HR: 0800h
AN: T31A-0493    [Abstracts]
TI: Emplacement of Oceanic Crust Can Continue for Several Hundred Thousand Years
AU: * Durant, D T
EM: ddurant@uoregon.edu
AF: U. of Oregon, Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AU: Toomey, D R
EM: drt@uoregon.edu
AF: U. of Oregon, Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AB: We report on the seismic imaging of magma-filled sills located ~20 km off axis near the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones. The seismic data were collected during the UNDERSHOOT experiment, which was designed to test competing magma supply models for the EPR by mapping the pattern of magma delivery from the mantle to the crust along the entire length of a transform-bounded ridge segment. Previously reported delay-time tomographic images reveal a mantle low-velocity zone (MLVZ) beneath the EPR that defines both on- and off-axis centers of magma accumulation spaced at along-axis intervals of 15 to 25 km. The tomographic results can be explained by a 1-3% change in melt fraction; however, melt concentrations could be greater if distributed anisotropically. To find evidence of off-axis magma bodies, we analyze seismic waveform data recorded on 12 three-component ocean-bottom seismometers (OBSs) located 10 and 20 km off axis. We search data for evidence of P-wave shadow zones and anomalous P-to-S conversions consistent with magma-filled sills. In a region where the MLVZ is located 10-15 km east of the rise, we find compelling evidence for anomalously low crustal and mantle velocities. Data from an OBS located 20 km east of the rise reveal an amplified secondary phase on the radial channel consistent with a solid-melt interface. Additionally, a crustal-level P-wave shadow zone is detected near the off-axis MLVZ. We find no evidence for off-axis low-velocity anomalies in regions where the MLVZ is located beneath the rise. To better map the location and physical properties of the anomalous features, we are using a combination of travel time and full waveform modeling. Our preliminary interpretation is that in regions where the MLVZ is located off axis, magmatic processes modify both the crust-mantle transition zone and the lower crust. An unusual aspect of this discovery is that there are no seamounts nor any other bathymetric features in the region that would indicate the presence of accumulated melt below the seafloor. We conclude that while the majority of oceanic crust is emplaced at the rise axis, the process of crustal formation can continue for at least a few hundred thousand years, particularly when mantle upwelling is not centered beneath the rise.
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 8150 Plate boundary: general (3040)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Tectonophysics [T]
MN: Fall Meeting 2005