Search

[1] The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non-coaxial shear with a top-to-the-south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non-coaxial shear with a top-to-the-north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post-date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust. Citation: Larson, K. P., L. Godin, W. J. Davis, and D. W. Davis (2010), Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet, Tectonics, 29, TC4013, doi: 10.1029/2008TC002393.

U-Pb baddeleyite ages and paleomagnetic poles obtained for dykes on Devon Island and Ellesmere Island in the Canadian Arctic and the Thule region of Greenland show that they are associated with the Franklin magmatic event. This study is the only one devoted to Franklin igneous rocks where a primary paleomagnetic remanente and U-Pb age have been obtained from the same rocks. Ages from this study range from 721 to 712 Ma, but paleomagnetic directional data show no clear age progression. The paleomagnetic poles from each of the two regional subsets are significantly different at the 95% confidence level from paleomagnetic results previously published for the Franklin event in the CanadianShield. The difference in the pole locations can be accounted for, to first approximation, by a simple model of early Cenozoic block rotations among the North American plate, Greenland, and a hypothesized ancient microplate comprising Ellesmere, Devon, Cornwallis, and perhaps Somerset islands. A new grand-mean paleopole for the Franklin event, including restoration of Greenland and the proposed 'Ellesmere microplate' to North America, is located at (8.4°N, 163.8°E, [A.sub.95] = 2.8°, N = 78 sites) and is a key pole for Neoproterozoic supercontinent reconstructions. Des ages U-Pb, determines sur de la baddeleyite, et des poles paleomagnetiques obtenus de dykes des files de Devon et d'Ellesmere, dans fArctique canadien, et de la region de Thule au Groenland, montrent que ces dykes sont associes a fevenement magmatique de Franklin. La presente etude est la seine consacree aux roches ignees de Franklin ou une remanente paleomagnetique primaire et des ages U-Pb ont ete obtenus des memes roches. Les ages dans cette etude varient de 721 a 712 Ma, mais les donnees sur les directions paleomagnetiques ne montrent aucune progression Claire de l'age. Les poles paleomagnetiques de chacun des deux sous-ensembles regionaux different de maniere significative, au niveau de confdance 95 %, des resultats paleomagnetiques publies anterieurement pour l'evenement de Franklin dans le Bouclier canadien. La difference d'emplacement des poles peut etre expliquee, en une premiere approximation, par un modele simple de rotations de blocs, au Cenozoique precoce, entre la plaque Nord-americaine, le Groenland et une ancienne plaque hypothetique qui comprenait les files d'Ellesmere, de Devon, de Cornwallis et peut-etre de Somerset. Un nouveau paleopole de grande moyenne pour l'evenement Franklin, incluant la restoration du Groenland et de la << microplaque d'Ellesmere >> proposee a l'Amerique du Nord, est situe a (8,4[degre]N, 163,8[degre]E, [A.sub.95] = 2,8[degre], N = 78 sites) et constitue le pole cle pour les reconstructions du super continent au Neoproterozoique. [Traduit par la Redaction], Introduction Precisely dated rocks with well-defined paleomagnetic poles are scarce in the Proterozoic record, yet are critical for reconstructing the past positions of continents and correlating what are today the [...]

The north-south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U-Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U-Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ~800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route. L'essaim de dykes Clarence Head, a tendance nord-sud, situe sur les iles Devon et Ellesmere dans le Grand Nord canadien, a une tendance orthogonale a l'essaim Franklin (Neoproterozoique) qui l'entoure. Les dykes Clarence Head ont ete dates par methode U-Pb sur de la baddeleyite et ils seraient ages entre 716 ± 1 et 713 ± 1 Ma, des ages apparemment plus jeunes que la plage d'age des dykes Franklin, mais a l'interieur des dates publiees pour ces derniers. Le recul alpha dans la baddeleyite pourrait constituer une explication possible pour les differences d'age, mais une comparaison des ages U-Pb sur des grains de taille semblable des deux essaims suggere que les distances de recul dans la baddeleyite soient inferieures a celles dans le zircon et que les dykes de Clarence Head constituent en effet un evenement plus jeune a l'interieur de la periode de magnetisme Franklin. Les dykes Clarence Head representent un vaste essaim qui est coge netique avec, et tangent a, un gigantesque essaim de dykes qui rayonne sur une distance d'environ 800 km de la source indiquee. Le mecanisme preferentiel pour la mise en place des dykes Clarence Head est l'exploitation de zones concentriques d'extension autour d'un panache source qui s'appauvrit et qui s'effondre. Bien que le paleomagnetisme de la plupart des dykes Clarence Head concorde avec celui des dykes Franklin, deux dykes ont des directions remanentes anomales, interpre tees comme une magnetisation chimique remanente portee par la pyrrhotite. La pyrrhotite a probablement ete depose e a partir de fluides mobilises vers le sud a partir de l'orogene ellesmerien (Devonien) au nord; ces fluides auraient utilise l'interieur des dykes comme conduits et auraient precipite la pyrrhotite en chemin. [Traduit par la Redaction], Introduction Giant mafic dyke swarms are common worldwide (Ernst et al. 1995), and many have radiating patterns of emplacement that appear to indicate their source at the focus, often considered [...]

A 25-70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650-875 km to the east at 1850 [+ or -] 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features. Keywords: Sudbury impactite, distal ejecta, precise U-Pb dates, Gunflint and Biwabik Formations, Ontario, Minnesota.

Surveys of the dental health of Native children in British Columbia, Canada, were conducted in 1980, 1984 and 1988 by Medical Services Branch, Health and Welfare Canada Data were gathered on children turning 5, 7, 9, 11, 13, and 15 yr of age in each survey year. This paper analyzes the findings related to dental caries and the treatment of caries. Comparisons were made between the 1980 and 1988 surveys using Student's -tests; data from the 1984 survey were included for comparison. Results of these surveys demonstrate a continuous improvement in the dental health of Native children between the years 1980 and 1988, but the improvement was limited to the permanent dentition. The deft for 5-yr-olds remained constant over the time interval. In contrast, the DMFT for each group significantly decreased from 1980 to 1988 (P<0.05). The percentage of 5-yr-old children who were caries-free in the primary dentition remained constant, and primary tooth mortality (tooth abscessed or with crown destroyed) in 5-yr-olds did not change. However, the percentage of children surveyed with caries-free permanent teeth im- proved significantly at each survey year and permanent tooth mortality significantly declined (P<0.05). The proportion of filled, compared with carious, primary and permanent teeth increased from 1980 to 1988. In contrast, the number of extracted primary teeth stayed relatively constant for 5-yr-olds, but the proportion of missing teeth declined significantly for all other age groups (P<0.05). The caries experience of this population of Native children was high relative to that of children of comparable age residing in the same region and to that of children who live in other parts of the world. [ABSTRACT FROM AUTHOR]

Comments on several issues related to nursing practice and nurses, including nurse and patient relationships.