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Buchan K.L.,Geological Survey of Canada
Precambrian Research | Year: 2013

Key paleomagnetic poles are poles that are well defined and precisely dated. The rock unit from which the pole is derived must have a precise (usually U-Pb) age and the pole itself must be demonstrated primary with a rigorous field test. The use of key poles is essential in defining reliable apparent polar wander paths (APWPs) and establishing continental reconstructions. Many hundreds of Proterozoic paleopoles have been published from around the globe, but only ~45 are from large craton interiors and pass the key pole criteria. Most key poles are from mafic dykes and sills in the Superior craton (pre-1.83. Ga) or Laurentia (post-1.80. Ga) or Baltica. As a result, with occasional exceptions, it is difficult to define or compare reliable APWP segments in order to test Proterozoic continental reconstructions. However, there are now sufficient age matches or approximate age matches for pairs of key poles from a number of cratons to help constrain their relative locations. In this analysis, Proterozoic key poles are identified and their use in constructing APWPs and testing continent and supercontinent reconstructions is discussed. This key pole database establishes a well constrained Superior craton-Laurentia APWP for much of the Proterozoic that can be used as a reference track against which a growing number of individual key poles from other cratons can be compared. There is now a robust Baltica-Laurentia reconstruction for ~330. m.y. between 1.59 and 1.26. Ga using this approach and potentially for ~570. m.y. between 1.83 and 1.26. Ga if additional key and non-key poles from well-dated units are considered. Key pole comparisons for several other cratons yield preliminary constraints on the relative movement of cratons (e.g., Slave and Superior cratons in the Paleoproterozoic) or on specific elements of continental reconstructions (e.g., Amazonia and Baltica in the Mesoproterozoic, South China craton and Australia in the Neoproterozoic, or Baltica and Laurentia also in the Neoproterozoic). © 2013. Source


Hyndman R.D.,Geological Survey of Canada
Bulletin of the Seismological Society of America | Year: 2015

This article provides a summary of the structure and tectonic history of the Queen Charlotte transform fault zone off western Canada, as background to understanding the 2012 Mw 7.8 thrust earthquake off Haida Gwaii. There was margin subduction prior to the Eocene. The fault zone then became the mainly transcurrent Pacific–North America boundary. There was mid-Tertiary oblique extension, then 15°–20° oblique convergence from ∼6 Ma to the present that resulted in underthrusting and subduction initiation. The total underthrusting has been too small for Benioff–Wadati seismicity or arc volcanics but is indicated by (1) a trench, the Queen Charlotte Trough, into which the oceanic plate bows downward and an offshore flexural bulge, the Oshawa rise; (2) the Queen Charlotte terrace, an accretionary sedimentary prism; (3) seismic receiver function delineation of the underthrusting Pacific plate; (4) heat flow decreasing landward as predicted for underthrusting; (5) low gravity offshore and high onshore, consistent with underthrusting; and (6) late Tertiary uplift and erosion of the west coast of the islands. Oblique convergence is partitioned into nearly marginnormal underthrusting (i.e., Mw 7.8 event) relative to the terrace, which is moving along the margin, and margin parallel on the Queen Charlotte strike-slip fault just off the coast that produced the 1949 Ms 8.1 earthquake. Landward on the mainland, Global Positioning System data suggest slow coast-parallel shear with no historical seismicity. The convergence rate decreases to the north of Haida Gwaii off Dixon Entrance, but large thrust earthquakes are possible. To the south, underthrusting of the Winona basin margin also could generate large earthquakes. © 2015 Seismological Society of America. All rights reserved. Source


Savard M.M.,Geological Survey of Canada
Environmental Pollution | Year: 2010

Hydrogen (δ2H), carbon (δ13C), oxygen (δ18O) and nitrogen (δ15N) isotopes of tree rings growing in field conditions can be indicative of past pollution effects. The characteristic δ13C trend is a positive shift generally explained by invoking closure of stomata, but experimental studies suggest that increased rates of carboxylation could also generate such trends. In many cases the δ18O and δ2H values decrease in trees exposed to pollution and exhibit inverse coinciding long-term trends with δ13C values. However, some trees exposed to diffuse pollution and experimental conditions can show an increase or no δ18O change even if δ13C values increase. These diverse responses depend on how stress conditions modify physiological functions such as stomatal conductance, carboxylation, respiration, and perhaps water assimilation by the root system. Recent studies suggest that δ15N changes in trees can be caused by soil acidification and accumulation of anthropogenic emissions with isotopic signals deviating from natural N. Crown Copyright © 2009. Source


Key paleomagnetic poles are poles that are well defined and precisely dated. The rock unit from which the pole is derived must have a precise (usually U-Pb) age and the pole itself must be demonstrated primary with a rigorous field test. The use of key poles is essential in defining reliable apparent polar wander paths (APWPs) and establishing continental reconstructions. Many hundreds of Proterozoic paleopoles have been published from around the globe, but only ~45 are from large craton interiors and pass the key pole criteria. Most key poles are from mafic dykes and sills in the Superior craton (pre-1.83. Ga) or Laurentia (post-1.80. Ga) or Baltica. As a result, with occasional exceptions, it is difficult to define or compare reliable APWP segments in order to test Proterozoic continental reconstructions. However, there are now sufficient age matches or approximate age matches for pairs of key poles from a number of cratons to help constrain their relative locations. In this analysis, Proterozoic key poles are identified and their use in constructing APWPs and testing continent and supercontinent reconstructions is discussed. This key pole database establishes a well constrained Superior craton-Laurentia APWP for much of the Proterozoic that can be used as a reference track against which a growing number of individual key poles from other cratons can be compared. There is now a robust Baltica-Laurentia reconstruction for ~330. m.y. between 1.59 and 1.26. Ga using this approach and potentially for ~570. m.y. between 1.83 and 1.26. Ga if additional key and non-key poles from well-dated units are considered. Key pole comparisons for several other cratons yield preliminary constraints on the relative movement of cratons (e.g., Slave and Superior cratons in the Paleoproterozoic) or on specific elements of continental reconstructions (e.g., Amazonia and Baltica in the Mesoproterozoic, South China craton and Australia in the Neoproterozoic, or Baltica and Laurentia also in the Neoproterozoic). © 2014. Source


Bédard (2006) proposed that Archaean cratons formed above large, long-lived mantle plumes, where the thick basaltic crust cannibalized itself to generate TTGs (tonalite-trondhjemite-granodiorite) and complementary eclogitic restites. In this model the dense eclogitic restites foundered into the depleted upper mantle and refertilized it, triggering generation of new basaltic melt pulses, and so eclogite represents a catalyst for coupled crust-mantle differentiation. Since most of the eclogite is destroyed in the upper mantle, voluminous hidden eclogitic reservoirs are not predicted. The model was not intended to explain the generation of overprinting fabrics and terrane assembly, but to account for chemical evolution of the coupled crust-mantle system in the initial stages of craton development. Wyman (2013) argues that the models and hypotheses presented in Bédard (2006) are unrealistic and irrelevant, and reaffirms his opinion that the Archaean world was dominated by plate tectonics. The criticisms and arguments of Wyman (2013) are invalid. © 2012 Published by Elsevier B.V. Source

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