Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral,NW Pakistan): Insights from phase equilibria modelling,and garnet–monazite petrochronology

The Karakoram–Hindu Kush–Pamir and adjacent Tibetan plateau belt comprise a series of Gondwana‐derived crustal fragments that successively accreted to the Eurasian margin in the Mesozoic as the result of the progressive Tethys ocean closure. These domains provide unique insights into the thermal and structural history of the Mesozoic to Cenozoic Eurasian plate margin, which are critical to inform the initial boundary conditions (e.g. crustal thickness, structure and thermo‐mechanical properties) for the subsequent development of the large and hot Tibetan–Himalaya orogen, and the associated crustal deformation processes. Using a combination of microstructural analyses, thermobarometry modelling and U–Th–Pb monazite and Lu–Hf garnet geochronology, the study reappraises the metamorphic history of exposed mid‐crustal metapelites in the Chitral region of the South Pamir–Hindu Kush (NW Pakistan). This study also demonstrates that trace elements in monazite (especially Y and Dy), combined with thermodynamical modelling and Lu–Hf garnet dating, provides a powerful integrated toolbox for constraining long‐lived and polyphased tectono‐metamorphic histories in all their spatial and temporal complexity. Rocks from the Chitral region were progressively deformed and metamorphosed at sub‐ and supra‐solidus conditions through at least four distinct episodes from the Mesozoic to the Cenozoic. Rocks were first metamorphosed at ~400–500°C and ~0.3 GPa in the Late Triassic–Early Jurassic (210–185 Ma), likely in response to the accretion of the Karakoram during the Cimmerian orogeny. Pressure and temperature subsequently increased by ~0.3 GPa and 100°C in the Early‐ to Mid Cretaceous (140–80 Ma), coinciding with the intrusion of calcalkaline granitic plutons across the Karakoram and Pamir regions. This event is interpreted as the record of crustal thickening and the development of a proto‐plateau within the Eurasian margin due to a long‐lived episode of slab flattening in an Andean‐type margin. Peak metamorphism was reached in the Late Eocene–Early Oligocene (40–30 Ma) at conditions of 580–600°C and ~0.6 GPa and 700–750°C and 0.7–0.8 GPa for the investigated staurolite schists and sillimanite migmatites respectively. This crustal heating up to moderate anatexis likely resulted in the underthrusting of the Indian plate after a NeoTethyan slab‐break off or to the Tethyan Himalaya–Lhasa microcontinent collision and subsequent oceanic slab flattening. Near‐isothermal decompression/exhumation followed in the Late Oligocene (28–23 Ma) as marked by a pressure decrease in excess of ~0.1 GPa. This event was coeval with the intrusion of the 24 Ma Garam Chasma leucogranite. This rapid exhumation is interpreted to be related to the reactivation of the South Pamir–Karakoram suture zone during the ongoing collision with India. The findings of this study confirm that significant crustal shortening and thickening of the south Eurasian margin occurred during the Mesozoic in an accretionary‐type tectonic setting through successive episodes of terrane accretions and probably slab flattening, transiently increasing the coupling at the plate interface. Moreover, they indicate that the south Eurasian margin was already hot and thickened prior to Cenozoic collision with India, which has important implications for orogen‐scale strain‐accommodation mechanisms.     

Vibroseis productivity: shake and go

We use both model and field data to compare three methods for increasing vibroseis productivity and decreasing acquisition costs. The first method, HFVS (high-fidelity vibratory seismic), allows us to separate the responses from individual vibrators when multiple vibrators are operating simultaneously. The data quality of the separated records is superior to that of conventional correlated data because they are processed with measured ground-force signals, but the number of sweeps must be greater than or equal to the number of vibrators. The second method, cascaded sweep, eliminates the listening time between multiple sweeps and partially mitigates harmonic noise observed at later times on near-offset traces. Finally, a combined method, continuous-HFVS (C-HFVS), allows source separation with a single, long, segmented sweep. Separation is as good as with HFVS and interference noise is limited to times near the end of a sweep-segment length. All three methods produce acceptable seismic images for post-stack and prestack amplitude interpretation.
The choice of which option to use depends upon the area being investigated. HFVS has numerous benefits, especially when fine sampling is required to mitigate static problems and elevation changes. Due to the ability to separate individual responses, fine sampling can be achieved without sacrificing productivity. For deeper targets, cascaded sweep can be more efficient but data quality suffers from harmonic noise. C-HFVS, which combines features of HFVS and cascaded sweep, has the potential to result in the highest productivity, without sacrificing either fine sampling or data quality.     

Quantifying gravel overlap and dislodgement forces on natural river bars: implications for particle entrainment

Equations for predicting particle entrainment typically assume that clast weight is the only factor resisting motion in the force balance on individual grains. In this work, increases in the force required to dislodge surface particles due to overlapping by surrounding clasts is quantified. Field data were collected at two subaerially‐exposed bars along the Colorado River in central Texas, USA, with median gravel diameters (D₅₀) of 37 and 64 mm. Clast size, shape, weight, the force required for vertical removal, and the fraction of clast area covered by surrounding grains were measured. Small hooks were glued to individual clasts without disrupting their positions and quasi‐static peak forces required to vertically dislodge each clast were measured using a force gauge. Clasts were also colored with dye before being dislodged, and image analysis was used to calculate the fraction of clast surface area covered by surrounding grains. The effect of overlap on the particle force balance is quantified by defining the ‘excess force ratio’ as the dislodgement force divided by the weight of the clast. Excess force ratio varies weakly but systematically with clast size: lifting larger clasts can require forces up to two times the clast's weight, while smaller clasts can require forces up to seven times their weight. The fraction of clast surface area covered by surrounding particles is also weakly correlated with excess force ratio. By assuming that critical shear stresses are proportional to the excess force ratio, the effect that overlap alone may have on particle entrainment is calculated. For a given size class, the most mobile grains should have critical shear stresses controlled only by their weight. However, clast overlap also causes broad distributions of critical stresses for partially‐exposed surface grains. The data quantify the significant fraction of bed area that should be less mobile than grain size alone would predict. Copyright © 2011 John Wiley & Sons, Ltd.     

Environmental cues and seasonal reproduction in a temperate estuary: a case study of Owenia collaris (Annelida: Polychaeta,Oweniidae)

Coordination of gametogenesis and spawning during restricted breeding seasons increases availability of mates, fertilization rates, and often success of offspring. Orton’s Rule predicts water temperature as the dominant environmental cue for gametogenesis or spawning in temperate invertebrates. Crisp’s Rule predicts that species producing planktotrophic larvae will time their reproduction to ensure optimal nutrition for the larvae. Owenia collaris (Annelida: Oweniidae) is a temperate polychaete that produces planktotrophic larvae that remain in the water column for several weeks. Reproductive phenology and its relationship to the environment were investigated in an estuarine population of O. collaris using field and laboratory studies. Owenia collaris produced mature gametes between March and September each year during the productive season between the spring and fall transitions when day lengths were 12 h or more, alkalinity was 8.1 or higher, and temperature was 11 °C or higher. Gamete production was not related to seasonal changes in salinity or benthic phytoplankton concentrations and gametes were present prior to the spring phytoplankton bloom. In laboratory experiments, production of gametes was influenced somewhat by manipulating day lengths but not adult food. The association with predictable and stable environmental cycles (day length) and a broad spawning season suggest that this species is unlikely to mis-match with larval food availability which would lead to reduced availability of recruits in any given year, supporting Crisp’s Rule. Our data do not support the hypothesis that reproductive timing in this species is driven by access to excess energy required by adults to initiate the production of gametes. Our data also do not support Orton’s premise that temperature is the primary controlling factor of reproduction in this temperate marine invertebrate.     

Occurrence of metal-free galaxies in the early Universe

The character of the first galaxies at redshifts z ≳ 10 strongly depends on their level of pre-enrichment, which is in turn determined by the rate of primordial star formation prior to their assembly. In order for the first galaxies to remain metal-free, star formation in minihaloes must be highly suppressed, most likely by H₂-dissociating Lyman–Werner (LW) radiation. We show that the build-up of such a strong LW background is hindered by two effects. First, the level of the LW background is self-regulated, being produced by the Population III (Pop III) star formation which it, in turn, suppresses. Secondly, the high opacity to LW photons which is built up in the relic H  ii regions left by the first stars acts to diminish the global LW background. Accounting for a self-regulated LW background, we estimate a lower limit for the rate of Pop III star formation in minihaloes at z ≳ 15. Further, we simulate the formation of a 'first galaxy' with virial temperature   T _vir≳ 10⁴ K  and total mass  ≳10⁸ M_⊙  at z ≳ 10, and find that complete suppression of previous Pop III star formation is unlikely, with stars of  ≳100 M_⊙  (Pop III.1) and  ≳10 M_⊙  (Pop III.2) likely forming. Finally, we discuss the implications of these results for the nature of the first galaxies, which may be observed by future missions such as the James Webb Space Telescope .     

Occurrence of Hydroclathrus clathratus in northern New Zealand waters

Hydroclathrus clathratus (C. Agardh) Howe (Phacophyta: Punctariales) is recorded from two harbours on the Northland east coast, North Island, New Zealand. The possibility of its being a recently introduced species is discussed.     

Commercial landings of the spiny lobster Jasus edwardsii (Hutton) at Chatham Islands,New Zealand (Crustacea: Decapoda: Palinuridae)

The spiny lobster Jasus edwardsii (Hutton) supports a thriving new fishery at the Chatham Islands, about 770 km east of New Zealand. Commercial fishing for J. edwardsii started in November 1965, and within 2 years the area became a major centre of New Zealand's large spiny lobster fishery, which earned >NZ$5,000,000 in 1967. Over 12 months (September 1966 to August 1967) landings at the Chatham Islands made up 39.8% of the total New Zealand spiny lobster catch of 16,600,0001b.

In the first year of fishing 2,177,923 lb of spiny lobsters were caught, and in the second 7,129,658lb—a 227% increase. Fishing effort also increased substantially: average number of boats fishing each month rose by 174% in the second year, and the total number of boat‐fishing days by 152%. Most specimens of J. edwardsii caught are large and mature, typical of a virgin stock.     

Using Tracers to Quantify Drilling Water Influence and Obtain Representative Groundwater Samples

This paper describes a practical field method of using applied tracers to determine how much purging is required to collect representative groundwater samples after the introduction of drilling water during borehole advancement. In general, the approach involves adding a tracer of known concentration to the drilling water and then measuring the tracer in the purge water until the tracer concentration declines to a defined target level. If necessary, the dilution effects of residual drilling water can be quantified, and the measured contaminant concentration can be corrected based on the measured tracer concentration. A project example is presented to demonstrate that this method is straightforward and reliable and that applied tracers can be used to quantify the influence of residual drilling water on formation water quality while also ensuring that purge times and volumes are not unnecessarily large.