Petrographic Studies of Rocks from The Chesapeake Bay Impact Structure (USA): Implication for Moderate Shock Pressures in Sedimentary Breccias

Shock petrographic investigations were carried out on samples collected from drill cores from the Chesapeake Bay impact structure (USA). The late Eocene Chesapeake impact structure is, at 85 km diameter, currently the largest impact structure known in the United States, buried at shallow to moderate depths beneath continental margin sediments underneath southeastern Virginia. To better define the variety of the samples collected from the shallow drill cores and the shock degrees experienced by the target rocks and breccias in the Chesapeake impact crater, thin section analyses were conducted on more than 50 samples from the various zones of the impact structure. The study involves measurements of the orientations of planar deformation features (PDFs) using a universal stage attached to a petrographic microscope. The aim of this study is to determine the shock pressures of various clasts in the shallow breccia fill of the crater. As a result, we note that the overwhelming numbers of shocked grains, which are now present in the sedimentary breccia, are derived from the basement granitoids. Our studies involved samples from four shallow drill cores (Exmore, Windmill Point, Kiptopeke, and Newsport News).The breccia fill is termed the Exmore breccia, which is dominated by particulates of silt, shocked and unshocked granitic fragments, shale, clay, and free shocked quartz grains. The Kiptopke and Windmill Point samples contained rare fragments showing a variety of different shock effects, whereas the Newporte News samples, show several fragments and impact melt with the evidence shock metamorphism was noted. The most abundantly observed shock indicators are shock fracturing, indicative of shock pressures of less than about 10 GPa, as well as 1-2 sets of PDFs in quartz grains, which is indicative of moderate shock pressures of up to about 20 or 25 GPa. Key words: Chesapeake Bay Crater, PDFs, Shock pressure, Universal stage, Impact structure

Simultaneous expansion and rotation of shear-free universes in modified gravity

We show in a fully covariant way that, there exist a class of $f(R)$ models for which a shear-free, almost FLRW universe can expand and rotate at the same time .Comment: 4 pages, no figures. Contribution submitted to the proceedings of the Spanish Relativity Meeting ERE2011, September 2011, Madrid, Spai

Shear Zone-Hosted Base Metal Mineralization near Abraha Weatsebaha-Adidesta and Hawzein, Tigray Region, Northern Ethiopia

Low-grade basement rocks of Neoproterozoic age with well developed shear zones and posttectonic granitic intrusives from Hawzien area of northern Ethiopia were studied for field characteristics, mineralogy, textures, alteration assemblages and geochemistry to explore theirpotential for base metal mineralization. The basement rocks includes metavolcanic (mafic to felsic), metavolcaniclastic and metasedimentary rocks. The intrusive post-tectonic granitoids mark the end of Proterozoic. Field observation and petrographic data indicate the presence of NS, NE-SW trending shear zones; hydrothermal quartz (±calcite) veins of different generations; malachite stains; alterations like chloritisation, kaolinization, epidotization, sericitization; and presence of base metal sulfides in association with quartz and calcite veins. Geochemistry of surface and drill core samples indicate enrichment of zinc in shear zones with low concentrations of copper, lead, gold, arsenic and silver. Zn-rich base metal mineralization with Pb-Cu-Fe (±Ag-As-Au) is related to D2 deformation. Barite veins are although common, conspicuously occurring in E-W trending veins. Post-tectonic granitoids are facilitating the  hydrothermal activity and the source rock compositions controlling the  nature of the mineralization. A paragenetic scheme is suggested on the basis of texture, mineral association, alteration and deformation. Keywords: Abraha-Weatsebaha, Tigray, Ethiopia, Neoproterozoic, Shear zones, Paragenesis, Sulfide mineralizatio

Evaluation of soil solarisation and bio-fumigation for the management of bacterial spot of tomato

Soil-borne plant pathogens cause heavy losses to all major crops, leading to reductions in both yield and quality. Soil solarisation and bio-fumigation offer disease management options that are safe and reduce the use of pesticides for soil-borne plant pathogens. Mustard plant releases antimicrobial hydrolysis products, notably isothiocyanates when used as a bio-fumigant. Bacterial spot of tomato caused by Xanthomonas campestris pv. vesicatoria (Xcv) can survive in soil and plant debris, which serve as a primary inoculum for infecting the next tomato crop. An experiment was carried out with the objective of evaluating effects of soil solarisation and the use of Ethiopian mustard (Brassica carinata A. Braun) as a bio-fumigant to control bacterial spot disease and on yield of tomato. The treatments consisted of six types of potted soil medium (solarised at Haramaya and Dire Dawa, bio-fumigated, biofumigated as well as solarised at Haramaya and Dire Dawa, and untreated control as non-solarised non-biofumigated pots). Treated tomato seeds were planted and fruit yields were compared among treatments. Potted soil was inoculated with the pathogen, Xcv, belonging to T2P2 race group. The total microbial and Xcv counts were done before as well as after setting up the experiment. The results revealed that solarisation reduced the population of Xcv from 10.68 to 8.79 CFU g-1, total bacterial population from 11.27 to 9.86 CFU g-1, and total actinomycete counts from 11.69 to 9.44 CFU g-1 while bio-fumigation had a non-significant effect on Xcv and total microbial counts. None of the treatments exhibited a significant effect on fungal counts. The fruit yield of tomato grown on biofumigated as well as solarised soil was the highest (91.18 t ha-1) as compared to the other treatments. It can, therefore, be concluded that solarisation and bio-fumigation cannot be used as a bio-rational option for effective management of Xcv on tomato but the two methods could be used to increase tomato yield in the presence of the pathogen.Key words: bio-fumigation, solarisation, bacterial leaf spo

Covariant gauge-invariant perturbations in multifluid f(R) gravity

We study the evolution of scalar cosmological perturbations in the (1+3)- covariant gauge-invariant formalism for generic $f(R)$ theories of gravity. Extending previous works, we give a complete set of equations describing the evolution of matter and curvature fluctuations for a multi-fluid cosmological medium. We then specialize to a radiation-dust fluid described by barotropic equations of state and solve the perturbation equations around a background solution of $R^{n}$ gravity. In particular we study exact solutions for scales much smaller and much larger than the Hubble radius and show that $n>2/3$ in order to have a growth rate compatible with the M\'esz\'aros effect.Comment: 32 pages, 3 figures, published versio

Soil and water managements and landscapes: Africa RISING science, innovations and technologies with scaling potential from the Ethiopian highlands

United States Agency for International Developmen