Surface-wave anisotropy of the Eastern North American Margin (ENAM)

Abstract

The eastern North American margin (ENAM) formed during the breakup of Pangaea 175–200 Ma, recording the onset of rifting. The ENAM Community Seismic Experiment (2014–2015) consisted of an onshore/offshore broadband and short-period ocean bottom seismometer (OBS) deployment aimed at better characterizing this relatively understudied region. In this study, we utilize the 28 ENAM broadband OBS to measure Rayleigh-wave phase velocities from ambient-noise (15-40 s period) and teleseisms (40-150 s). Tilt noise is especially high due to the Gulf Stream current; thus, tilt is removed from the vertical component of both teleseismic and ambient-noise waveforms. We observe Rayleigh-wave azimuthal anisotropy of ~2–3% from 15-40 s period with margin-parallel fast directions, orthogonal to predictions of olivine alignment generated by corner-flow at the paleo-ridge. Previous shear-wave splitting observations showed similar margin-parallel splitting, which was attributed to present-day margin-parallel asthenospheric flow. Our measurements are mostly sensitive to the lithosphere, suggesting that this signal is not purely due to present-day asthenospheric flow, but instead, is embedded in the fossilized fabric of the oceanic lithosphere. This observation has implications for the mantle flow field during and immediately after the rifting of Pangaea and subsequent opening of the Atlantic Ocean.