Mantle Flow Beneath the Juan de Fuca and East Pacific Rise Spreading Centers and Adjacent Plates

Observations of seismic anisotropy are a principal means of inferring the direction of mantle flow beneath tectonic plates. Azimuthal anisotropy of mantle head waves (Pn) observed in mid-plate settings, for example, has been used to infer that beneath oceanic crust the mantle flow that is frozen in is parallel to the paleospreading direction. While the agreement between historical measurements of azimuthal anisotropy and paleospreading direction is good, the combined uncertainties in experimental results (many of which date back 30 to 50 years) and in inferring the paleospreading direction are often 10-15°. In contrast to historical results from mid-plate settings, recent studies of Pn anisotropy beneath the East Pacific Rise (EPR) and the Mid-Atlantic Ridge reveal that the fast-direction of seismic anisotropy - and by inference the direction of mantle flow - is skewed with respect to the current spreading direction. This result indicates that sub-ridge mantle flow is not an entirely passive response to plate spreading. Here we use data from recent active-source seismic experiments to investigate azimuthal anisotropy of Pn arrivals in two near-ridge settings. These modern experiments, which use dense arrays of ocean-bottom seismometers (OBSs) and well-navigated seismic shooting lines, can constrain azimuthal anisotropy to within ±1°. One data set is from the multi-scale Endeavour seismic tomography experiment (ETOMO) that took place in September 2009. Seismic data were collected using 68 four-component OBSs at 64 sites and the 6600 in3 airgun array of the R/V Marcus G. Langseth. The study includes 5567 shots covering 90 km along-axis and 50 km across. The second data set is from the UNDERSHOOT experiment, which was conducted at the EPR between the Siqueiros and Clipperton transforms, a section of ridge that is sub-divided by the 9°03'N overlapping spreading center (OSC). Seismic data were collected using a combination of four-component OBSs and single-component hydrophones at 57 sites and the 8500 in3 airgun array of the R/V Maurice Ewing. The study includes ~5000 shots covering 200 km along-axis and 100 km across. For each data set we document evidence for mantle anisotropy using Pn arrivals that propagate beneath the axis of spreading and for paths that sample solely the Pacific, Juan de Fuca and Cocos plates. We also present results for subsets of the data that are grouped geographically. For the EPR we select data subsets that sample three regions, one each north and south of the 9°03'N OSC and one centered on the OSC. For these subsets, the azimuth of anisotropy is effectively identical to that reported previously for the entire data set (~10° anticlockwise of the spreading direction). We thus conclude that the orientation of seismic anisotropy does not vary with local plate boundary geometry, e.g. the trend of ridge segments which differ to either side of the OSC or the presence of the OSC itself. Instead, the anisotropy is a regional-scale signal related to the azimuth of mantle divergence. Our results will further test the hypothesis that beneath spreading centers the axes of mantle upwelling and asthenospheric flow are skewed with respect to the plate spreading direction.