HR: 0800h
AN: V21A-0592    [Abstracts]
TI: Field-based Constraints on Lower Crustal Flow From the World's Largest Exposure of Lower Continental Crust, Northern Saskatchewan, Canada
AU: * Dumond, G
EM: gdumond@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts-Amherst 611 North Pleasant Street, Amherst, MA 01003 United States
AU: Gonclaves, P
EM: philippe.goncalves@univ-fcomte.fr
AF: Département des Géosciences, Université de Franche-Comté 16 route de Gray, Besancon, 25030 France
AU: Williams, M L
EM: mlw@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts-Amherst 611 North Pleasant Street, Amherst, MA 01003 United States
AU: Bowring, S A
EM: sbowring@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AB: Predictions about the behavior and geometry of lower continental crust during orogenesis have included: it is rheologically weak; it flows under the influence of a tectonic or topographic load; and it is characterized by pervasive shallow fabrics produced by high-temperature deformation mechanisms. Arguably the world's largest exposure of lower continental crust that still preserves much of its deep crustal deformation history is the central portion of the Snowbird tectonic zone in the western Canadian Shield. Recent fieldwork along a ca. 100 km-long transect of this exposure is characterized by an early, penetrative shallow fabric. A 40 km-long segment of this transect, dominated by charnockite and granodiorite orthogneisses, is characterized by km-scale domains of shallow, granulite-grade gneissic foliation (S₁) with a spectacular rodding lineation (L₁) defined by: 1) discontinuous ribbons of recrystallized Pl + Qtz + Hb + Cpx + Opx, in addition to mm- to cm-scale core-and-mantle structure in Pl and Kfs, and 2) near-continuous, 10s of cm-long rods of compositional banding. Isoclinally-folded layering is locally preserved perpendicular to (L₁). We interpret (L₁) as a composite lineation with both intersection and extension components. Thermobarometric data, microstructural, and kinematic observations are compatible with high-grade (700-800°C) ductile, top-to-the-ESE flow during production of S₁ at 1.0-1.1 GPa (30-40 km paleodepths in the Neoarchean. S₁ is variably transposed into upright, open, shallowly-plunging F₂ folds with sub-horizontal, NW-striking enveloping surfaces. The weakly folded S₁ is locally overprinted by <10 m-wide penetrative high-strain zones in which S₁ has been transposed into steeply-dipping, NE-striking foliation (S₂). D₂ high-strain zones contain shallow SW-plunging stretching lineations (locally L-tectonites) and dextral, oblique-slip kinematics. D₂ low-strain zones preserve Type 2 (mushroom-crescent) fold interference patterns resulting from superposition of upright F₂ folds with sub-vertical NE-striking axial planes onto isoclinal, recumbent F₁ folds. Metamorphic reactions that led to Grt-production during development of S₁ were intrinsically syn-kinematic, with garnet growing in the Na-rich recrystallized mantles of Pl-porphyroclasts. Relatively H₂O-poor and/or CO₂-rich conditions are required by the preservation of fine-grained microstructures and absence of grain-coarsening or recrystallization in the S₁ tectonite. We speculate that the shallow S₁ tectonite exposed in the central Snowbird tectonic zone transect represents an important and unique field-based analog for the nature of deep crustal reflectivity and lower crustal flow in collisional orogens. Furthermore, our results suggest that the strength of the lower continental crust is dynamic and evolving. In this particular case, flow of relatively weak lower crust during production of S₁ was followed by a period of near-isobaric cooling and strengthening. Subsequent deformation events produced steep fabrics (e.g. S₂), 10s of m- to 100s of km-scale moderately- to steeply-dipping shear zones, and local reactivation of S₁, reflecting the dramatic effects of strain partitioning in a heterogeneous and anisotropic medium.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1020 Composition of the continental crust
DE: 1115 Radioisotope geochronology
DE: 3660 Metamorphic petrology
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005