Jamieson, Rebecca A.
Permanent URI for this collectionhttps://hdl.handle.net/10222/22093
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Item Open Access Last Gasp of the Grenville Orogeny - Thermochronology of the Grenville Front Tectonic Zone Near Killarney, Ontario(1993-09) HAGGART, MJ; Jamieson, Rebecca Anne; REYNOLDS, PH; KROGH, TE; BEAUMONT, C.; CULSHAW, NGWe present U-Pb (titanite, zircon) and Ar-40/Ar-39 (hornblende, mica, K-feldspar) data from a transect across the western part of the Grenville Front Tectonic Zone (GFTZ) near Killarney, Ontario. High-grade metamorphic assemblages (approximately 1450 Ma) in this part of the GFTZ pre-date the Grenvillian orogeny and were primarily exhumed, with little or no metamorphic overprinting, by Grenvillian deformation. The titanite and zircon data form a discordant array with an upper intercept of 1454 +/- 8 Ma and a lower intercept of 978 +/- 13 Ma. These data are interpreted in terms of partial lead loss during a short-lived thermal event that increased in intensity from west to east across the transect. Ar-40/Ar-39 data from hornblende indicate cooling through approximately 450-degrees-C at approximately 993-979 Ma, multiple diffusion domain models for the interpretation of discordant K-feldspar spectra indicate cooling through approximately 365-340-degrees-C at 990-960 Ma, and muscovite data indicate cooling through approximately 320-degrees-C at approximately 930 Ma. Biotite data are not easily interpreted owing to the effects of partial resetting and/or excess Ar-40. The thermochronological data suggest that a thermal event with peak temperatures of 500-600-degrees-C affected the GFTZ at approximately 980 Ma, followed by very rapid cooling to approximately 350-degrees-C. We interpret the data in terms of a tectonic model involving rapid exhumation of GFTZ rocks (in response to erosion) in the hangingwall of a crustal-scale shear zone developed during a approximately 980 Ma episode of convergence.Item Open Access Devonian extension in Northwestern Newfoundland: Ar-40/Ar-39 and U-Pb data from the Ming's Bight area, Baie Verte Peninsula(2001-03) Anderson, SD; Jamieson, Rebecca Anne; Reynolds, PH; Dunning, GRThe Ming's Bight Group of northwestern Newfoundland, an outlier of Humber Zone continental margin rocks, is entirely surrounded by ophiolitic rocks of the Dunnage Zone. Structures in the Ming's Bight Group and adjacent units record three main phases of deformation. The earliest structures relate to Silurian sinistral transpression previously documented in the region. Two later phases of extensional deformation produced a series of dextral oblique-normal shear zones and faults that now separate the Ming's Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. Ar-40/Ar-39 and U-Pb data constrain the times of oblique- normal shear and cooling. Metagabbro in the Point Rousse Ophiolite Complex, which lies in the hangingwall, preserves disturbed Ordovician hornblende Ar-40/Ar-39 ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record Ar-40/Ar-39 ages of 390-380 Ma. Synkinematic titanite and rutile porphyroblasts from an extensional shear zone on the northwestern margin of the Ming's Bight Group have been dated by the U-Pb method at 388 and 380 Ma, interpreted as growth and cooling ages, respectively. The titanite and hornblende ages suggest that the main phase of ductile oblique- normal shear was underway at 405-385 Ma. Ming's Bight Group schists and pegmatites produced concordant muscovite Ar-40/Ar-39 ages averaging 362 Ma, interpreted as the time of footwall cooling below 350 degreesC. We suggest that the Ming's Bight Group is a mid-Devonian symmetrical core complex formed within a local transtensional regime developed during dextral oblique transcurrent movement along the Baie Verte Line. The timing and tectonic setting of extension do not support recent models for "extensional collapse" in the northern Appalachians.Item Open Access Evolution of orogenic wedges and continental plateaux: insights from crustal thermal-mechanical models overlying subducting mantle lithosphere(2003-04) Vanderhaeghe, O.; Medvedev, S.; Fullsack, P.; Beaumont, C.; Jamieson, Rebecca AnneThe links between an early phase of orogenesis, when orogens are commonly wedge shaped, and a later phase, with a plateau geometry, are investigated using coupled thermal-mechanical models with uniform velocity subduction boundary conditions applied to the base of the crust, and simple frictional-plastic and viscous rheologies. Models in which rheological properties do not change with depth or temperature are characterized by growth of back-to-back wedges above the subduction zone. Wedge taper is inversely dependent on the Ramberg number (Rm; gravity stress/basal traction); increasing convergence velocity or crustal strength produces narrower and thicker wedges. Models that are characterized by a decrease in crustal viscosity from eta (c) to eta (b) with depth or temperature, leading to partial or full basal decoupling of the crust from the mantle, display more complex behaviour. For models with a moderate viscosity ratio, eta (b) /eta (c) similar to 10(-1), the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust. A reduction in the viscosity ratio (eta (b) /eta (c) similar to 10(-2) ) is sufficient to cause a transition of the central wedge region to a plateau. This transition depends on the basal traction, therefore the thickness of the weak basal layer also affects the transition. Further reduction of the viscosity ratio (eta (b) /eta (c) similar to 10(-4) ) leads to full basal decoupling and the development of plateaux in all cases considered. In most models, the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust. Where the crust is fully decoupled, large-scale model geometries for both depth- and temperature-dependent rheologies are similar with gravity-driven flow concentrated in the low-viscosity region. However, strong lateral temperature gradients within these models, controlled by the interaction of horizontal and vertical thermal advection, diffusion and heterogeneous thickening of the radioactive crustal layer, lead to differences in the velocity and deformation fields between the two cases, particularly at the plateau margins. The results suggest that simple depth-dependent viscosity models may be reasonable approximations for describing the large-scale geometry of fully developed plateaux, but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry.