HR: 16:15h
AN: T24B-02    [Abstracts]
TI: Mantle Temperature Variations in Active Back-Arc Basins Inferred from Seismology, Petrology, and Bathymetry
AU: * Wiens, D A
EM: doug@seismo.wustl.edu
AF: Dept. of Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Kelley, K
EM: kelley@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Dept. of Earth Sciences, Boston University, Boston, MA 02215 United States
AB: Active backarc spreading centers show wide variations in axial depth and petrological characteristics. We investigate the correlation between these characteristics and the seismological structure of the North Fiji, Mariana, Lau, and S. Scotia backarc basins. These basins are chosen because they have large spatial dimensions and good waveform paths allowing accurate determination of the average upper mantle structure. For each backarc, vertical and transverse seismograms are inverted from several representative paths traversing the basin, with source to station distances of 650-1300 km. A variety of earthquake depths permits greater resolution of deeper structure. Synthetics are calculated using a reflectivity method, and the nonlinear inversion is carried out using a niching genetic algorithm, which allows alternative local minima to be explored. The inversion solves for separate path-averaged SH and SV velocity structures, with a penalty function to ensure that any polarization anisotropy is required by the data. The results show that all four backarc systems are characterized by low seismic velocity and moderate levels of polarization anisotropy (1-4.5%), with the anisotropy largely confined to the upper 100 km. Substantial differences in seismic velocity between the backarc regions occur at depths of 40-100 km, with differences of up to 7% between the slowest (Lau) and the fastest (Mariana) structures. The mantle seismological structures correlate with major element systematics and bathymetry of the basins, indicating differences in average upper mantle temperatures. The Lau basin shows shallow bathymetry and low Na8, high Fe8, and high Ca/Al, all indicating higher mantle temperatures (potential temperature of 1450\deg C from the major elements). The Mariana Trough shows the opposite trends in all these variables, indicating low upper mantle temperatures (1360\deg C), and the North Fiji basin is intermediate (1410\deg C). The South Sandwich backarc shows intermediate values of seismological structure and ridge elevation, but major elements indicate colder mantle temperatures (1350\deg C). The petrological indicators suggest a ~100\deg C range in mantle potential temperature, but these differences are not large enough to explain the seismological or axial depth data. Some variation in the percentage of partial melt between the hot and cold spreading centers could enhance the differences in seismic velocity. In addition, some contribution from temperature variations below 100 km depth is required to satisfy the ridge elevation data.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting