Johnson P.R.,6016 SW Haines Street |
Andresen A.,University of Oslo |
Collins A.S.,University of Adelaide |
Fowler A.R.,United Arab Emirates University |
And 4 more authors.
Journal of African Earth Sciences | Year: 2011
During the late Cryogenian-Ediacaran (650-542. Ma), the Arabian-Nubian Shield (ANS) underwent final assembly and accretion to the Saharan Metacraton concurrent with the assembly of eastern and western Gondwana. At the end of the Precambrian it lay at one end of the East African Orogen, with its northern margin (present coordinates) forming a low-relief stable shelf facing an open ocean; to the south the ANS transitioned into the Mozambique Belt. The geologic history of the ANS during this period provides insight into the closing developmental stages of one of the world's largest accretionary orogens. Following a 680-640. Ma orogenic event reflecting amalgamation of a core grouping of island-arc terranes (the proto-Arabian-Nubian Shield; pANS), the region underwent extensive exhumation, erosion, and subsidence. Depositional basins formed in the northern and eastern pANS, with those in the east below sea level and connected to an ocean. Periodic basin closure and formation of new basins in other parts of the ANS followed. Many basins were filled by terrestrial, molasse-type sediments interfingering with subordinate to predominant amounts of volcanic rocks. Magmatism was extensive throughout the period, initially characterized by tonalite-trondhjemite-granodiorite (TTG) and granite (monzogranite, syenogranite), but also characterized, from ~610. Ma on, by increasing amounts of alkali-feldspar granite and alkali granite. The plutons are largely undeformed, except where cut by brittle-ductile shear zones. The magma sources of the late Cryogenian-Ediacaran granitoids were dominated by juvenile crust and(or) depleted mantle and magmas mostly originated in anorogenic, post-collisional, commonly extensional, settings. They were derived by melting and fractionation of anhydrous high-grade metamorphosed lower crust, mafic- to intermediate calc-alkaline crust, and(or) subduction-modified mantle wedges associated with slab break-off or delamination.By ~630. Ma, the region was affected by oblique (transpressional) convergence of continental blocks that formed eastern and western Gondwana-the pANS was approaching the Saharan Metacraton; north-trending shear and shortening zones developed in the southern ANS; and northwest-trending strike-slip shear zones of the Najd fault system dominated farther north. In the northwestern ANS, convergence and Najd transpression buckled the crust causing structural highs with domes of gneissic infracrust overlain by supracrust composed of ophiolitic and volcanosedimentary assemblages dating from the Tonian-middle Cryogenian period of island-arc activity. The supracrust was extensively translated to the northwest above a high-strain zone. Extension and tectonic escape augmented exhumation of the gneissic infracrust particularly between ~620-580. Ma. In the northeastern ANS, linear belts of gneiss formed from reworked older intrusive bodies or syntectonic intrusions that were emplaced along Najd faults. By ~620. Ma a marine basin on the eastern margin of the pANS (present coordinates) was beginning to close. A thick sedimentary assemblage (Abt formation) in this basin underwent metamorphism and folding, and subduction-related magmatism and volcanism farther into this basin (Al Amar arc; >690-615. Ma) was coming to an end. Amalgamation of the Abt formation, Al Amar arc, and the pANS occurred between ~620 and ~605. Ma, and terminal collision between the pANS and the Saharan Metacraton was complete by ~580. Ma. At this time, the ANS was fully assembled. Granite magmatism continued until ~565-560. Ma and orogeny ceased by ~550. Ma. During these terminal events, the region underwent strong chemical weathering and became a vast low-relief surface on which Lower Paleozoic sandstone was eventually deposited. © 2011 Elsevier Ltd.
Said Razouki S.,Nahrain University |
Salem B.M.,Columbus State Community College
International Journal of Pavement Engineering | Year: 2015
A laboratory investigation and analysis on the resilient modulus of gypsum-rich roadbed sand with a gypsum content of 38.8%, subjected to cyclic soaking and drying with a cycle length of 14 days, is presented and discussed. Eleven pairs of California bearing ratio (CBR) soil specimens were prepared for determining the resilient modulus using the wave propagation technique. After the application of this technique, the CBR load–penetration test was performed on each specimen to make use of the corresponding curve for determining the resilient modulus too. The wave propagation resilient modulus results are generally in good agreement with those based on CBR full load–penetration curve. The paper reveals that the cyclic soaking and drying process converges to an equilibrium state after the fifth cycle resulting in a resilient modulus which is much less than that for the common four days of soaking or that obtained from published correlation studies. © 2015 Taylor & Francis
Razouki S.S.,Nahrain University |
Salem B.M.,Columbus State Community College
Transportation Geotechnics | Year: 2016
Presented is a study of the effect of frequency of cyclic soaking and drying on the time variation of resilient modulus MR of gypsum-rich roadbed sand having a gypsum content of about 39%.Three different frequencies are adopted. The high frequency is represented by a short cycle length of 14 days, while for the low and medium frequencies, cycle lengths of 180 and 60 days were chosen. Generally, thirteen pairs of CBR specimens were prepared for each frequency and each specimen was subjected to a surcharge load of 200 N. The wave propagation technique was used to determine the MR for each CBR specimen at the begin and end of each phase.The paper reveals that the MR decreases during the soaking phase and increases during the drying phase and the process converges to an "equilibrium state" after the fourth cycle. An increase in cycle length decreases soil stiffness. © 2016 Elsevier Ltd.
Hague A.L.,Columbus State Community College
Journal of Evidence-Based Dental Practice | Year: 2011
Subjects The participants, ages 2 to 6 years, were recruited from the dental clinics at the Departments of Pediatric Dentistry at Boston Universitys Goldman School of Dentistry and Tufts University School of Dental Medicine, and Cambridge Health Alliance. All of the participants were medically healthy with a full primary dentition and no history of antibiotic use 3 months before enrollment in the study. The mean age for participants without caries (n = 39) and with severe early childhood caries (S-ECC) (n = 72) was 3.65 years and 3.90 years, respectively. Most of the participants were male and born in the United States. Key Risk/Study Factor The key risk factor was use of a short diet survey to differentiate S-ECC from caries-free children. The survey addressed mealtime and between-meal beverages (milk, juice, and water), frequency of food/beverage intake, and cariogenicity of food intake (categories included caries-protective, noncariogenic, low cariogenicity, liquids, or solid/retentive foods) for the previous day or a day that was more typical of his or her diet. Cariogenicity was estimated based on a weighted sum of the number of food items for each food category divided by the total food and beverage items. The data were collected by trained clinicians using a nondirective interviewing approach. Main Outcome Measure Themain outcome measure was dental caries activity. S-ECC represented extensive lesions in the primary dentition, whereas a caries-free dentition indicated no lesions or visible white spots on the enamel. Plaque samples were obtained from both S-ECC and caries-free participants. The samples were analyzed for Streptococcus mutans, Streptococcus sobrinus, and Bifidobacterium species using polymerase chain reaction technology. S-ECC participants were monitored for new caries lesions at 3- to 6-month intervals for 12 months. Main Results S-ECC participants had an average of 13.6 ± 0.54 (SEM) carious teeth. S-ECC participants ate more frequently (P = .0002) and drank more between-meal juice (P = .007) than the caries-free participants. S-ECC participants also had higher scores for cariogenic liquids (P = .003), solid/retentive foods (P = .0004), and estimated food or beverage cariogenicity (P = .0001) than caries-free participants. S-ECC participants with lesion recurrence ate fewer caries-protective foods compared with those without new lesions (P = .05). S mutans and S sobrinus were associated with S-ECC (P <.0001) and lesion recurrence (P < .05). Participants who tested positive for S mutans had high estimated food cariogenicity scores (P = .03). Attrition at follow-up was because of a family move or failure to return for appointments (n = 22). Conclusions The authors concluded that dietary factors, derived from an open-ended survey, can be used to differentiate S-ECC from caries-free children, and that dietary factors are associated with the detection of specific strains of bacteria associated with S-ECC. Subsequently, the prevention and treatment of S-ECC should address food frequency, putative cariogenicity of foods/beverages, and suppression of cariogenic bacteria. © 2011 Elsevier Inc. All rights reserved.
Schaefer J.L.,Columbus State Community College
Educational Gerontology | Year: 2010
The success of older adult students seeking higher education degrees depends, in part, upon fulfillment of critical support needs. This phenomenological study explored the experiences of nine contemporary Older Baby Boomer students (ages 50-62) who are pursuing bachelor's degrees at a Midwestern university. Qualitative data was gathered from individual interviews and reflection questionnaires. Nancy Schlossberg's transition model provided a theoretical framework for the findings, which suggest that most of these learners (a) are first generational college students with deficient information about formal higher education process, (b) are primarily motivated by career aspirations, and (c) experience complex support needs. © Taylor & Francis Group, LLC.