HR: 1330h
AN: S22A-0423    [PDF]
TI: How Sharp is the Base of the Lithosphere?
AU: * Rychert, C A
EM: Catherine_Rychert@Brown.edu
AF: Brown University, Box 1846 Department of Geologucal Sciences, Providence, RI 02906 United States
AU: Rondenay, S
EM: rondenay@mit.edu
AF: MIT, Department of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Avenue, Cambridge, MA 02139-4307 United States
AU: Fischer, K M
EM: Karen_Fischer@Brown.edu
AF: Brown University, Box 1846 Department of Geologucal Sciences, Providence, RI 02906 United States
AB: Precisely determining the depth and the velocity gradient at the base of the continental lithosphere has proved challenging with existing body and surface wave tomography. Higher resolution imaging of this boundary can be achieved through the analysis of teleseismic P-to-S (Ps) converted phases generated at the discontinuity. In this study we have found a Ps phase which appears to emanate from the base of the lithosphere beneath eastern North America. We have used the timing, amplitude, and frequency dependence of the phase to invert for the properties of this discontinuity. To image the discontinuity, waveforms recorded at single stations were decomposed into their P and S components and migrated to depth through least-squares simultaneous deconvolution in each of 9 epicentral bins. Data from stations HRV in Massachusetts, LMN in New Brunswick, and PAL and BINY in New York reveal a negative velocity contrast at $\sim$90-100 km depth over a $\sim$450 km swath of the Appalachian orogen. This depth range is consistent with the thickness of the lithosphere found in previous surface wave tomography studies. At HRV, where the phase is most clearly observed, its amplitude decreases as the low-pass cut-off frequency increases, indicating that the velocity contrast occurs over a non-zero depth range. Crustal thickness and velocity and an observed mid-lithospheric discontinuity were first modeled to ensure that their effects on the phase of interest are accounted for. Finally, we inverted frequency-dependent waveforms containing the phase from the base of the lithosphere in different epicentral bins. The dominant period of the incident P-wave was constrained by comparing observed auto-deconvolved P-waves to synthetic data at various periods, and during the inversion the period of the incident waveform was allowed to vary within the error bars determined in the auto-deconvolution test. After each inversion iteration, the migration model was adjusted to match the best-fitting model, and the data were remigrated and reinverted to confirm that the last result remained identical to the preferred model. Initial inversions indicate a 7-10% velocity drop that occurs over no more than 5 km located at $\sim$95km depth. Such a strong and sharp velocity contrast at the base of the lithosphere is not consistent with a purely thermal gradient and suggests the influence of other factors such as volatile variations.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9350 North America
SC: Seismology [S]
MN: 2003 Fall Meeting