HR: 11:50h
AN: T51H-06    [PDF]
TI: Mid-Ocean Ridge Mantle Processes Constrained by the FAIM Seismic Refraction Experiment
AU: * Collins, J A
EM: jcollins@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Falmouth, MA 02543 United States
AU: Lizarralde, D
EM: danl@eas.gatech.edu
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340 United States
AU: Gaherty, J B
EM: gaherty@eas.gatech.edu
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340 United States
AU: Gaherty, J B
EM: gaherty@eas.gatech.edu
AF: Lamont-Doherty Earth Observatory, Box 1000, Palisades, NY 10964 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Falmouth, MA 02543 United States
AU: Kim, S
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340 United States
AB: The seismic structure (e.g. velocity, velocity gradients, anisotropy) of the shallow upper mantle of oceanic lithosphere constrains the degree of melt extraction at mid-ocean ridges and the shear deformation of the lithosphere. The variation of fine-scale structure with depth is difficult to resolve with teleseismic data but, with some notable exceptions, seismic refraction techniques - which have excellent depth resolution - have not been used to elucidate oceanic upper mantle structure. Here we report further results from the FAIM (Far-offset Active-source Imaging of the Mantle) seismic refraction experiment conducted along an 800-km-long flow-line transect and a 150-km-long ridge-parallel transect in the western north Atlantic. Sixteen short-period, vertical-component ocean-bottom seismometers (OBS) were deployed along the spreading-parallel transect in 3-km-separated pairs spaced 80-120 km apart, and 3 OBS were deployed orthogonal to the primary transect to a maximum distance of 350 km. Shot spacing was 1 km. Coherent P-wave arrivals were observed to 350-km distance, and the amplitude of this phase and its increasing and fast apparent-velocity suggest that energy is propagating to $\sim$24 km depth below the Moho. Two-dimensional travel-time analyses predict velocities of $\sim$8.25-8.55 km/s over the uppermost 24 km of the mantle on the flow-line-parallel transect and velocities of $\sim$8.05-8.25 km/s over the uppermost 10 km of the mantle on the ridge-parallel transect. At face value, these values indicate P-wave anisotropy is at least $\sim$3.0% throughout the upper 10 km of the mantle. However, velocities in only two orthogonal directions cannot constrain the precise direction and magnitude of anisotropy. To improve this estimate we analyzed the travel times of P-waves with good signal to noise from shots at ranges of 75-250 km, with an azimuth range spanning $\sim$100\deg. When plotted as a function of propagation azimuth, these travel times display a strong variation, with slower travel times arriving near an azimuth of 100-120$\deg$ relative to the fossil flow-line, i.e. nearly perpendicular to the fossil spreading direction. Assuming that azimuthal anisotropy in the region is constant, we can estimate its magnitude and direction by fitting delay times with a function that is periodic in 2$\theta$. The best-fitting model has a fast propagation direction at $\theta$ = 10$\deg$, with a peak-to-peak $\delta$t variation of 0.3 s, which corresponds to a P-wave anisotropy of 4.0%, approximately 50% smaller than that observed in comparable experiments on lithosphere formed at faster spreading rates in the Pacific ocean. The large positive velocity gradients measured along both transects are not readily explained with existing predictions of upper mantle deformation. Likewise, the observations are unlikely to be due to depth variations in cracking because cracking in the mantle might be expected to have a preferred direction. One possible explanation for the large gradients is the presence of gabbro plutons in peridotite due to melt retention in the shallow mantle.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting