fault zone is nearly 40 m thick (at Ocean Drilling Program Site 948), and its upper part passes through smectite-rich deposits. A sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement. There is a consistent increase in percent smectite with distance above the base of the decollement, but the top of the decollement is poorly defined by clay mineralogy. The intrinsic mechanical weakness of strata with abundant smectite-group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the Atlantic abyssal plain. A second inherited parameter is the local abundance of radiolarians, which contribute to higher than normal porosities. Sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic. The principal cause of excess pore pressure seems to be updip fluid advection; in theory, however, decreases in pore-fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals. The imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions. If fresher pore water migrates to the propagating tip of the decollement, its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more. The location and evolution of the decollement, therefore, are controlled by a complicated interplay of static factors inherited from the abyssal Atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry.

Spatial variations of the décollement/protodécollement zone and their implications: A 3‐D seismic inversion study of the northern Barbados accretionary prism

AbstractWe conducted a 3‐D seismic inversion study to investigate spatial variations of physical properties of the décollement zone (DZ) and protodécollement zone (PDZ) under the northern Barbados accretionary prism. Significant spatial variations of physical properties were observed in the PDZ seaward of the thrust front from the inversion data. The density generally increases southward with a few localized low‐density patches. A lower density commonly corresponds to a thicker PDZ, suggesting that the paleomorphology may at least partially control the variations of the physical properties. Similar low‐density patches were also found in the DZ. These features may be inherited from those of the PDZ and enhanced after subduction through localized arrested consolidation. Under the prism toe, the density of the DZ increases landward. This trend may mainly result from shear‐induced consolidation of the DZ but may also be related to landward increasing tectonic loading. Significant north–south differences in density and, thus, porosity and strength of the PDZ, are observed and these differences may continue into the DZ. A stronger DZ is likely responsible for a larger prism taper observed in the southern area of the prism toe. The larger taper, thus more horizontal shortening, coupled with a thinner sediment sheet above the PDZ in the southern area, may cause a relative retreat of the thrust front and a pronounced change in strike of the sequence thrusts south of seismic Line 690. The north–south differences may ultimately have originated in the approach of a structurally higher segment of the Tiburon Rise. The Tiburon Rise affects regional morphology and, thus, it controls the sedimentation and physical properties of the PDZ. It may also control sediment accumulation above the PDZ. Therefore, the sedimentational change induced by the structural high of the Tiburon Rise, in turn, resulted in structural change of the prism in the southern area.

The role of turbidites on compaction and dewatering of underthrust sediments at the toe of the northern Barbados accretionary prism: new evidence from Logging While Drilling, ODP Leg 171A

Recently obtained Logging While Drilling data sets from ODP Leg 171A enable a detailed correlation of sediments among drill sites and an estimate of the change of bulk densities among them. In this analysis, three different lithologies from the underthrust sediments of the Barbados accretionary prism were picked up and volumetric strains were calculated by comparing bulk densities of correlated intervals. Results showed that the interbedded mudstones in turbidites have experienced larger volumetric strain while massive claystones have not. The mudstone beds, sandwiched both above and below by relatively highly permeable sandstones, are interpreted to have undergone dewatering because laterally continuous sandstones have acted as effective drains for dewatering. The existence of the turbidite sandstones in the underthrust package and the migration of warm fluid along the sandstones might explain why high heat flow is observed at the ocean floor at Site 672, where turbidite sandstones exist, but not at Site 543, where turbidites were not deposited.

Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust

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 × 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.

Dilatant clayey microstructure in the Barbados décollement zone

Dilatant clayey microstructures in cores from ODP Leg 156, which drilled the basal décollement zone of the northern Barbados accretionary prism, are obtained from freeze-dried samples. The microstructure in the scaly fabric portion shows heterogeneous strain characterized by domains with strong preferred orientation of clay minerals and with sigmoidal, spindle, or tube-shaped coarse pores in fabric domains with random particle orientation. These dilatant microstructures have not been observed in clayey sediments from the Barbados décollement zone or other active shear zones. The dilatancy of the microstructure might be formed under the condition of high pore-fluid pressure in the course of shearing clays and due to tectonic stress.

Deformation and dewatering of the subducting plate beneath the lower slope of the northern Barbados accretionary prism

Detailed structural and stratigraphic analysis of a three‐dimensional seismic data set reveals two scales of structural fabric in the westward dipping oceanic basement under the toe of the northern Barbados accretionary prism. The large‐scale fabric consists of three NNE trending structural highs with relief of about 300 m and spacing of 5–6 km. The small‐scale structure is characterized by a series of NNE trending normal faults, resulting in horsts and graben with relief of 50–200 m and spacing of 0.2–2 km. The structural fabric at both scales is preexisting. Structures in the underthrust sediment are mostly secondary, having formed by differential compaction. Three distinctive seismic stratigraphic units are defined in the underthrust sedimentary section. The upper Cretaceous to lower Eocene unit mostly fills in basement lows, resulting in great thickness variations. The thickness of the Oligocene and middle to upper Eocene units is less variable. We estimate that the upper two sedimentary units have lost 25% of their total initial void space owing to the load of the accretionary prism within the first 3.5 km landward of the thrust front. This suggests a current fluid expulsion rate of 1008 m3/yr per kilometer of strike length from the toe. The fluid discharge is expected to increase to 1092 m3/yr within 64,000 years as a thicker sediment section is subducted. Our results also suggest that the basement indirectly controls fluid movement in the underthrust Oligocene unit by creating secondary normal faults that act as major fluid conduits between the overlying décollement and the underlying, more permeable middle to upper Eocene turbidite‐bearing section. Ponded sediments between basement horst blocks are not faulted and may compact more slowly.

Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism

Research Article| September 01, 1998 Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism J. Casey Moore; J. Casey Moore 1Earth Sciences Department, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Adam Klaus; Adam Klaus 2Ocean Drilling Program, Texas A&M University, College Station, Texas 77845 Search for other works by this author on: GSW Google Scholar Nathan L. Bangs; Nathan L. Bangs 3Institute for Geophysics, University of Texas, Austin, Texas 78759 Search for other works by this author on: GSW Google Scholar Barbara Bekins; Barbara Bekins 4U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Christian J. Bücker; Christian J. Bücker 5Niedersaechsisches Landesamt fuer Bodenforschung Geowissenschaftliche, Gemeinschaftsaufgaben (GGA) Stilleweg 2, 30631 Hannover, Germany Search for other works by this author on: GSW Google Scholar Warner Brückmann; Warner Brückmann 6GEOMAR, Christian-Albrechts-Universität, Kiel 24148, Germany Search for other works by this author on: GSW Google Scholar Stephanie N. Erickson; Stephanie N. Erickson 7Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar Olav Hansen; Olav Hansen 8STATOIL, 7501 Stjoerdal, Norway Search for other works by this author on: GSW Google Scholar Thomas Horton; Thomas Horton 9Anadrill Schlumberger, Youngsville, Louisiana 70592 Search for other works by this author on: GSW Google Scholar Peter Ireland; Peter Ireland 9Anadrill Schlumberger, Youngsville, Louisiana 70592 Search for other works by this author on: GSW Google Scholar Candace Olson Major; Candace Olson Major 10Lamont-Doherty Earth Observatory, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Gregory F. Moore; Gregory F. Moore 11SOEST, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar Sheila Peacock; Sheila Peacock 12School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom Search for other works by this author on: GSW Google Scholar Saneatsu Saito; Saneatsu Saito 13Ocean Research Institute, University of Tokyo, Tokyo 164, Japan Search for other works by this author on: GSW Google Scholar Elizabeth J. Screaton; Elizabeth J. Screaton 14Geology Department, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar John W. Shimeld; John W. Shimeld 15Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar Philip Henry Stauffer; Philip Henry Stauffer 1Earth Sciences Department, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Tuncay Taymaz; Tuncay Taymaz 16Jeofizik Mühendisligi Bölümü, Istanbul Teknik Üniversitesi, Maslak, Istanbul 80626, Turkey Search for other works by this author on: GSW Google Scholar Philip A. Teas; Philip A. Teas 1Earth Sciences Department, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Tomochika Tokunaga Tomochika Tokunaga 17Department of Geosystem Engineering, University of Tokyo, Tokyo 113, Japan Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (9): 811–814. https://doi.org/10.1130/0091-7613(1998)026<0811:CPDIAE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation J. Casey Moore, Adam Klaus, Nathan L. Bangs, Barbara Bekins, Christian J. Bücker, Warner Brückmann, Stephanie N. Erickson, Olav Hansen, Thomas Horton, Peter Ireland, Candace Olson Major, Gregory F. Moore, Sheila Peacock, Saneatsu Saito, Elizabeth J. Screaton, John W. Shimeld, Philip Henry Stauffer, Tuncay Taymaz, Philip A. Teas, Tomochika Tokunaga; Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism. Geology 1998;; 26 (9): 811–814. doi: https://doi.org/10.1130/0091-7613(1998)026<0811:CPDIAE>2.3.CO;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 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Elevated fluid pressure and fault zone dilation inferred from seismic models of the northern Barbados Ridge decollement

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.

Episodic and constant flow models for the origin of low‐chloride waters in a modern accretionary complex

Some low‐chloride pore waters observed in accretionary complexes are thought to result from clay dehydration and subsequent migration of the released water along faults or sand layers. We test this hypothesis with a two‐dimensional flow and transport model for a cross section of the northern Barbados accretionary complex. The model flow system is driven by consolidation of the accreted sediments and by fluids from smectite clay dehydration. Steady state simulations result in concentrations that are too high along the décollement fault and too low near the seafloor. In a transient model we simulate buildup and release of fluids by assuming that strain or hydrofracture along the fault causes an instantaneous increase in décollement permeability of 2–3 orders of magnitude. With such an increase, the observed concentrations can be achieved in 100–1000 years. Also pressures along the fault rise to near lithostatic values in 10–100 years and remain high for 1000–10,000 years. This pressure rise may represent a mechanism for sustaining high fault permeabilities long after the initial increase.

Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: implications for fluid migration and fault strength : T.H. Shipley, G.F. Moore, N.L. Bangs, J.C. Moore & P.L. Stoffa, Geology, 22(5), 1994, pp 411–414

Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: implications for fluid migration and fault strength : T.H. Shipley, G.F. Moore, N.L. Bangs, J.C. Moore & P.L. Stoffa, Geology, 22(5), 1994, pp 411–414