HR: 11:05h
AN: T22B-04    [Abstracts]
TI: Upper mantle anisotropy and shear wave velocity structure beneath the Slave craton inferred from Rayleigh waves
AU: * Chen, C
EM: cwchen@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Rondenay, S
EM: rondenay@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Weeraratne, D S
EM: weeraratne@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC, 20015 United States
AU: Yang, Y
EM: yingjiE_yang@brown.edu
AF: Department of Geological Sciences, Brown University, P. O. Box 1846, Providence, RI 02912 United States
AU: Snyder, D B
EM: dsnyder@nrcan.gc.ca
AF: Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A 0E9 Canada
AB: The seismic structure and depth extent of cratonic lithosphere is crucial to understanding the dynamics of continental evolution and stabilization, a subject that remains poorly understood. In this study, Rayleigh wave phase and amplitude data are analyzed to constrain velocity structure in the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We measure phase velocities for periods between 20-142 s (with greatest sensitivity at depths of 28-200 km, but providing resolution down to ~300 km) of crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. To optimally resolve lateral velocity variations under this region, we employ an inversion technique that considers the multipathing and finite frequency scattering effects of surface waves. The technique recovers shear-wave (S) velocities and azimuthal anisotropy for each period, thus providing constraints on the depth of anisotropic structure that shear-wave splitting alone cannot recover. Phase velocities obtained for the Slave province are comparable to values from other cratons at shorter periods, but exceed global average by ~2% at periods above 60 s. One-dimensional inversion of phase velocities yields high S-wave velocities (4.72±0.02 km/s) representative of cratonic lithosphere between 50-200 km depths. The depth to the base of the cratonic lithosphere is estimated at ~200 km, in agreement with petrological constraints from mantle xenolith analyses. Two-dimensional phase velocity maps indicate noticeable lateral variations. In the western part of the Slave, which corresponds to the oldest portion of this craton, high velocities are consistently observed between 25-33 s. In contrast, lower velocities are observed at periods 25-50 s in the central part of the craton, a region that hosts an important cluster of kimberlite pipes. We investigate whether these anomalies are crustal signals or may extend to the deeper mantle. Inversion for anisotropic parameters reveals two distinct trends in the Slave's lithosphere. A fast direction of N59°E±20° is found at periods above 29 s, consistent with the direction of absolute plate motion. At shorter periods 20-25 s, the fast direction is N10°E±20°, corresponding mainly to crustal structure. These anisotropic characteristics are in remarkable agreement with results from shear-wave splitting analyses supporting a previously proposed two-layer model, and suggest a significant anisotropic structure in the Slave craton's lithosphere and perhaps sub-lithospheric mantle that is likely influenced by asthenospheric flow.
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8125 Evolution of the Earth (0325)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8180 Tomography (6982, 7270)
SC: Tectonophysics [T]
MN: Fall Meeting 2005