Far-offset Airgun Imaging of the Mantle: Lithospheric Anisotropy of the North Atlantic

Melt extraction and associated mantle flow beneath a spreading center produces coherent fabric in olivine-rich mantle rocks that is retained in the mantle lithosphere as it translates away from the ridge. Observations of seismic anisotropy provide a means to map this mantle fabric and thus flow associated with ridge processes. Refraction and surface-wave studies have successfully delineated the anisotropic structure of Pacific lithosphere, and to first order, these results agree with simple passive-spreading models of the mid-ocean ridge. Numerical models suggest that slow-spreading lithosphere may be characterized by anisotropy that is very different than that observed in the Pacific due to the possible influence of buoyancy-driven upwelling and along-axis flow. Unfortunately, observations of anisotropy in the Atlantic and other slow-spreading environments are limited to long-period surface waves, and the details of lithospheric fabric in such regions are largely unknown. We investigate the anisotropic structure of old Atlantic lithosphere using refraction data recorded during the FAIM (Far-offset Airgun Imaging of the Mantle) experiment. This experiment consisted of airgun shots to a 700-km-long linear array of 16 ocean-bottom seismometers (OBS), with an additional 3 OBS deployed perpendicular to the main array, located 150 km SW, 75-km SW, and 75-km NE from the line. The shots were also recorded by a broad-band seismometer deployed on Bermuda, perpendicular to the line approximately 350 km NE. Preliminary analysis of one of the off-line instruments indicates that refracted arrivals were successfully recorded to a source-receiver distance of over 250 km. These data will provide azimuthal record sections that span up to 160 degrees. The travel-time residuals from these data will be corrected for along-path heterogeneity using the results of the primary refraction profile, and the remaining azimuthal variation will be modeled for the direction, magnitude and depth distribution of anisotropy in the Atlantic lithosphere at a length scale of a few hundred km. By comparing the signal from the different instruments, we hope to assess the impact of the Bermuda swell on the travel-time observations. We will also compare lithospheric anisotropy inferred from the refraction data with that implied by shear-wave-splitting analysis of up to 40 teleseismic events recorded on Bermuda.