HR: 12:05h
AN: S31H-08    [PDF]
TI: Crustal Structure and Mantle Anisotropy from Florida to Edmonton
AU: * Fischer, K M
EM: Karen_Fischer@brown.edu
AF: Department of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: Syracuse, E
AF: Department of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: Rondenay, S
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AU: Wysession, M E
AF: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 United States
AU: Salas, M
AF: Department of Geology, University of Puerto Rico, Mayaguez, PR 00681 United States
AU: Doermann, L
AF: Department of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: Welsh, M
AF: Department of Geological Sciences, Brown University, Providence, RI 02912 United States
AB: New constraints on crustal thickness and velocity and mantle anisotropy beneath the east-central United States and southern Canada were obtained using data from the Florida to Edmonton Broadband Seismometer Experiment (FLED). This array included 28 broadband IRIS/PASSCAL seismometers that were in the field from May, 2001 to October, 2002, and adjacent permanent stations from the IRIS/GSN, the USNSN and the CNSN. Crustal properties were determined using Ps phases scattered from the Moho and, where possible, crustal reverberations. Waveforms at each station were corrected with an inverse free surface transform, simultaneously deconvolved, migrated to depth in one dimension, and modeled using migrated propagator matrix synthetic seismograms. Crustal thickness varies from 30 km to 50 km along the array. Significant features include zones of thickened crust beneath the southern Appalachian mountains and the late-Proterozoic Mid-Continent Rift in Iowa. The thickness of the southern Appalachian crustal root relative to surface topography is consistent with the root to topography ratio found in the northern Appalachians of Pennsylvania using data from the Missouri to Massachusetts Broadband Seismometer Experiment. However, both are large when compared to root/topography ratios in young orogens. These results are consistent with global trends in orogenic crustal thickness, mountain topography and gravity anomalies that suggest a systematic decrease with age in the buoyancy of crustal roots relative to the mantle. To constrain anisotropy beneath the FLED array, shear-wave splitting was measured in SKS and other teleseismic core phases. At southeastern stations located between the continental margin and the Appalachian Front and at stations within the central U.S. craton, where the lithosphere defined by tomographic studies is significantly thicker, shear-wave splitting fast directions are roughly parallel to North American absolute plate motion, and splitting times range from 0.6 s to 1.4 s. One exception occurs in the Mid-Continent Rift Zone in Iowa where a significantly different (roughly N) fast direction was obtained. Apart from this latter observation, the measurements are consistent with splitting found elsewhere in the craton and southern Appalachians by previous studies. Fast directions over this broad region may in general be explained by lattice-preferred orientation of olivine in the asthenosphere as it flows beneath and around the base of the lithosphere, although contributions from anisotropy due to past deformation in the lithosphere are also possible. In contrast, the N fast direction in the Mid-Continent Rift is not easily modeled by asthenospheric flow alone and is evidence for a local lithospheric fabric possibly associated with extension at roughly 1.1 Ga.
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Seismology [S]
MN: 2003 Fall Meeting