HR: 10:50h
AN: T32A-03    [Abstracts]
TI: Understanding Low Velocity Zones and Melt Reservoirs
AU: * Savage, B
EM: savage13@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., NW, Washington, DC 20015 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., NW, Washington, DC 20015 United States
AB: Low velocity zones within the mantle are poorly characterized seismically, especially in the upper mantle. They do, however, play a significant role in understanding mantle convection and the Earth's thermal evolution. In the recent years numerous investigators [Revenaugh and Sipkin, 1994; Thybo and Perchuc, 1997; Vinnik and Farra, 2002; Vinnik et al., 2003; Savage et al. 2003; Song et al., 2004] have reported substantial velocity decreases between the crust and 410 km discontinuity that have been interpreted as sublithospheric partial-melt zones. Encompassing all tectonic settings, these studies have relied upon a varying techniques from receiver functions to waveform modeling in order to identify the velocity decreases. We seek to develop a systematic procedure that is optimized for detecting sublithospheric low velocity zones that may reflect the presence of partial melt. As a first step, we use the reported low velocity zone models noted above, and create synthetic data sets in 1,2,and 3 dimensions to determine the distinguishing characteristics of low velocity zones when probed with a variety of techniques. It is well known that first-arrival travel times are not an appropriate data set for low velocity zone investigations. Rather, wavefield propagation in the horizontal (later-arriving phases from local and regional earthquakes) and the vertical (receiver function and mantle reflectivity profiles from teleseisms) directions provide important and complementary constraints on the thickness, lateral extent as well as the absolute velocity of these regions. This investigation aims to combine these two approaches to provide useful characteristics of low velocity zones so they may be mapped worldwide in conjunction with upper mantle heterogeneity. Further, it has been suggested [Silver et al. 2004] that large, long-lived reservoirs of melt may exist just beneath old cratonic lithosphere, and are hypothesized to source for the flood basalts created during catastrophic diking events. Our initial areas of interest are thus cratonic areas possessing broadband seismic data from regional and teleseismic events, such as the Kaapvaal craton in southern Africa and cratonic South America. Using these data, we test for the presence of melt reservoirs by looking for extensive low velocity zones in the sublithospheric upper mantle.
DE: 8110 Continental tectonics--general (0905)
DE: 8159 Rheology--crust and lithosphere
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting