Slow Extension Rate Research Articles

Shear‐Dominant Continental Rifting in Northern Ryukyu Revealed by Ambient Noise Tomography

AbstractIn this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc‐oblique, right‐lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear‐wave velocity models using data recorded by an ocean‐bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary‐prism origin of the latter. The resolved azimuthal anisotropy exhibits arc‐parallel and arc‐perpendicular fast shear‐wave polarization directions in the upper to mid‐crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc‐parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right‐lateral motion of the Ryukyu Arc. The underlying arc‐perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc‐perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear‐dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.

Miocene climate cooling and aridification of Antarctica may have enhanced syn-extensional magmatism in the western Ross Sea

Continental rift systems are commonly characterized by volcanism with parental basaltic magmas sourced from the mantle. Erosion of the rift shoulders and sedimentation in the adjacent basins can affect the stress and thermal fields at depth, thereby affecting partial mantle melting. However, the sensitivity of magmatic activity to such surface forcing is elusive. Geological observations from the western Ross Sea, Antarctica, suggest rift onset in the Cretaceous with a transition from wide-rifting to narrow-rifting at the boundary between the Antarctic craton and the Transantarctic Mountains. Miocene climate cooling during rifting in the western Ross Sea, in addition, leads to an abrupt decrease in sedimentation rate, synchronous to the emplacement of the McMurdo Volcanic Group. This represents the largest alkali province worldwide, extending both inland and offshore of Transantarctic Mountains and western Ross Sea, respectively. Here, we use coupled thermo-mechanical and landscape evolution numerical modeling to quantify melt production in slowly stretching rift basins due to changes in erosion/deposition rates. The model combines visco-elasto-plastic deformation of the lithosphere and underlying mantle during extension, partial rock melting, and linear hillslope diffusion of the surface topography. The parametric study covers a range of slow extension rates, crustal thicknesses, mantle potential temperatures and diffusion coefficients. Numerical simulations successfully reproduce the ∼150–200-km-wide extension of western Ross Sea and Miocene-to-present asthenospheric melt production (McMurdo Volcanic Group). Results further show that slow rifts magmatism is highly sensitive to sediment deposition within the basin, which inhibits mantle decompression melting and delays the crustal breakup. Regional climate-driven sedimentation rate changes are thus likely to have affected the syn-rift magmatic history of the western Ross Sea, Antarctica, supporting the relevance of interactions between surface and deep-seated processes across extensional settings.

Evidence for a Prolonged Continental Breakup Resulting From Slow Extension Rates at the Eastern North American Volcanic Rifted Margin

AbstractWhen continental rifting is accompanied by localized magmatism under extensional stress, the breakup duration can be short and the continent/ocean transition sharp, as mantle melts are thought to be efficient at heating and weakening the lithosphere. This mode of rifting has been invoked for the Eastern North American Margin (ENAM) based on the existing geophysical data. Here, we present results from multichannel seismic profiles from the ENAM Community Seismic Experiment offshore North Carolina, U.S. Our survey area encompasses both the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA), which lies ~200‐km farther seaward. Our prestack depth‐migrated seismic images reveal major changes in the structure of the igneous crust across the BSMA. Between the ECMA and BSMA, we image a proto‐oceanic domain of rough, faulted, and thin igneous crust. The roughness of this oceanic crust is similar to modern ultraslow spreading environments which involve the continued presence of a pre‐existing lithospheric lid. Seaward of the BSMA the basement is smooth, and the crust is relatively thick, which is typical for Jurassic oceanic crust. Across the BSMA, we image a step up in basement and crustal root, which we interpret to represent complete lithospheric breakup and a transition to steady‐state seafloor spreading in agreement with coincident refraction results. Our results would also indicate low extension rates in the final stages of rifting that may have influenced the thermal structure of the lithosphere and could explain the delay for continental breakup. All of these observations show that although continental rifting between eastern North America and northwest Africa was assisted by magmatic activity, it did not lead to rapid localization of extensional strain as previously thought.

Geochronology of Cadmium (Cd), Cuprum (Cu), and Arsenics (As) in Annual Band of Coral Porites lutea at Pantai Kondang Merak, Malang

Coral reef is a massive natural building block that mainly composed of hard coral. In ecological view coral reef is the center of biological activity for shelter, foraging and feeding place. Porites lutea is a common coral in reef flat area such as in Pantai Kondang Merak. This coral can form a massive with hillocky colony surface and has a slow linear extension rate, that made this coral has long longevity. The annual growth band of coral skeleton provide information of pollutants in the coastal extending back over several years ago due to ability of coral aragonite traped trace metal from environment. The aim of this research was to determine the concentration of trace metals of Cd, Cu and As along with the coral annual banding. The heavy metals detection was performed using ICP-OES (iCAP 7400 Series). The concentration of Cd, Cu and As in sample 1 (KM1) were 2.236 mg/kg, 9.726 mg/kg, and 2.474 mg/kg, while sample 2 (KM2) were 1.989 mg/kg, 19.157 mg/kg, and 2,064 mg/kg respectively. Two ways mechanism of trace metals to be trapped in the coral skeleton are by direct mechanism when trace metals in a form dissolved ion that were uptaken by coral then stored into coral skeleton and by indirect mechanism when particulate metals ingested by plankton then eaten by coral through coral tissue. The tracing of heavy metal in coral is provided important information of environment condition of the sea from 2009 - 2015 that may be used for authority decision regarding pollutant ambient in the sea environment.

Dental development in living and fossil orangutans

Numerous studies have investigated molar development in extant and fossil hominoids, yet relatively little is known about orangutans, the only great ape with an extensive fossil record. This study characterizes aspects of dental development, including cuspal enamel daily secretion rate, long-period line periodicities, cusp-specific molar crown formation times and extension rates, and initiation and completion ages in living and fossil orangutan postcanine teeth. Daily secretion rate and periodicities in living orangutans are similar to previous reports, while crown formation times often exceed published values, although direct comparisons are limited. One wild Bornean individual died at 4.5 years of age with fully erupted first molars (M1s), while a captive individual and a wild Sumatran individual likely erupted their M1s around five or six years of age. These data underscore the need for additional samples of orangutans of known sex, species, and developmental environment to explore potential sources of variation in molar emergence and their relationship to life history variables. Fossil orangutans possess larger crowns than living orangutans, show similarities in periodicities, and have faster daily secretion rate, longer crown formation times, and slower extension rates. Molar crown formation times exceed reported values for other fossil apes, including Gigantopithecus blacki. When compared to African apes, both living and fossil orangutans show greater cuspal enamel thickness values and periodicities, resulting in longer crown formation times and slower extension rates. Several of these variables are similar to modern humans, representing examples of convergent evolution. Molar crown formation does not appear to be equivalent among extant great apes or consistent within living and fossil members of Pongo or Homo.

Fault linkage across weak layers during extension: an experimental approach with reference to the Hoop Fault Complex of the SW Barents Sea

A series of analogue experiments utilizing sequences of sand with interlayered silicone polymer have been performed to investigate the effects of multistage extension on rock sequences of different strength, with particular reference to the Hoop Fault Complex of the Barents Sea. It was found that the width and style of the graben systems as seen in map view depend strongly on the extension velocity. Wide areas of graben formation are promoted by fast extension, whereas narrowly constrained deep-graben structures are typical of slow extension rates. Furthermore, the decoupling strata are likely to be characterized by flow rather than by distinct detachment faults. The scaled experiments produced units of contrasting fault frequencies and styles in individual sand layers positioned between layers of silicone polymer. It was also found that the fault segments developed from different levels were related to varying extents (hard-linked, soft-linked, firm-linked, unlinked). The fault configurations and the general fault pattern obtained in the experiments is similar to that observed in natural faults where salt or unconsolidated mudstone separate sequences of sand.

Present‐day kinematics of the East African Rift

The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present‐day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.

Melting during late-stage rifting in Afar is hot and deep

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.

Deformation distribution and type in the Main Ethiopian Rift (MER): A remote sensing study

The Main Ethiopian Rift (MER) is magmatically segmented and seismically active. The magmatic segments are arranged en echelon (e.g. Ebinger, C.J., Casey, M., 2001. Continental breakup in magmatic provinces: An Ethiopian example. Geology Society of America Bulletin, vol. 12, pp. 527–530). Furthermore they are about 50 km long and up to 20 km wide, orientated about N10°–20°E, and are therefore oblique to the main direction of Mid-Miocene border faults (N35°E). In this paper we propose that these segments not only represent magmatic but also tectonic segmentation. We use the association of the high spectral resolution of Landsat data (principal component analysis and classifications) and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Additionally, several digital elevation models of metric resolution have been produced at different locations along the segments in order to quantify and describe the faulting. We define 4 tecto-magmatic segments (Gedemsa, Bosetti, Kone and Fantale), 40–70 km, 10–15 wide, arranged en echelon and N10°–20°E orientated. Each tecto-magmatic segment (TMS) is separated by 2–18 km wide domains exhibiting minor deformation. Furthermore, the TMS overlap in a N–S direction and show similar symmetrical deformation patterns. Within an individual segment deformation varies from its centre, where it is essentially magmatic, to its tips, where brittle deformation is predominant. The largest volumes of recent basalt are erupted in the centres and nearby. In contrast, recent magma injections are absent in the tip domains. It seems probable that dikes connect laterally to, and are fed by, the magmatic centres. Dikes are sub-parallel to the segment axis and therefore orientated normal to the recent extension direction. Within the TMS, aligned cones, atypical fault geometries, en echelon fault segments, and atypical displacement–length ratios all suggest that faulting is dike-induced. Along axis propagation of the TMS generate interactions between younger (Quaternary) intra-rift faults and older (Mid-Miocene) border faults. Thus, at the deformed tips, cross-cutting and long curved faults are produced. The fault interactions in the MER confirm that the border faults have been inactive during the recent stage of rifting. We discuss the implications of oblique rifting with moderate obliquity ( α = 27°, angle between the extension direction and the trend perpendicular to the rift) at slow extension rates on active rifting processes associated with intermediate continental crust. The tecto-magmatic segmentation mirrors crustal segmentation beneath the rift axis, as indicated from 3-D-tomography (Daly, E., Keir, D., Ebinger, C., Stuart, G., Ayele, A., Waltham, D., 2007. Crustal structure of the northern Main Ethiopian Rift from a tomographic inversion of local earthquakes. Geophysical Journal International, in press; Keranen, K., Klemperer, S.L., Gloaguen, R., EAGLE Working Group, 2004. Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology 32 (11), 949–952). These long and narrow zones (40–70 km long and 10–15 km wide), where deformation is generated principally by diking and is associated with magmatic centres, have similarities with along-axis segmentation at mid ocean ridges. Consequently, deformation distribution and type in the MER can be considered the outcome of a transition between continental and active oceanic rifting.

The role of gravitational instabilities, density structure and extension rate in the evolution of continental margins

Abstract Formation of rifted continental margins is associated with localized thinning and breakup of the continental lithosphere, driven or accompanied by the ascent of the lithosphere–asthenosphere boundary. Thinning creates sharp density and viscosity contrasts and steep boundaries between cold deformed lithosphere and hot upwelling asthenosphere, thus providing conditions for the development of positive (asthenosphere) and negative (mantle lithosphere) Rayleigh–Taylor (RT) instabilities. The evolution of many continental margins (e.g. Newfoundland margin and Iberian margin) is characterized by slow spreading rates. This allows the RT instabilities to grow at the timescale of rifting. The impact of positive RT instabilities (asthenospheric upwelling) is well studied. The negative RT instabilities, associated with mantle down-welling, remain an overlooked factor. However, these instabilities should also affect the rift evolution, in particular, they may cause mantle thinning or thickening below the rift flanks. Our numerical experiments suggest that the ratio of the RT-growth rate to the extension rate controls the overall rift geometry and evolution. Even if the effect of negative RT instabilities is more important for slow extension rates of 2×5 mm year −1 (Deborah number, De <1), it is still significant for 2–3 times higher extension rates of 2×15 mm year −1 ( De <10). The numerical experiments for extension rates of 2×15 mm year −1 and mantle–asthenosphere density contrasts of 10–20 kg m −3 demonstrate a number of structural similarities with continental margins characterized by low De (e.g. Flemish Cap and Galicia margin). In particular, rift asymmetry results from interplay between the RT instabilities and differential stretching at De <1. Formation of interior basins occurs at De ≈1–3. The best correspondence with the observed geometry of rifted margins is obtained for chemical density contrast of 20 kg m −3 and extension rate of 2×15 mm year −1 , which is twice that of the averaged values inferred from the observations. This suggests that margins may initially (prebreakup stage) extend at higher rates than the average extension rates characterizing rift evolution. The influence of RT instabilities is strongly controlled by extension rate, density, rheology and thermal structure of the lithosphere; this implies that we need better constraints on these parameters from the observations.

Comparison of Stress Corrosion Cracking Characteristics of CP Ti, Ti-6Al-7Nb, Ti-6Al-4V, and Ti-15Mo

Abstract The purpose of this research was to evaluate and compare the stress corrosion cracking (SCC) of Ti-15 Mo (ASTM F2066), Ti-6Al-7Nb (ASTM1295), Ti-6Al-4V ELI (ASTM F136), and Grade 4 CPTi (ASTM F67). Evaluation of alloy composition, microstructure, Vickers microhardness, and tensile properties was performed to determine compliance with the appropriate ASTM specification. For SCC testing, smooth tensile samples with a gauge length of 10 mm and a gauge diameter of 4 mm and notched samples with a notch root diameter of 4 mm (Kt=3.2) were prepared using low stress grinding procedures. Three smooth and three notched samples were tested in distilled H2O and Ringer's solution at 37°C. Testing was performed using the slow extension rate method at a stroke rate of 10−5 mm/s according to the guidelines established in ASTM G129. The ratio of the percent elongation (PER) and reduction of area (ROAR) of smooth and notched samples tested in Ringer's solution and distilled water were evaluated. The fracture surfaces of representative samples were also examined for fracture mode identification using SEM. Results showed that all alloys complied with the appropriate ASTM specification. SCC may be considered to occur in a material if the ductile properties in an aggressive media are inferior to the ductile properties in a non-aggressive media. Evaluations of the PER and ROAR ratios in smooth and notched samples for each alloy in Ringer's and distilled water showed no indication of SCC failure mechanisms. SEM examination of the fracture surfaces showed no differences in the fracture morphology regardless of the testing solution. These results were consistent with the mechanical testing data. It is therefore concluded that SCC mechanisms were not operating or contributing to the fracture of these alloys under the conditions evaluated.

Evaluating the use of the massive coral Diploastrea heliopora for paleoclimate reconstruction

To date, coral‐based paleoclimate research in the Pacific has primarily utilized core samples from the genus Porites and has been most successful reconstructing past variability on interannual timescales, particularly the El Niño Southern Oscillation (ENSO). The Indo‐Pacific coral genus Diploastrea, however, owing to its slower extension rate, denser structure, and longer lifespan, can potentially preserve geochemical proxy records 2–3 times longer than Porites cores of the same length. Before its potential can be realized, Diploastrea must first be calibrated and its climate signal assessed. We present oxygen isotope (δ18O) and Sr/Ca results from two Diploastrea cores collected in Fiji (16°49′S, 179°14′E) that allow for simultaneous evaluation of this coral's paleoclimatic utility and the reproducibility of each tracer at this site. Comparison to a Porites record from the same location allows for further evaluation of Diploastrea as a paleoclimatic archive. We demonstrate that Diploastrea's septal and columellar material yield similar δ18O‐SST relationships and that despite some sacrifice of the seasonal δ18O amplitude, bulk sampling of either region is satisfactory for resolving interannual and lower frequency modes of climatic variability. Therefore paleoclimate reconstructions employing either a septal or columellar sampling regime of this genus may be useful at filling in spatial and temporal sampling gaps which currently hinder the reconstruction of long‐term changes in major climate fields in the western Pacific.

Embrittlement of laser surface-annealed 17-4 PH stainless steel

Stress corrosion cracking and fatigue crack growth behavior were determined in 17-4 PH stainless steel under various metallurgical conditions, including the H900 (482°C/1 h), H1025 (552°C/4 h) aging and laser surface annealing treatments. Peak-aged (H900) specimens locally irradiated by laser beam consisted of a portion of composite region (CR), in which comprised of soft laser-annealed (LA) zones on the outer surfaces and the hard base metal in between. Slow extension rate tensile tests were performed at room temperature in a saturated H 2S solution to evaluate the hydrogen embrittlement susceptibility of various specimens. H900 specimens show an obvious improvement in impact toughness after irradiating by laser. Regardless of testing environments, H900 specimens exhibited better tensile properties than the other specimens. Experimental results also indicated that H900 specimens had the highest fatigue crack growth rates among the specimens, particularly at low stress intensity factor range. The retardation of crack growth in the region ahead of the CR in the LA specimens was rather pronounced. For compact tension specimens tested in gaseous hydrogen, enhanced crack growth was correlated with quasi-cleavage fracture in contrast to transgranular fatigue fracture in air.

The mechanical properties of human alar and transverse ligaments at slow and fast extension rates

Objective. To study biomechanics and failure characteristics of the alar and transverse ligaments of the upper cervical spine at two different extension rates. Design. In vitro biomechanical study using human and alar and transverse ligament specimens. Background. Previous studies of these ligaments have been at relatively slow extension rates, even though the real life trauma occurs at significantly higher extension rates. Methods. Fresh human cadaveric alar and transverse ligaments were subjected to slow (0.1 mm/s) and then fast (920 mm/s) extension rates. Force and elongation curves were recorded during the testing, and several biomechanical parameters were computed and compared. First the specimen was stretched at the slow extension rate up to 70 N for the alar ligament and 140 N for the transverse ligament. Then the specimens were stretched until failure at the fast extension rate. Rasults. Avarage initial lengths of the alar and transverse ligaments were 11.2 and 18.0 mm, respectively. At 70 N, the average alar ligament strain decreased from 16.4 to 0.3%, the stiffness increased from 80 to 2316 N/mm and the energy absorbed decreased from 47.4 to 1.3 Nrom, as the extension rate increased from the slow to the fast. At failure, the average strain, force and energy absorbed at the fast extension rate were respectively 3.1%, 367 N and 123 Nmm. For the transverse ligament at 140 N, the strain decreased from 12.5 to 0.6%, the stiffness increased from 141 to 1472 N/mm, and the energy absorbed decreased from 96.1 to 7.6 Nmm as the extension rate went from slow to the fast. At failure, strain, force and energy absorbed for the transverse ligament at the fast extension rate were, respectively, 2.3% and 436 N and 101 Nmm respectively. Conclusion. Within the physiological limits, the strain and energy absorbed decreased to less than one tenth, while the stiffness increased to greater than ten times as the extension rate increased, for both the alar and transverse ligaments. When failed at the faster rate, the alar ligament, although weaker of the two, absorbed greater energy to failure because of its higher failure strain. Relevance Injuries to the upper cervical spine have extremely serious clinical consequences because of the anatomic location. Alar and transverse ligaments provide most of the stabilit in this region. This is the first study to determine the mechanical properties of these ligaments at an extension rate that is closer to real life trauma.

Nitride Precipitation in Duplex Stainless Steel Weld Metal.

Duplex stainless steel base material was welded using gas tungsten arc welding with an Ar-10%H2 shielding gas and laboratory-made filler wires were employed to deposit duplex and fully ferritic weld metals having different nitrogen contents. Weld metal slow extension rate tensile (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nitrogen content in fully ferritic stainless steel weld metal increased the density of precipitates and the hydrogen-induced cracking susceptibility. The facets on the quasi-cleavage fracture surfaces of broken WM-SERT test specimens were parallel to the {100} plane in ferrite. Scanning and transmission electron microscope observations revealed the crystallographic features and morphology of the precipitates. The precipitates were rod-like Cr2N nitrides. Many of them had directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr2N precipitates and ferrite suggested that the axes of the Cr2N precipitates were parallel to direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr2N precipitates could act as sinks for hydrogen and may be preferential sites for initiation of hydrogen cracking; this could promote crack propagation on {001} cleavage planes in ferrite on which Cr2N precipitates are located. Higher densities of Cr2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.

Neoproterozoic sedimentation on the northeast margin of Laurentia and the opening of Iapetus

AbstractVery thick shallow water sedimentary sequences were deposited in Neoproterozoic time along the future margins of Laurentia. On the eastern margin these include the Eleonore Bay and Hecla Hoek sequences of Greenland and Svalbard; these are described and their geotectonic context briefly reviewed. They present both geotectonic and geodynamic problems: why did subsidence continue for some 300 Ma prior to the opening of Iapetus, and how could 15–20 km of sediment be accommodated in an ensialic environment?Prolonged slow stretching appears to have affected the eastern margin of Laurentia while the western (Cordilleran) margin progressed through the rift-drift transition as the Pacific opened. It is proposed that expansion of the Pacific was associated with both the convergence of Proto-Gondwanan continental terranes during the Pan-African orogeny (the extended SWEAT hypothesis) and also the maintenance of very slow extension rates on the future Iapetus margin. The strain-hardening effect of slow stretching may have been inhibited by a continuous basin-fill of juvenile heat-producing Grenville detritus. The onset of subduction in the Pacific freed up this margin; major rifting took place between East Greenland and possibly the Tornquist margin of Baltica in Vendian time, followed by the opening of northern Iapetus.

The [formula omitted]-temperature relationship in coralline aragonite: Influence of variability in [formula omitted] and skeletal growth parameters

This paper provides an evaluation of two of the most likely pitfalls of Sr Ca thermometry, i.e., the effect of biogenic cycling of Sr vs. Ca in the surface ocean and the effect of variable extension rate on Sr incorporation in coralline aragonite. We also report calibration of the Sr Ca -temperature relationship for three coral species, Porites lobata, Pocillopora eydouxi and Pavona clavus, collected from the Hawaiian and Galapagos islands. Analyses of seawater samples show significant spatial and depth variability in the Sr:Ca ratio. The uncertainty introduced by this effect is estimated to be <0.2°C for corals located in tropical oligotrophic waters, and potentially larger for corals located in upwelling areas. Sr Ca analyses along two different growth axes of a Galapagos Pavona clavus, with annual extension rates of ~6 and 12 mm/y, respectively, indicate an offset of 1–2°C, with higher Sr Ca values associated with slower extension rates. The offset observed between the two growth axes may be the result of variations in extension and/or calcification rate.

Hydrogen compatibility of Femnal alloys

Five Femnal alloys with various ferrite/ austenite were prepared by mainly adjusting the carbon content. Hydrogen compatibility of these alloys was investigated. A slow extension rate tensile test in one atmosphere of hydrogen gas showed that hydrogen embrittlement (HE) increased with the content of ferrite in the alloys. The alloy with a full austenite phase was strong, ductile, and essentially immune to HE, whereas the one with full ferrite was weaker, very brittle, and suffered serious damage in hydrogen. A sustained load test was further conducted in hydrogen gas at 25°C to 75 °C for the alloys with 10, 35, and 65 pct ferrite. No subcritical crack growth occurred in the alloy with 10% ferrite. Typical three-stage environment-assisted crack growth was observed for the alloys with 35 and 65 pct ferrite. A kinetics study of the subcritical crack growth indicated that the cracking rate was basically controlled by hydrogen transport through the ferrite grains. Austenite phase could be embrittled provided sufficient hydrogen was dissolved. To reduce HE in Femnal alloys, the content of ferrite should be reduced; this can be achieved mainly by controlling the carbon content in the alloys.

A comparison of fungal communities in straw decomposing in different soil types and under different cultivation practices

SynopsisThe composition of the mycoflora growing on wheat straw decomposing in three arable soil types and following two methods of incorporation was monitored throughout one cropping season. Fusarium species, Trichoderma species and Chaetomium globosum were the most frequently isolated cellulose-decomposers during the autumn and early spring on all treatments. Around some straws the brown colour of lignin was replaced by a dark red coloration: if this was cleared subsequently, it was considered to indicate the presence of lignin-decomposing species. Colonies of a Typhula sp., a lignin-decomposing basidiomycete, developed from some of the straws surrounded by cleared zones. In late spring more lignin-decomposing basidiomycetes were isolated, none of which had previously been described on, nor isolated from, wheat straw decomposing in the field. These basidiomycetes, characterised by slower extension rates than Typhula, were more frequent on straw ploughed-in at two sites. Typhula was more numerous where the straw had been incorporated by tined implements. The composition of the cellulolytic fungal community differed between treatments. Inoculation of straw with pure and mixed cultures of Fusarium culmorum, Trichoderma viride, Chaetomium globosum and the lignin-decomposing basidiomycetes had varied effects on the decay rate.

Quantitative examination of stress corrosion cracking of alloy 600 in high temperature water

Stress corrosion cracking (SCC) examination of Inconel 600 steam generator tubing has continued at Brookhaven National Laboratory, using U-bends, constant load and slow extension rate tests, leading to Arrhenius plots of failure times versus inverse temperature for crack initiation and propagation. Effect of applied load can be expressed as log-log curves for failure times versus stress. Variations in environment and cold work are included in all the experiments. Microstructure and composition of oxide films on Inconel 600 surfaces were examined after exposure to pure water at 365°C, and stripping with the bromine-methanol method. Results are consistent with a mechanism of transient creep, film rupture and a mass-transport-limited anodic process.