HR: 09:30h
AN: T21C-07    [Abstracts]
TI: Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities
AU: * Snyder, D B
EM: dsnyder@NRCan.gc.ca
AF: Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A 0E9, Canada
AB: A 20-station seismic array in NW Canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the Mohorovicic and Lehman discontinuities. Following wave field decomposition, groups of seismograms were source-normalized through simultaneous deconvolution to estimate the near-receiver impulse response and thus detect major discontinuities beneath each seismic station. Stations within the Lac de Gras kimberlite field display an unusually strong negative impulse on the radial component within the NW quadrant and two moderate impulses on the transverse component. Forward modeling of these impulses suggests a mantle layer dipping at 22° to the southeast with a mildly anisotropic upper discontinuity at 120 to 135 km depth and another mildly anisotropic discontinuity at about 170 km depth. Superimposed on these layers is another, stronger anisotropic layer between 110 and 180 km depths that dips to the west. Stations outside of the Lac de Gras field, but within the southeastern Slave craton, display more numerous, but weaker impulses. The most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed further north. The prominent negative impulse on the radial component is interpreted to arise from structural- preferred orientation in the form of a stockwork of wehrlite dykes beneath the Lac de Gras field. Interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000-2900 Ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 Ma. The layer beneath the SE Slave has a similar, but distinct tectonic history of underthrusting associated with the 2635 to 2615 Ma Defeat Suite of plutonism. Taken together, these interpretations indicate that the Slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 Ma: each block is 90 to 120 km thick and abut across a near-vertical boundary beneath MacKay Lake in the central Slave craton. Significant amounts of carbon could have become incorporated into the central Slave mantle within the diamond stability field during the proposed underthrusting of lithosphere.
DE: 5102 Acoustic properties
DE: 7218 Lithosphere (1236)
DE: 8103 Continental cratons
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting