HR: 1340h
AN: S53B-0207    [Abstracts]
TI: Evidence for Localized Down Welling of Lithospheric Material Through the Upper Mantle Transition Zone Beneath Southern California
AU: Lewis, J
AF: Texsa Tech, Dept of Geosciences, Texas Tech, Lubbock, TX 79409 United States
AU: * Gurrola, H
EM: harold.gurrola@ttu.edu
AF: Texsa Tech, Dept of Geosciences, Texas Tech, Lubbock, TX 79409 United States
AB: Common conversion point stacking of receiver functions has been applied to data from 21 seismic stations located throughout southern California to produce an image of the upper mantle transition zone. This method has gained wide acceptance in the investigation of the upper mantle transition zone (TZ) which is generally believed to be the depth range in the mantle where olivine undergoes a series of phase changes. The olivine to spinel phase change is elieved to be responsible for the 410 km discontinuity and the perovskite and magnesiowstite is generally accepted to be associated with the 660 km discontinuity. An investigation of depth variations in the 410 and 660 km discontinuities is generally considered to be a means to identify variations in mantle temperatures associated with the respective phase change. That is that the 410 will become shallow in cooler regions of the mantle and deeper in warm regions. The 660 will behave oppositely in response to thermal variation in the mantle. As a result we would find a thick transition zone in cooler regions and a thin transition zone in warm regions. The transverse ranges in southern California have been subject to much investigation because they do not appear to have an adequate crustal root to account for observed topography. Humphreys and Hager (1990) have found evidence of very localized subduction or a "drip" of lithospheric material beneath this region, as a result of convergence due to a kink in the San Andreas fault system, which can account for the observed topography. Using seismic tomography they imaged a high velocity anomaly (generally considered to be evidence for cooler mantle temperatures) directly beneath the transverse ranges to a depth of almost 300 km. We have found in receiver function images that there is an anomalously thick TZ directly beneath this "drip" beneath the transverse range which would indicate a small localized low temperature anomaly no more than 100 km across. We interpret this feature as evidence that the "drip" feature penetrate the TZ or that the drip feature became detached and is stagnant in the TZ. Our TZ image also suggests other localized down-welling features beneath the Southern Sierra Nevada Range.
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting