Edge Of Block Research Articles

"In vitro" dissolution of coral in peritoneal or fibroblast cell cultures.

Previous studies have shown that in vivo coral resorption involves a biphasic process: First, the edges of the coral block become powdery, then extracellular fluid and phagocytosis contribute to the dissolution of the crystals. The authors examined some types of cells that could be involved in phagocytosis, particularly the ability of both dermal fibroblasts and mouse-resident peritoneal cells to phagocytose and dissolve coral powder "in vitro". Radioactive coral was incubated for 24, 48, or 72 hrs with cells in the presence or absence of cytochalasin B (a phagocytic inhibitor) or chloroquine (a lysosomotropic agent). Furthermore, to specify the role of crystal cell contacts in the solubilization process, they incubated radioactive coral in conditioned media (obtained from two-day human fibroblastic or macrophagic cell culture in the presence or absence of non-radioactive coral) or at a distance from the cells using culture inserts. Measurements of the radioactivity in the different supernatants were performed. Transmission electron microscopy was carried out on the cells cultivated in the presence or absence of radioactive coral. The data suggest that both fibroblasts and macrophages dissolve the coral, and that the intracellular degradation in phagolysosomes is one of the mechanisms explaining coral powder dissolution.

AERODYNAMIC NOISE GENERATION BY A STATIONARY BODY IN A TURBULENT AIR STREAM

When a vortex street formed at the trailing edge of an upstream plate is arranged to impinge upon the leading edge of a small downstream block of similar thickness, a very powerful aerodynamic noise source of tonal quality is formed. Such an arrangement in which the down stream block is suspended on taut wires has been used to investigate Curle's prediction of the relation between the fluctuating lift force exerted by the block on the stream and the radiated sound power in the frequency domain for flow speeds ranging from 70 m/s to 190 m/s.The possible effect of additional stiff constraint to the motion of the clock in the direction of the fluctuating lift force on the radiated noise has also been investigated. The apparent effect of the additional constraint was to increase the magnitude of the fluctuating force on the radiating block but in either the case of constrained or unconstrained block the measured sound power data tends to agree with Curle's prediction at flow speeds above 140 m/s but gradually to diverge upward at lower flow speeds toward a prediction based upon a small vibrating sphere. In all cases the measured sound power levels tend to be bounded by Curle's prediction below and 9·5 dB above by a prediction based upon a small vibrating sphere.

Monitor unit calculation in 6 MV irregularly shaped beams--accuracy in clinical practice.

The results of an investigation of the accuracy of monitor unit (MU) calculation in clinical shaped beams are presented. Measured doses at the reference depth on the beam central axis (isocentre) or on a beam axis representative of the irradiated area (when the isocentre lies under a block or near the edges of the block's shadow) were compared with the expected doses when calculating MUs, by applying different methods normally used in clinical practice. Empirical (areas weighted, Wrede) and scatter summation (Clarkson) methods as well as a pencil-beam based algorithm were applied. 40 irregular fields (6 MV X-rays, CLinac, Varian 6/100), divided into six categories, were considered. Dose measurements were performed with a NE2571 ionization chamber in an acrylic 30 x 30 x 30 cm3 phantom. The depths in acrylic were converted into water-equivalent depths through a correction factor derived from TMR measurements. The method of dose measurements in acrylic was found to be sufficiently accurate for the purpose of this study by comparing expected and measured doses in open square and rectangular fields (mean deviation +0.2%, SD = 0.5%). Results show that all the considered methods are sufficiently reliable in calculating MUs in clinical situations. Mean deviations between measured and expected dose values are around 0 for all the methods; standard deviations range from 1% for the Wrede method to 0.75% for the pencil-beam method. The differences between expected and measured doses were within 1% for about 3/4 of the fields when calculating MUs with all the considered methods. Maximum deviations range from 1.6% (pencil-beam) to 3% (Wrede). Slight differences among the methods of MU calculation were revealed within the different categories of blocked fields analysed. The surprising agreement between measured and expected dose values obtained by using empirical methods (area weighted and Wrede) is probably due to the fact that the reference points were positioned in a "central" region of the unblocked areas.

Pain sensitivity, exercise and stoicism.

Habitual recreational runners engage in strenuous physical activity undeterred by fatigue, pain, or adverse environmental conditions. Some continue to run despite stress fractures1 or the onset of myocardial infarction2. Even without injuries, however, regular training produces regular discomfort. These observations raise two questions: are athletes generally less sensitive to pain than normally active persons, and does exercise reduce pain sensitivity? As the presence or magnitude of allegedly noxious natural stimuli cannot be controlled, laboratory pain tests are employed. By presenting uniform, calibrated noxious stimuli to athletes and controls, or to athletes under different experimental conditions, one can reasonably study differences in pain responsivity. This paper reviews evidence of differences in pain responsivity between habitual exercisers and normally active people and of changes in pain responsivity during exercise. It further discussses the notion of 'stoicism' with regard to these findings. Various laboratory pain tests have been utilized to document the effects of exercise on pain behaviour. To facilitate later discussion, these stimuli will be described briefly. The tourniquet ischaemia pain test3 usually involves inflating a pressure cuff above systolic pressure on the upper arm, after which the subject exercises the hand for several minutes. Sensation and pain reports are elicited at regular intervals, and the subject is encouraged to tolerate the stimulus for as long as 10 or 15 min. Cold pain is produced by the cold pressor test, in which the subject immerses his hand in an icewater slurry for as long as 3 to 5 min. During this immersion, sensation and pain reports are elicited at regular intervals, and the time to withdrawal is noted. Heat pain is often delivered by a contact thermode4, but also with focused light5 and lasers6, either in a continuous ramp or as temporally discrete stimuli. The subject either rates each discrete stimulus on a rating scale, or indicates when particular levels of sensation or pain have been achieved along the ramp. Electrical stimuli are delivered to the skin, usually of the fingers or forearm7, but also to the teethV10. The stimulation and response protocols are analogous to those described for thermal stimulation. Pressure pain is usually produced by placing the subject's finger under the dulled edge of a lucite block; different forces press the edge against the dorsal surface of the finger, and the subject reports when particular levels of sensation or pain have been achievedl 1. Responses are then summarized by various psychophysical measures. Most commonly, the pain threshold is taken as the mean stimulus intensity evoking the minimal report of pain. Tolerance is taken as the mean maximal intensity endured on the trials. Another model, sensory decision theory (SDT) 2, provides two indices of perceptual performance. Discriminability indexes the accuracy in separating higher from lower intensity stimuli, and response bias indexes the most probable report category. Since most of these indices can vary independently of one another, the use of SDT separates measures of average report level from sensitivity to differences in level. Analgesics such as local nerve blocks or IV morphine5 reduce both the number of pain reports and discriminability, while placebos only reduce the number of pain reports13.

Geoelectric signals in China and the earthquake generation process

We present original apparent resistivity records for stations located in the Chinese provinces of Hebei and Sichuan, along with seismicity patterns, fault maps, and limited hydrologic data. These records are part of a 30‐year database of geoelectric signals that have been recorded in China by a nationwide network of more than 100 stations set up by the State Seismological Bureau of the People's Republic of China. The database includes the time period during which the M 7.8 1976 Tangshan and M 7.2 1976 Songpan earthquakes occurred. Resistivity data recorded in and around the epicentral region of the Tangshan event show coherent changes, both temporal and in areal distribution, years and months before and after the earthquakes. A regional change in water table occurred more than 3 years prior to the Tangshan earthquake. The resistivity and water table signals appear to correlate in the 2‐ to 3‐year period prior to the earthquake. Rainfall records from the region around Tangshan, extending to the cities of Beijing and Tianjin, do not appear to account for the resistivity changes. Regional seismicity and active fault patterns in north China indicate that deformation occurs mainly along the borders of large tectonic blocks. The source region for the geoelectric signals appears to lie within a large shear zone, at the edge of a tectonic block undergoing rotation. The Tangshan epicentral area is thus only part of a larger zone of deformation. We suggest that dilational strain associated with the opening of fractures may lower resistivity and water table. For shallow crustal depths, such as those monitored by the Chinese geoelectric network, dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle. Our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension. The Chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes.

On failure phenomenon in single-fibre pull-out tests

This letter seeks to provide a theoretical explanation of a commonly observed failure phenomenon in the single-fibre pull-out test, namely the presence of a matrix cone at the emergent end of the extracted fibre. The single fibre pull-out test is commonly used [1,2] to characterize fibre reinforced polymer composites. This is because the test is simple to conduct and can be Used for almost any fibre-matrix composite system. An advantage [1] of the pull-out test is that in addition to the interfacial bond strength and interfacial toughness, other interfacial properties, such as the matrix shrinkage pressure on the fibre, the interfacial shear stress and the work done in pulling the fibre from the matrix, can be determined. The last factor is important, since the significant increase in fracture toughness of fibrous composites has been attributed to the fibre pull-out process during failure. Four types of failure [3] may occur in the specimen during a single-fibre pull-out test. These are: (i) specimen failure due to matrix failure away from the fibre-matrix interface, (ii) specimen failure by fibre fracture along the external free length of the fibre, (iii) partial debonding followed by specimen failure due to fibre fracture within the embedded length of the fibre and (iv) specimen failure due to complete debonding and extraction of the debonded fibre from the matrix. The first three types of failure represent unsuccessful pull-out tests. Data from specimens that exhibit these types of failure should not be included in pull-out analysis. Only specimens that exhibit failure of type (iv) represent successful pull-out tests. Thus, only data from such specimens should be utilized for analysis. This letter concerns specimens that exhibit this type of failure. A matrix "cone" is frequently observed at the emergent end of the extracted fibre after specimen failure. Such a typical matrix cone on the extracted fibre of a glass fibre/epoxy matrix pull-out specimen is shown in Fig. l a. The region below the matrix cone is the exposed debonded fibre surface. The point at which the cone is widest represents the original location of the edge of the matrix block that was bonded to the fibre. The part of the matrix cone above this location represents the menscus formed by the epoxy on the glass fibre. The matrix cone of an extracted fibre from a specimen with short embedded length is shown in Fig. lb. All of the features described above for a specimen with larger embedded length can be seen clearly in Fig. lb. Such matrix cones have been reported in the literature for both thermoset-based [3, 4] and thermoplastic-based [5] single-fibre pull-out specimens. None of the existing models [6-8] can account for the formation of such a matrix cone during type (iv) failure of a single-fibre pull-out specimen. It will be shown in this letter that radial and circumferential stresses in the matrix are responsible for the formation of the matrix cone. Radial stresses O'rm are usually neglected in the existing models, which predict a maximum finite value of the interfacial shear stress ~'i at the fibre emergent end since oS~ is much smaller [6] "ri. Therefore O~m is expected to have little effect on debonding crack initiation at the emergent end of the single-fibre pull-out specimen. However, the prediction of a finite maximum

Of Hearths and Houses

During the 1993 East Texas Archeological Field School conducted at the Tyson site (41SY92) in western Shelby County, the junior author had an opportunity to participate in the excavation of a Caddoan hearth. The work was directed by Linda Lindsay, a graduate student in Anthropology at Southern Methodist University. This paper describes our findings and a few features of hearths and houses. One goal of the 1993 Field School was to explore the area around Feature 3 looking for evidence of a house. This was accomplished by opening a 6 meter by 6 meter unit referred to as Block 1. Feature 3 had been excavated in 1992 and found to be a 1.2 meter in diameter, round, basin shaped pit containing a large amount of daub, bone, and Caddoan pottery sherds. Near the bottom of the pit was a zone of ash. Charcoal and mussel shell from Feature 3 yielded three calibrated radiocarbon dates of about 1425 AD. When Block I was completely exposed, a number of other pits and postholes were seen in plan view. Our activity focused on Feature 9 on the western edge of Block 1. This 1.1 5 meter by O. 9 meter oval hearth was first revealed at 20 cm depth when ash was encountered. The feature contained large amounts of ash from in situ burning, nuggets of fired clay, a small amount of bone, and several burned sherds with ash adhering to their surfaces. The hearth was slightly basin-shaped and approximately 15 cm thick. A discontinuous thin layer of bright orange clay near its bottom was observed. The hearth had been prepared for use by digging a very shallow pit but no intentional "clay lining" was seen. Two large postholes were found in the area of Feature 9. Feature 17 was discovered beneath the eastern end of the hearth. It was 30 cm in diameter and had a smoothly rounded bottom at 75 cm below ground surface. Feature 12 was a very distinct posthole of similar proportions just west of the hearth. The diameter of F 12 was 27 cm and the depth was 65 cm below surface. How do we understand this feature? Specifically, does Feature 9 represent the central hearth of a Caddoan house? This question is currently difficult to answer because the outside wall of a putative house has not been identified. Possibly, Block l lies entirely inside a large house. The question may be easier to answer after reviewing accounts written by early Europeans visiting the area and reviewing the archeological findings at other East Texas Caddoan sites.

Measurement of Solid-liquid Contact Angle and Liquid Surface Tension. Method Using Inclined Plate.

A method is proposed for measuring either the solid-liquid contact angle or the liquid surface tension. The method depends on the break-down of the liquid meniscus formed under an inclined test plate fixed on an inclined surface of a support block. The plate and the support block are once immersed into a liquid bath and then lifted up gradually, thus making the meniscus slide downward on the inclined plate. When the constricted portion of the meniscus makes contact with the side wall of the support block, the meniscus immediately breaks, causing a spontaneous falling down of the liquid along the side wall. The contact angle θ can be calculated theoretically from the measured critical height HBcr of the upper edge of the support block at the moment of the meniscus break. The liquid surface tension σ also can be determined by the same principle with the aid of a glass plate which is wetted by the liquid zero contact angle. The feasibility and the usefulness of the method were experimentally examined.

A preliminary survey of Kounounkan Forest, Guinea, West Africa

Biological studies in Guinea have been very patchy, leaving much of the country almost unsurveyed. In 1992 the Fauna and Flora Preservation Society funded a 10-day field visit to Kounounkan, a relict forest at the northern edge of the Upper Guinea Forest Block. The survey found a variety of habitats and high biodiversity, including two vulnerable and one endangered species. Hunting pressure appears to be low and the site is very well suited for the development of further conservation initiatives.

Frequency and distribution of resistance to oxytetracycline in micro-organisms isolated from marine fish farm sediments following therapeutic use of oxytetracycline

the background level of resistance to oxytetracycline in sediments free of anthrogogenic influences was determined on 2216 V agar with 25 μg·g −1 oxytetracycline. The mean frequency of resistance in 153 samples taken in Galway Bay was 1.2 ±1.8%. The impact of oxytetracycline therapy on the frequency of resistance in the sediments under a marine fish farm was investigated on two occasions. In the first investigation, oxytetracycline was detected at a concentration of 9.9 ± 2.9 μg·g −1 in the sediments under a cage that received 865 g oxytetracycline per day for 10 days, but no significant rise in resistance frequency was detected. In the second investigation, oxytetracycline was detected at a concentration of 10.9 ± 6.5 μg·g −1 in the sediments under a cage block that received 175 kg oxytetracycline over 12 days. The frequency of resistance reached 16.0 ± 8.9% after the treatment. The frequency declined at an exponential rate ( r 2=0.89) with a half-life of 26 days. At 73 days after the end of therapy the frequency, in under-cage samples, was not significantly higher than the background level. At the end of the therapy elevated frequencies of resistance were detected up to 75 m from the edge of the cage block and in samples where the levels of oxytetracycline were below the limit of detection (1.2 μg·g −1). Thirty-three days after the end of the therapy the frequency of resistance in all samples not directly under the cages was not significantly higher than in samples taken from sediments free of anthropogenic influence.

Mating disruption to control damage by leafrollers in Virginia apple orchards

AbstractMating disruption for control of variegated leafroller (VLR), Platynota flavedana (Clemens), tufted apple bud moth (TBM), P. ideausalis Walker, and redbanded leafroller (RBL), Argyrotaenia velutinana (Walker), was studied in Virginia apple orchards in 1989 and 1990. In 1989, each dispenser (1000/ha) contained 190 mg of 67.2% E11–14:Ac, 28.8% Z11–14:Ac, 1.4% E11–14:OH, 0.6% Z11–14:OH, and 2% Z9–12:Ac (a putative generic leafroller disruption blend). Trap captures of VLR, TBM and RBL were reduced by 97%, 51% and 55%, respectively. Average leafroller injury in the interior and edge of the pheromone block was 3.8% and 2.7%, respectively. The conventional control and abandoned blocks had 0.05 and 27.5% injury, respectively. Dispensers containing E11–14:OH (70%) and Z11–14:OH (30%) (close to the natural blend of VLR), more effectively disrupted orientation to pheromone traps by both Platynota spp. than did the generic blend. In 1990, dispensers containing 150 mg of E11–14:OH (70%) and Z11–14:OH (1000/ha) were placed in two 2‐ha blocks and one 4‐ha block. One pheromone‐treated block was sprayed in August with phosmet for codling moth, not timed for leafrollers. Trap captures of VLR and TBM were reduced by almost 100% and 69%, respectively. RBL captures were not reduced by VLR pheromone permeation. Injury from Platynota spp. in pheromone block middles and edges ranged from 0.3–1.7% and 0.3–2.3%, respectively. Injury in conventional blocks ranged from 0–1.1%. RBL injury in pheromone block middles ranged from 0–6.1%, and in edges, 1.7–4.8%. Injury in control blocks ranged from 0–1.1%. Combined leafroller injury in an abandoned block was 18% (s.e. 0.3). Release rates averaged 30 and 32 mg/ha/h for the VLR and generic pheromone dispensers, respectively.

Tectonics of the Himalayan collision between the Indian Plate and the Afghan Block: a synthesis

Abstract A synthesis of the available tectonic, structural, magmatic and geochronologic data from Afghanistan and western Pakistan suggests that collision between the Indian Plate and the Afghan Block (a collage of blocks assembled during the Cimmeride orogeny), took place during the Pliocene. Prior to this, the thinned western margin of the Indian Plate, together with the transitional to oceanic crust which separated the Indian Plate from the Afghan Block, was overlain by a wedge of Tertiary clastic sediments deposited in a large flexural basin that included what is now the Katawaz basin as well as the Sulaiman foredeep. At least three phases of collision can thus be recognised in the northwest Himalaya: the first, at about 100 Ma involved the suturing of the Kohistan-Ladakh-East Nuristan island arc to southern Asia; the second, at about 55 Ma involved India-Asia collision and the third, during the Pliocene, involved India-Afghanistan collision. During the Eocene and Miocene, the Afghan block was displaced southwards in response to Indian indentation into Asia. Simultaneous counterclockwise rotation of the Afghan Block resulted in intracontinental extension causing alkaline magmatism and right lateral displacements along the Herat Fault. As the Indian Plate moved northward with respect to Afghanistan, some of the wedge of clastic sediments deposited on the Indian transitional crust were accreted to the leading edge of the Afghan block and displaced sinistrally with respect to the rest of the sediments in the wedge. These displaced sediments now form the Makran sequence. Collisional structures on both sides of the India-Afghan collision zone in the Helmand block and the Sulaiman-Kirthar Ranges probably represent the inversion of continental margin basins rather than thrust belts with large shortening amounts. After initial inversion, the dominant feature of this highly oblique collision has been transpressional tectonics. Prior to its final docking with the Afghan Block, the Kabul block was a continental slice, completely surrounded by oceanic crust, and located northwest of continental India. Palaeocene collision between India and the Kabul block could have led to thickening of the Indian Plate prior to initiation of India-Asia collision.

An update on incision for ilioinguinal lymph node dissection

The wound morbidity after ilioinguinal lymph node dissections can be significantly reduced by precise planning of both the incision and the extent of flap elevation. Flap elevation extends no farther than the edges of the quadrilateral block of Anson, and exposure is best obtained with a bipedicle incision with broad-based pedicles. The broad bases ensure the greatest possibility for the sustentation of the flaps by the remaining microcirculation. All suction drains are placed far laterally, not through the base of any flaps, to avoid the high bacterial count on the skin of the groin, genitals, perineum, and anus. Closure is undertaken with a running suture, which allows for a seal that will facilitate suction. Suction continues until drainage is less than 25 mL/d.

Collision and fan‐shaped compressional stress pattern in the Izu Block at the northern edge of the Philippine Sea Plate

The Izu block in central Japan, which is surrounded by the Nankai‐Suruga trough and the Sagami trough, is considered to be colliding with the Eurasian (EUR) plate at the northern edge of the Philippine Sea (PHS) plate. The stress state in the Izu block is investigated from focal mechanism data mainly obtained by the seismic network of the National Research Institute for Earth Science and Disaster Prevention. The predominant focal mechanisms in the central to eastern part of the Izu block indicate strike‐slip faulting with P axes oriented N–S to NW–SE and T axes oriented E–W to NE–SW, whereas focal mechanisms in the western part are strike‐slip and reverse faulting with P axes oriented N–S to NE–SW, indicating that the compressional axes remain horizontal throughout the region. The compressional stress trajectories estimated from the P axes show a fan‐shaped pattern, radiating from the northern edge of the Izu block where the collision is considered to be occurring. The fan‐shaped pattern of compressional stress trajectories is interpreted to be the result of the collision of the Izu block with EUR plate. Plane stress analysis by the finite element method supports this conclusion. The stable distribution of the T axis directions in the central to eastern part is basically interpreted as resulting from the slab pull force by the subducting PHS plate along the Sagami trough. Furthermore this model can explain the seismic quiescence in the northern part of the Suruga Bay region.

The Naga Hills and Andaman ophiolite belt, their setting, nature and collisional emplacement history

The Indo-Burmese Range and the Andaman-Nicobar Island Arc, form a continuous arcuate trend along which several ophiolite occurrences have been reported. In Naga Hills (NHO) and Andaman (ANO), these ophiolites are represented by dismembered mafic and ultramafic rocks with closely associated oceanic pelagic sediments. They occur as folded thrust slices occupying the highest tectonic levels and are brought to lie over distal shelf sediments of Eocene to Oligocene age. Ophiolites are unconformably overlain by ophiolite-derived clastics of Middle to Late Eocene age. The ophiolites preserved along this belt are remnants of a continuous, narrow, one or several intra-continental ocean basin(s) of broadly comparable age, created during the Late Mesozoic rifting of the Greater India Gondwana continent. Rifting and creation of oceanic crust date between Cretaceous and Early Eocene. In the initial stages, the ocean floor had been deeper than Carbonate Compensation Depth (CCD). Subsequently it had become uneven, when oceanic crust was being added through several seamounts or seamount chains and on top of which calcareous pelagic sediments were deposited. Both tholeiitic and alkaline volcanic rocks are present in these ophiolites. In NHO, the two groups of lavas have generated from different sources in different tectonic settings. The alkalic and some tholeiitic lavas in NHO are similar to off-axis seamount basalts. Tholeiitic lavas from ANO and some NHO resemble MORB or backarc basin basalts and on the basis of certain chemical characters these are suggested to have generated in marginal basin setting. Significant volume of acid differentiates are associated in ANO which also support the marginal basin character of the basalts. The suite of rocks in ANO indicates fractionation in a shallow level magma chamber. Closure of the small ocean basin(s) and emplacement of ophiolites took place in two stages. In the initial stage, the seamount chain brought to the subduction zone collided with the Burmese block prior to Middle Eocene. Part of the ophiolites represent clipped seamounts which got accreted to the leading edge of the eastern continental block. With continued closure, this eastern block with accreted ophiolite slices was brought in juxtaposition with distal shelf sediments of the western block marking the terminal continent-continent collision. The thrust front of ophiolitic rocks apparently advanced further westward in Andaman to the south compared to the northern sector, and thus an imbricated zone and melange involving the Eocene floor sediments (Lipa Fm) has been created, whereas in the Naga Hills the floor sediments (Disang Fm) remained virtually passive. The time of terminal continental collision is represented as the regional Late Oligocene unconformity. The entire thrust stack got deformed and folded into upright geometry after being blocked. The present subduction of oceanic crust beneath the Andaman island arc appears to be a westward jump of subduction zone due to sustained post-collisional NE drive of the Indian plate.

Dead horse graben: A west Texas accommodation zone

The Cretaceous rocks of the Dead Horse graben area underwent two superposed deformations. Laramide (latest Cretaceous to early Tertiary) left‐transpression structures are overprinted by a Basin and Range (Miocene to Recent) accommodation zone. During Laramide shortening, two northwest trending sets of monoclines developed: an en echelon set over a left‐slip basement fault and a single monocline draped over the uplifted edge of a rotated basement block. Basin and Range extension caused northerly trending half‐grabens that have opposing asymmetry: downthrown‐to‐the‐east north of the study area and downthrown‐to‐the‐west south of the area. These half‐grabens are separated by an accommodation zone, expressed as a northwest trending, right‐lateral pull‐apart graben. The normal faults of the pull‐apart graben flatten at a shallower depth than those of the half‐grabens. This suggests not only that accommodation zones can be mechanically layered but that this layering can occur on a scale different from the adjacent half‐grabens. The northwest trend and superposition of both sets of structures implies a tectonic control by the preexisting structural fabric of the Texas Lineament.

Geochronological constraints on the tectonic evolution of a late Archaean greenstone belt, Wabigoon Subprovince, Northwest Ontario, Canada

U-Pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the Kakagi Lake-Savant Lake volcanic belt (KSVB) within the Wabigoon Subprovince, and the English River Subprovince in northwest Ontario, Canada. Two tonalite gneisses intruded by mafic dykes at Tannis Lake and Caribou Lake have minimum zircon ages of 3051 Ma and 3075 Ma, respectively. This contrasts with a 2775 ± 1 Ma age on zircon from a felsic tuff located within a lower, predominantly mafic volcanic sequence in the interior of KSVB. These data, combined with previous studies, indicate that KSVB is presently flanked by older sialic crust. In the Sioux Lookout and Kashaweogama Lake areas, lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc-alkaline volcanic units. Titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 ± 4 Ma. This late age is confirmed by a 2703 ± 2 Ma zircon age for an unconformably underlying porphyry intrusion. One clast gives a zircon age of 2903 ± 16 Ma, suggesting that the sediments were at least partly derived from older material in the English River Subprovince. Data from previous studies show that the structurally overlying calc-alkaline volcanic sequences are at least 30 Ma older than the sediments. The absence of zircon inheritance in all but the youngest KSVB sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment, in isolation from older sialic material. The intervening oceanic crust must therefore have been consumed by subduction, which led to arc magmatism and eventual collision between the arc (KSVB) and continental material (English River Subprovince). The sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial, continental environment. Since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment, they must have been obducted onto the edge of a continental block. This was perhaps followed by nappe and thrust emplacement on top of the sediments of calc-alkaline sequences derived from earlier arc magmatism. Continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions.

Interpretation of Footwall (Lowside) Fault Traps Sealed by Reverse Faults and Convergent Wrench Faults

Lowside (footwall) closures sealed by reverse-slip faults and convergent strike-slip faults offer opportunities for significant field extension and new field prospects in basins deformed by contraction. The faults have reverse separation in cross section and transverse closure (in the direction of reservoir dip) is often provided by dip of beds away from the fault at structural upturns. The upturns are common and form at the edge of the footwall block as a consequence of block-edge folding, fault drag, and shortening transverse to fault strike. Effective fault seal and longitudinal closure (parallel to reservoir strike) are the most uncertain trap controls. Fault seal may be provided by the juxtaposition of older, less permeable rocks against the down-dropped reservoir or by impermeable material within the fault zone. Fault-zone barriers to fluid flow include shaly smear gouge, cataclastic gouge, mineral deposits, or asphalt or tar impregnation. Longitudinal closure is most commonly formed by a broad positive warp or bowing at the edge of the footwall block or by stratigraphic reservoir terminations. Secondary faults, intersections of primary block faults, and en echelon folds may also provide longitudinal closure. Prospects can range in importance from secondary extensions of existing highside closures to large traps unrelated to hanging-wall structure. The variety of geometries, relationships that provide transverse and longitudinal closure, and important geologic parameters that determine fault seal are illustrated with examples from oil fields in Sumatra and southern California. These fields can be used as models for the recognition and delineation of prospects in other basins.

Averaging methods for numerical simulations of flows through heterogeneous porous media

Many natural rock systems contain small patches of different permeability which affect the flow of fluids through them. As these heterogeneities become smaller and more numerous, they become harder to model numerically. We consider how to reduce the computational effort required in simulations by incorporating their effects in the boundary conditions at the edges of each grid block. This is in contrast with current methods which involve often arbitrary changes in the fluid properties. The method is restricted to the case of widely-spaced patches, which simplifies interaction effects. The system then reduces to an array of dipoles, and two averaging methods are proposed for finite grid blocks. Several infinite systems, including vertical and horizontal bands, are also considered as further approximations. There is a great wealth of existing results from different fields which lead to identical mathematical problems and which can be used in these cases. Finally, we consider how to use these techniques when the precise configuration of the grid block is not known, but only its statistical properties. This can lead to results which are very different from the deterministic case.

Expected Paleozoic Stratigraphy Beneath Western Part of Metamorphic Overthrust in Southern Appalachians: ABSTRACT

A stratigraphic cross section from Roanoke, Virginia, to Alabama shows the formations and facies of the easternmost Valley and Ridge outcrops. This cross section, coupled with 3-dimensional insights on facies changes, siliciclastic sources, and regional unconformity patterns, delimits stratigraphic expectations directly beneath the metamorphic over-thrust of the Blue Ridge-Great Smoky-Piedmont terrain as far east as the Brevard zone. These strata are in the Saltville structural block south of central Tennessee and in the Pulaski block to the north. The stratigraphic column in the block directly beneath the metamorphic overthrust is up to 5,100 m (17,000 ft) thick and contains Upper Precambrian (Chilhowee Group) to middle Mississippian beds. The Chilhowee in the block beneath the metamorphic overthrust is probably less shaly and thinner than equivalent Chilhowee exposed along the leading edge of the metamorphic overthrust block. The Shady Formation becomes less dolomitic eastward, passing into dark, shaly limestone east of the carbonate bank edge. Rome-Conasauga siliciclastics change to carbonates eastward and northeastward. The Knox Group changes to limestone beneath the overthrust, and the post-Sauk unconformity probably disappears. Middle Ordovician siliciclastics coarsen eastward toward the Blount delta. The Wallbridge discon inuity expands eastward, so that Silurian and Lower Devonian strata probably do not occur beneath the metamorphic overthrust. The pre-Upper Devonian unconformity truncates more section eastward, so that Chattanooga Shale locally rests on Middle Ordovician Bays Formation in southeastern Tennessee. In Tennessee, Georgia, and possibly North Carolina, Mississippian strata include sandstone, dark shale, limestone, and bedded chert passing beneath the Great Smoky-Cartersville fault. End_of_Article - Last_Page 470------------