HR: 0830h
AN: S31C-0782    [PDF]
TI: Mantle Anisotropy Beneath the Alaska Range Inferred From S-wave Splitting Observations: Results From BEAAR
AU: * Christensen, D H
EM: doug@giseis.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, P.O. Box 757320, Fairbanks, AK 99775-7320 United States
AU: Abers, G A
EM: abers@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215 United States
AU: McKnight, T L
EM: tlmcknig@mtu.edu
AF: Michigan Technological University, Department of Geology, Houghton, MI 49931 United States
AB: The tectonics of southern and central Alaska are dominated by the subduction of the Pacific plate beneath the North American plate. The Broadband seismic Experiment Across the Alaska Range (BEAAR) was a 2.5-year PASSCAL deployment utilizing 36 closely spaced (10-15 km) broadband sensors. These instruments were deployed across the Alaska Range and above the subducting Pacific plate in roughly north-south and east-west lines. Much of the study region is underlain by a mantle wedge, which gradually pinches out at the southernmost stations. The depth of the subducting slab increases to the northwest with the deepest earthquakes occurring at 150 km, just north of the Alaska Range. Teleseismic SKS and S phases, in addition to S waves from local earthquakes in the subducting plate, were used to study the anisotropic behavior of the upper mantle in central Alaska. Two distinct regions of anisotropy are observed, reflecting both flow in the direction of plate convergence and flow parallel to the strike of the subducting plate. The transition between these two zones appears to be very sharp occurring near the 70-75 km contour of the subducting plate. Waves arriving through material southeast of this line (through the shallow corner of the mantle wedge) show fast directions that are parallel to the plate convergence direction. North of the 70-75 km slab contour the fast directions abruptly change orientation to roughly parallel the strike to the subducting plate. This direction is also parallel to the trend of several major tectonic features in the area, including the Alaska Range, and the right-lateral strike slip Denali and Tintina faults. Stations that are located above the 70-75 km contour of the subducting Pacific plate show characteristics of both regions, depending on the back azimuth of the arrivals. These observations set strong constraints on the location of the transition zone between the two regions. While teleseismic observations may represent anisotropy (or flow) in the mantle wedge, in the subducting Pacific plate, or beneath the subducting plate, observations from local slab events indicate that a significant portion of the signal comes from the mantle wedge and that the sharp change in direction is a feature of the mantle wedge.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Seismology [S]
MN: 2003 Fall Meeting