HR: 08:15h
AN: T41F-02    [Abstracts]
TI: High-Resolution Seismic Constraints on Oceanic Crust and Sediment Subduction
AU: * Horleston, A C
EM: ach17@le.ac.uk
AF: Seis-UK, Geology Department, University of Leicester, University Road, Leicester, LE1 7RH United Kingdom
AU: * Horleston, A C
EM: ach17@le.ac.uk
AF: University of Bristol, Department of Earth Sciences, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Helffrich, G
EM: george@gly.bris.ac.uk
AF: University of Bristol, Department of Earth Sciences, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AB: Low-velocity layers observed at the top of subducting slabs have long been thought to consist of some or all of the subducted oceanic crust and sediment. However, previous estimates of the thickness of these layers have been poorly constrained giving thicknesses between 2 and 10 km. These results provide only broad constraints on possible subduction factory models and chemical recycling budgets that we seek to improve upon. Here we present a new method for directly determining the thickness of these layers based on observations of S-to-P converted phases from local slab earthquakes. We apply frequency-domain polarisation filtering to isolate mode-converted (P→S and S→P) energy in the seismograms. Two S-to-P converted phases are possible from the upper and lower interfaces of the layer, but they are distinguishable by their relative polarity to the direct P phase. Using differential travel times obtained by measuring these phases, we apply a fitting technique to determine the low-velocity layer thickness. Synthetic tests indicate that thicknesses may be determined to better than 250 m, but the uncertainty depends on event distribution. We present results from two study areas, the Shumagin Islands, Alaska and Guam in the Marianas. These regions were chosen due to the geometry of the slabs, the location of seismic stations and in light of previous work done in each region. In the Shumagins, the thickness of the low-velocity layer is 7 ± 0.25 km, and in the Marianas it is 6 ± 0.5 km. Given sediment input to these trenches, the results in both locations suggest that complete subduction of the oceanic crust and sediment layer occurs. We show an example of how the results may be used to update element recycling budgets through subduction zones.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 7220 Oceanic crust
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005