Fluid Pressure Buildup Research Articles

CO2 storage and potential fault instability in the St. Lawrence Lowlands sedimentary basin (Quebec, Canada): Insights from coupled reservoir-geomechanical modeling

Coupled reservoir-geomechanical (TOUGH-FLAC) modeling is applied for the first time to the St. Lawrence Lowlands region to evaluate the potential for shear failure along pre-existing high-angle normal faults, as well as the potential for tensile failure in the caprock units (Utica Shale and Lorraine Group). This activity is part of a general assessment of the potential for safe CO2 injection into a sandstone reservoir (the Covey Hill Formation) within an Early Paleozoic sedimentary basin. Field and subsurface data are used to estimate the sealing properties of two reservoir-bounding faults (Yamaska and Champlain faults). The spatial variations in fluid pressure, effective minimum horizontal stress, and shear strain are calculated for different injection rates, using a simplified 2D geological model of the Becancour area, located ∼110km southwest of Quebec City. The simulation results show that initial fault permeability affects the timing, localization, rate, and length of fault shear slip. Contrary to the conventional view, our results suggest that shear failure may start earlier for a permeable fault than for a sealing fault, depending on the site-specific geologic setting. In simulations of a permeable fault, shear slip is nucleated along a 60m long fault segment in a thin and brittle caprock unit (Utica Shale) trapped below a thicker and more ductile caprock unit (Lorraine Group) – and then subsequently progresses up to the surface. In the case of a sealing fault, shear failure occurs later in time and is localized along a fault segment (300m) below the caprock units. The presence of the inclined low-permeable Yamaska Fault close to the injection well causes asymmetric fluid-pressure buildup and lateral migration of the CO2 plume away from the fault, reducing the overall risk of CO2 leakage along faults. Fluid-pressure-induced tensile fracturing occurs only under extremely high injection rates and is localized below the caprock units, which remain intact, preventing upward CO2 migration.

Oil-Gas Transformation Induced Subcritical Crack Propagation and Coalescence in Petroleum Source Rocks

The present work describes a multi-physics model to investigate subcritical propagation of initially oil-filled, sub-horizontal collinear microcracks driven by the excess pressure induced by the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance, propagation duration, crack coalescence and excess pressure in the crack are determined using a finite difference scheme that couples linear elastic fracture mechanics, oil-gas transformation kinetics and an equation of state for the gas. The numerical results for a shale source rock with typical properties show that when the crack spacing parameter b/a0 is greater than 3, where a0 is the half crack length and b the half distance between the crack centers, the cracks do not coalesce, and the duration of gas-driven crack propagation is governed by the transformation kinetics because the oil-gas conversion rate is much slower than the subcritical crack propagation rate. The collinear cracks coalesce for smaller crack spacing and the crack propagation duration may reduce significantly due to crack interactions. The multi-physics model presented in this work together with our previous model for crack propagation during the conversion of solid kerogen to oil indicates that self-propagating microcracks resulting from the buildup of excess fluid pressure during hydrocarbon generation may serve as effective pathways for primary migration of hydrocarbons.

Tracking fluid pressure buildup from focal mechanisms during the 2003–2004 Ubaye seismic swarm, France

Recent studies have shown that the Ubaye seismic swarm that occurred in the French southwestern Alps in 2003–2004 was triggered by fluid overpressures. This contribution provides additional constraints on the temporal and spatial changes in fluid overpressure during this swarm. The orientations of the double‐couple nodal planes of an extended set of 74 focal solutions, spanning the whole 2003–2004 episode, are compared with the regional stress field. Based on a Mohr‐Coulomb analysis, these comparisons provide estimates of fluid pressures along seismic fault planes. We show that the fluid overpressures required to reactivate the cohesionless fault planes vary through time, with values close to 35 MPa at the inception of the swarm. Overpressures then increase up to 55 MPa during the burst of seismic activity and lastly decrease down to 20 MPa at the end of the crisis. We also show that the fluid overpressures are developed as patches along two parallel faults bordering a releasing bend structure characterized by low to null overpressure. The development of moderate fluid overpressure at the swarm inception enables the reactivation of normal, transtensional, and strike‐slip faults while the development of larger fluid overpressures during the burst of seismic activity progressively enables the reactivation of further misoriented normal, transtensional, and transpressional faults. In order to reconcile the spatial and temporal evolution of the fluid overpressures and the seismic activity, we propose that creep compaction could be the process allowing the successive development of fluid overpressure and the migration of seismicity.

Two-domain mechanics of a spherical, single chamber heart with applications to specific cardiac pathologies

Continuum approximations of tissue consider responses averaged over many cells in a region. This simplified approach allows consideration of macroscopic effects, such as deformation or action potential propagation. A bidomain (sometimes known as biphasic) approach retains the macroscopic character of a continuum approximation while allowing one to consider microscopic effects; novel behavior arising from interactions between the intracellular and extracellular spaces can also be noted. I consider a spherical, single chamber heart with the new mechanical bidomain model in four separate pathologies: hypertension, hypovolemic hypotension, and hypertrophic and dilational cardiomyopathies. Analytic solutions of intracellular and extracellular displacements and hydrostatic pressures are presented; the distributions describe elastic deformation and hydrostatic fluid pressure buildup of the extracellular collagen matrix and the intracellular muscle under simplified spherical geometry. Potential applications, such as stretch activated membrane channels, are also noted.

Nonmodified Silica Nanoparticles Decrease Water Invasion Into Atoka Shale

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 146979, ’Decreasing Water Invasion Into Atoka Shale Using Nonmodified Silica Nanoparticles,’ by J. Cai, SPE, China University of Geosciences; M.E. Chenevert, SPE, and M.M. Sharma, SPE, The University of Texas at Austin; and Jim Friedheim, SPE, M-I Swaco, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The complete paper was published in the March 2012 issue of SPE Drilling & Completion, Pages 103-112. Shales account for 75% of the all footage drilled and are responsible for 90% of wellbore-stability problems. Maintaining wellbore stability is one of the more critical aspects of oil and gas drilling. The main cause of shale instability for both soft and hard shales is water absorption and subsequent swelling and sloughing of the wellbore. Laboratory data shows the positive effect of adding commercially available, inexpensive, nonmodified silica nanoparticles (particle size varies from 5 to 22 nm) to water-based drilling muds and their effect on water invasion into shale. Experimental Methods Materials Used . Nanoparticles. A total of 22 different types of nanoparticles were obtained from four manufacturers. Of these samples, only seven—NP-G, NP-D, NP-A, NP-E, NP-B, NP-C, and NP-F—were nonmodified, nonacidic, and commercially available (Table 1). Shale Properties. Shale is a sedimentary rock that mineralogically consists of clays, quartz, and other silicate and carbonate minerals. Because of their high clay content, shales tend to absorb water from a water-based mud, which results in swelling and sloughing of the wellbore. In this study, a hard and preserved Atoka shale was used. Great care was taken to preserve the shale at its native water activity. The shale was not exposed to air for any period of time over a couple of minutes. It was stored in oil, and, during preparations for testing, it was stored in a controlled-humidity desiccator. Use of shales that have dried out or that are reconstituted is not acceptable for the tests that were run in this study. Muds Used. Three types of water-based muds were used in this study: a freshwater mud (FWM), a thick bentonite mud, and a low-solids mud (LSM). It was found that, in all tests, the presence of 10 wt% concentration of nanoparticles reduced the permeability of the shale. Test Equipment for Measuring Shale Permeability. A pressure penetration (PP) test was used to measure shale permeability. The test consisted of three steps. Seawater was first flowed through the shale sample, followed by a base mud, and then by a base mud that contained 10 wt% nanoparticles. The experimental setup (Fig. 1) consisted of several devices used to achieve a continuous flow of the test fluid across the top face of the shale sample while the simulated pore fluid was kept in contact with the lower face of the shale. The simulated pore fluid was a 4 wt% solution of sea salt that had a 0.98 water activity. A test fluid was flowed across the top of a shale sample (Fig. 2) at a constant pressure (approximately 250 psi), and the buildup of fluid pressure in a small sealed chamber located at the bottom of the shale was recorded.

A versatile flow phantom for intravoxel incoherent motion MRI

Although there have been many advancements in cancer research, much is still unknown about the heterogeneous tumor microenvironment. Diffusion-weighted MRI has proven to be a viable and versatile microstructural probe. Diffusion-weighted sequences specifically sensitive to intravoxel incoherent motion (IVIM) have seen a recent resurgence of interest as they promise to provide a valuable window on the vascular microenvironment. To understand, test, and optimize IVIM-sensitive approaches, a complex flow phantom was constructed to mimic certain characteristics of the tumor microenvironment such as tortuous microvasculature, heterogeneous vascular permeability, and interstitial fluid pressure buildup. Results using this phantom on a clinical scanner platform confirmed IVIM sensitivity to microscopic flow effects. Biexponential fitting of signal decay curves enabled quantitative extraction of perfusion fraction, IVIM-related pseudodiffusivity, and tissue diffusivity. Parametric maps were also generated, illustrating the potential utility of IVIM-sensitive imaging in clinical settings. The flow phantom proved to be an effective test-bed for validating and optimizing the IVIM-MRI technique to provide surrogate markers for microvascular properties.

Hydrothermal Breccia Textures and Processes: Lisca Bianca Islet, Panarea Volcano, Aeolian Islands, Italy

Panarea is a largely submarine to partly subaerial Quaternary lava dome complex-stratovolcano with a long-lived, active, shallow hydrothermal system, located in the Aeolian Islands volcanic arc of southern Italy. The emergent top of the volcano forms a small archipelago, made up of calc-alkaline basaltic andesite to rhyolite lava domes (ca. 150–20 ka). We document the facies outcropping on Lisca Bianca islet, Panarea archipelago, based on grain size, clast fabric, and degree of hydrothermal alteration, identifying coherent facies, boulder breccia facies, cobble breccia facies, pebble breccia facies, and pervasively altered andesite facies (alunite-marcasite-sulfur). The breccias all have ubiquitous jigsaw-fit clast textures, and are variably hydrothermally altered. The breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics. The breccias formed through a cyclical process, involving the following: stage 1: progressive build-up of fluid pressure toward the level of the tensile strength of the host andesite; stage 2: incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions; stage 3: pervasive fracturing of the host andesite, leading to an increase in permeability as a network of fractures develops; stage 4: declining pressure, with fluid flow rates that lead to infilling and sealing of fractures by natroalunite, thereby reducing permeability, leading to progressive build-up of fluid pressure again, and the beginning of a new cycle.

Geomechanical stability of the caprock during CO2 sequestration in deep saline aquifers

Sequestration of carbon dioxide (CO2) in deep saline aquifers has emerged as a mitigation strategy for reducing greenhouse gas emissions to the atmosphere. The large amounts of supercritical CO2 that need to be injected into deep saline aquifers may cause large fluid pressure buildup. The resulting overpressure will produce changes in the effective stress field. This will deform the rock and may promote reactivation of sealed fractures or the creation of new ones in the caprock seal, which could lead to escape paths for CO2. To understand these coupled hydromechanical phenomena, we model an axisymmetric horizontal aquifer-caprock system. We study plastic strain propagation patterns using a viscoplastic approach. Simulations illustrate that plastic strain may propagate through the whole thickness of the caprock if horizontal stress is lower than vertical stress. In contrast, plastic strain concentrates in the contact between the aquifer and the caprock if horizontal stress is larger than vertical stress. Aquifers that present a low-permeability boundary experience an additional fluid pressure increase once the pressure buildup cone reaches the outer boundary. However, fluid pressure does not evolve uniformly in the aquifer. While it increases in the low-permeability boundary, it drops in the vicinity of the injection well because of the lower viscosity of CO2. Thus, caprock stability does not get worse in semi-closed aquifers compared to open aquifers. Overall, the caprock acts as a plate that bends because of pressure buildup, producing a horizontal extension of the upper part of the caprock. This implies a vertical compression of this zone, which may produce settlements instead of uplift in low-permeability (k≤10-18 m2) caprocks at early times of injection.

Introduction to special section on Phenomenology, Underlying Processes, and Hazard Implications of Aseismic Slip and Nonvolcanic tremor

This paper introduces the special section on the “phenomenology, underlying processes, and hazard implications of aseismic slip and nonvolcanic tremor” by highlighting key results of the studies published in it. Many of the results indicate that seismic and aseismic manifestations of slow slip reflect transient shear displacements on the plate interface, with the outstanding exception of northern Cascadia where tremor sources have been located on and above the plate interface (differing models of the plate interface there also need to be reconciled). Slow slip phenomena appear to result from propagating deformation that may develop with persistent gaps and segment boundaries. Results add to evidence that when tectonic deformation is relaxed via slow slip, most relaxation occurs aseismically but with seismic signals providing higher‐resolution proxies for the aseismic slip. Instead of two distinct slip modes as suggested previously, lines between “fast” and “slow” slip more appropriately may be described as blurry zones. Results reported also show that slow slip sources do not coincide with a specific temperature or metamorphic reaction. Their associations with zones of high conductivity and low shear to compressional wave velocity ratios corroborate source models involving pore fluid pressure buildup and release. These models and spatial anticorrelations between earthquake and tremor activity also corroborate a linkage between slow slip and frictional properties transitional between steady state and stick‐slip. Finally, this special section highlights the benefits of global and multidisciplinary studies, which demonstrate that slow phenomena are not confined to beneath the locked zone but exist in many settings.

Coupled hydromechanical modeling of CO 2 sequestration in deep saline aquifers

Sequestration of carbon dioxide (CO 2) in deep saline aquifers has emerged as an option for reducing greenhouse gas emissions to the atmosphere. The large amounts of supercritical CO 2 that need to be injected into deep saline aquifers may cause large fluid pressure increases. The resulting overpressure may promote reactivation of sealed fractures or the creation of new ones in the caprock seal. This could lead to escape routes for CO 2. In order to assess the probability of such an event, we model an axisymmetric horizontal aquifer–caprock system, including hydromechanical coupling. We study the failure mechanisms, using a viscoplastic approach. Simulations illustrate that, depending on boundary conditions, the least favorable moment takes place at the beginning of injection. Initially, fluid pressure rises sharply because of a reduction in permeability due to desaturation. Once CO 2 fills the pores in the vicinity of the injection well and a capillary fringe is fully developed, the less viscous CO 2 displaces the brine and the capillary fringe laterally. The overpressure caused by the permeability reduction within the capillary fringe due to desaturation decreases with distance from the injection well. This results in a drop in fluid pressure buildup with time, which leads to a safer situation. Nevertheless, in the presence of low-permeability boundaries, fluid pressure continues to rise in the whole aquifer. This occurs when the radius of influence of the injection reaches the outer boundary. Thus, caprock integrity might be compromised in the long term.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Formation of abnormal high pressure and its application in the study of oil-bearing property of lithologic hydrocarbon reservoirs in the Dongying Sag

The mechanisms of abnormal high pressures are studied in this paper, and it is concluded that the undercompaction, hydrocarbon generation and stratum denudation are obviously effective to fluid pressure buildup. Because of the episodic difference, the hydrocarbon generation and stratum denudation are the main factors influencing oil-gas migration. On the basis of basin evolutionary analysis in the Dongying Sag, it is considered that the undercompaction mainly caused the abnormal pressure before the first denudation by the uplift in Late Paleogene, while hydrocarbon generation was the main factor of abnormal pressure after the denudation. The second denudation occurred in Late Neogene, which changed the pressure field and induced the fluid migration. The development of overpressures is the necessary condition to the formation of lithologic hydrocarbon reservoirs, which have positive correlations to overpressures. According to the fullness of the present reservoirs, the quantitative relations between oil-bearing property and driving forces of reservoir formation were determined, the latter were decided by dynamic source, reservoir capillary pressure, fluid pressure of surrounding rocks and the dynamic attenuation in different conducting systems.

Quasi‐static fault slip on an interface between poroelastic media with different hydraulic diffusivity: A generation mechanism of afterslip

We theoretically study the generation mechanism of afterslip, assuming a two‐dimensional in‐plane fault on a bimaterial interface that separates poroelastic half‐spaces with different hydraulic diffusivities; the deformation is assumed to be quasi‐static. Our study shows that the coseismic faulting triggers quasi‐static fault tip growth effectively because of positive feedback between the evolving fault slip and fluid pressure buildup. A similar study was made by Yamashita (2007), but he assumed the difference to be only in the Biot‐Willis coefficient and undrained and drained Poisson's ratios. A comparison with the model of Yamashita (2007) shows that the diffusivity contrast is much more effective for the generation of afterslip. We also find that the fault tip is likely to extend unilaterally in the direction of slip in the medium of higher diffusivity; the duration of fault growth is longer for a larger diffusivity contrast. We find a scaling relationship in which the moment released by the quasi‐static fault extension is approximated well by a linear function of the duration of fault growth. This is largely different from the expectations from a 2‐D classical dynamic fault model; the moment is proportional to the square of duration of fault growth if the classical fault model is assumed.

Dissociation Behavior of CO2 Hydrate in Sediments during Isochoric Heating

As CO2 is sequestered into sediments in the oceanic environment, CO2 hydrate can form as a byproduct. This study explored the dissociation behavior of CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size. We synthesized CO2 hydrate in three types of particulate sediments: glass beads, fine sand, and crushed silt. We then dissociated them through isochoric heating. We observed the excess pore fluid pressure build-up and self-preservation behavior, in which the pressure-temperature state evolves along the hydrate phase boundary until either it reaches the second quadruple point or all hydrates dissociate. The pore fluid pressure evolution is limited, however, by the CO2 vapor-liquid phase equilibrium boundary due to the liquefaction of CO2. The presence of CO2 liquid in sediments forces the pressure-temperature evolution to follow the CO2 vapor-liquid phase equilibrium boundary, regardless of hydrate formation and dissociation processes. CO2 hydrate in fine-grained sediments experiences capillary pressure-induced melting point depression, but this effect vanishes when the pores exceed approximately 1 microm, such as in coarse-grained sediments. In particular, any fracture generation in sediments which involves the local release of confinement eliminates the melting point depression induced by the capillary effect.

Fault weakening due to CO 2 degassing in the Northern Apennines: short- and long-term processes

Abstract The influx of fluids into fault zones can trigger two main types of weakening process that operate over different timescales and facilitate fault movement and earthquake nucleation. Short- and long-term weakening mechanisms along faults require a continuous fluid supply near the base of the brittle crust, a condition satisfied in the extended/extending area of the Northern Apennines of Italy. Here carbon mass balance calculations, coupling aquifer geochemistry to isotopic and hydrological data, define the presence of a large flux ( c . 12 160 t/day) of deep-seated CO 2 centred in the extended sector of the area. In the currently active extending area, CO 2 fluid overpressures at ∼85% of the lithostatic load have been documented in two deep (4–5 km) boreholes. In the long-term, field studies on an exhumed regional low-angle normal fault show that, during the entire fault history, fluids reacted with fine-grained cataclasites in the fault core to produce aggregates of weak, phyllosilicate-rich fault rocks that deform by fluid assisted frictional–viscous creep at sub-Byerlee friction values (μ<0.3). In the short term, fluids can be stored in structural traps, such as beneath mature faults, and stratigraphical traps such as Triassic evaporites. Both examples preserve evidence for multiple episodes of hydrofracturing induced by short-term cycles of fluid pressure build-up and release. Geochemical data on the regional-scale CO 2 degassing process can therefore be related to field observations on fluid rock interactions to provide new insights into the deformation processes responsible for active seismicity in the Northern Apennines.

Press forming of commingled yarn based composites: The preform contribution

The process of press forming commingled yarn composites is studied and modelled based on the theory of porous media. Two sub processes are considered: (1) the wetting and compaction of individual bundles and (2) the overall preform deformation. An experimental method is introduced, to study these two sub processes separately. The experiment suppresses the fluid pressure build-up, and thus the process of wetting and bundle compaction, by draining the resin into a porous wall. In consequence, the measured response consists only of the preform contribution. Finally, the total response is simulated and tested against independent consolidation experiments. It was demonstrated that the present approach gives better predictions compared to models based on the bundle response only.

Hydraulic, water quality, and isotopic characterization of Late Cretaceous-Tertiary Ardley coal waters in a key test-well, Pembina-Warburg exploration area, Alberta, Canada

Abstract A detailed evaluation of the hydraulic parameters and formation fluids of a key test-well completed in the uppermost Ardley coal was undertaken in the Pembina-Warburg exploration area, Alberta Basin, Alberta, Canada. The testing method isolated the target coal zone from the overlying Paskapoo Formation with an intermediate casing set in the coal. Measurements showed a build-up of fluid (water) pressure within the coal to approximately 1,470 kPa. Detailed hydrogeological testing showed that the permeability of the coal is clearly less than 5 mD and likely on the order of 1 mD. Formation waters sampled during the drawdown and build-up phases were Na-HCO3 type with salinities of approximately 1,920 mg/L TDS (calculated). Sodium and chloride concentrations were approximately 835 and 250 mg/L, respectively. Detailed analyses of the formation waters showed that dissolved and total trace metal concentrations fall within Canadian drinking water quality standards with the exception of iron and manganese. Organic compounds including monocyclic aromatics (Benzene, Toluene, Ethyl-benzene, Xylene, BTEX) were not detected, with the exception of trace toluene concentrations. Anomalously high bicarbonate concentrations of up to 1650 mg/L (δ13CDIC +22.50 ‰) and dissolved methane concentrations of up to 36 mg/L (δ13CCH4 −48.32 ‰) point to the presence of secondary biogenic gas. Formation waters are non-tritiated; δ18O and δ2H values plot along the local groundwater line for Edmonton, Alberta indicating a meteoric origin. Data from the test-well suggest that depressurization (i.e., diversion of fluids) of the coal zone is necessary for gas production. If depressurization is contained to within the coalbeds, relatively small quantities of produced fluids would be anticipated. Although most of the groundwater for consumption within the immediate area of the test well is obtained from shallower parts of the flow systems in the uppermost parts of the Paskapoo Formation, under the current regulatory framework a Preliminary Groundwater Assessment as defined by the Alberta Environment and the EUB (2004) would be necessary prior to diversion.

Mapping the fluid front and pressure buildup using 4D data on Norne Field

Norne Field, in the southern part of the Nordland II area in the Norwegian Sea approximately 100 km north of Aasgard Field, is producing from an FPSO. The main field is a horst block approximately 9 × 3 km (Figure 1). The reservoir rocks are sandstones of Lower and Middle Jurassic age. The hydrocarbon reserves consist of a gas cap (75 m), mainly situated in Garn Formation, and an oil leg (110 m), mainly situated in Ile and Tofte formations. The sandstones are very good quality with porosities and permeabilities of 25–32% and 200–2000 mD, respectively. Net-to-gross is close to 1 for most reservoir zones. Oil production started in 1997. The first 4D seismic survey was acquired in 2001, and 4D information has been actively used in subsequent reservoir management.

Possible resetting of quartz OSL signals during earthquakes—Evidence from late Pleistocene injection dikes, Dead Sea basin, Israel

Clastic dikes are formed either by passive deposition of clastic material into pre-existing fissures (depositional dikes), or by fracturing and injection of clastics during earthquakes (injection dikes). We proposed to use optically stimulated luminescence (OSL) dating to distinguish between the two modes of formation and hypothesized that (1) depositional dikes filled from above show OSL ages younger than the host rock; and (2) injection dikes filled from below show the same OSL ages as that of the host rock. We studied the mechanisms of clastic-dike formation and their ages within the seismically active Dead Sea basin, where hundreds of dikes crosscut the late Pleistocene (∼70–15 ka) lacustrine sediments of the Lisan Formation. Field observations and analysis of magnetic tensors show unequivocally that most of these dikes were emplaced by injection, inferred to be due to seismically triggered fluidization–liquefaction during earthquakes. Twenty-eight samples were collected from the Lisan source material and dikes that, based on field observations, are unmistakably either depositional dikes or injection dikes. Quartz single aliquot OSL ages of the source Lisan layers are between 43 and 34 ka, and are typical for the Lisan Formation. The ages of both depositional and injection dikes are between 15 and 17 ka, younger than the Lisan host rock. Depositional dikes show a highly scattered distribution of single grain ages, suggesting several episodes of infill. Single grain ages of injection dikes are of latest Pleistocene to Holocene, and do not contain recently bleached grains that infiltrated from above. These results imply that the OSL signals were reset at the time of fluidization–liquefaction and buildup of fluid pressure within the injection dikes. If this resetting mechanism has a physical ground, then OSL dating is an important tool for constraining the ages of earthquake-induced injection dikes and recovering paleoseismic data from them.

About the Cover.

Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverMany invertebrates form colonies of genetically identical individuals in which some individuals become specialized for different functions. One such animal is Membranipora membranacea, a sheet-like bryozoan commonly found encrusting kelp blades in Puget Sound and many other temperate coastal areas.The main cover image shows two colonies of M. membranacea, the larger of which is 2 cm in diameter. Each of the rectangular, skeletal boxes in the colony houses an individual zooid that bears a crown of ciliated tentacles. The tentacles generate water currents that bring suspended food particles to the colony. Groups of zooids form openings (chimneys) that allow water to flow out of the colony after it has been filtered through the colony's feeding tentacles to capture food. Zooids in the center of chimneys lack feeding tentacles, but the tentacles of individuals surrounding chimneys are raised above those of their neighbors. Chimneys enhance feeding by preventing fluid pressure build-up within the colony. In the cover picture, chimneys appear as dark patches within the canopy formed by the tentacles.Because of its simple geometry and rapid growth, M. membranacea provides an excellent opportunity to investigate how fluid flow affects the development of fluid transport systems. Previous authors suggested that phenotypic plasticity in chimney spacing could be explained if water flow through the colony controls where chimneys form. To test whether water currents affect chimney formation, Michelangelo von Dassow, in a study described on pages 76-82 in this issue, injected water under the canopy of tentacles to enhance flow out the edge of the colony. In the colony shown in the upper portion of the cover image, a sheet of red light was positioned so that only the raised tentacles around chimneys were lit. As the colony grew outward, a new chimney-like opening formed distal to the polyethylene tube through which water was injected (toward the top of the image). The new opening distal to the site of water injection (right-hand tube) formed sooner than the new opening distal to the control site without water injection (left-hand tube). The ends of the tubes in the upper image were 0.8 cm apart. The observation that enhancing excurrent flow at the colony edge induced the formation of new chimneys supports the hypothesis that fluid flow controls the development of this fluid transport system. These results provide insight into the role of fluid flow in patterning animal colonies and in the development of fluid transport systems.Credits: Photos, Michelangelo von Dassow; Cover layout, Beth Liles (Marine Biological Laboratory). Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 211, Number 1August 2008 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv211n1cover Views: 39 © 2006 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.