Seismic Imaging of Oceanic Lithosphere: The PI-LAB Experiment at the Equatorial Mid-Atlantic and the VoiLA Experiment in the Lesser Antilles

Catherine Rychert*1, Nicholas Harmon1, Mike Kendall2, Saikiran Tharimena3, Matthew Agius4, Petros Bogiatzis1, Ben Chichester1, Stephen Hicks1, Steve Constable5, Shunguo Wang5

1Ocean and Earth Science, University of Southampton,UK (c.rychert@soton.ac.uk), 2Department of Earth Sciences, University of Bristol, UK, 3NASA Jet Propulsion Lab, USA, 4Department of Earth Sciences, University of Malta, Malta, 5Scripps Institution of Oceanography, University of California, San Diego, USA

Ocean lithosphere is ideal for studying the definition of the tectonic plate and mantle dynamics. Here we present results from 2 large broadband passive seismic deployments on Atlantic seafloor. On and around the mid-Atlantic Ridge Sp receiver functions image a 4 to 8 km thick oceanic crust and a negative discontinuity, which deepens progressively away from the western ridge segment from 30 to 80 km beneath 0 to 40 My old lithosphere. Surface waves and magnetotelleurics image thickening with age although not necessarily monotonically, with a punctuated anomaly structure. Magnetotelleurics show ponded melt channels beneath the plate in some locations, but not all. Ps images thinning of the transition zone by 5 to 20 km in the western part of our study region, suggesting a connection to the deeper mantle. Anisotropy is consistent with aligned melt near the ridge and weak, 2% APM directions elsewhere. Where the plate subducts in the Lesser Antilles Arc receiver functions image the Moho that varies from 29 to 40 km depth. In the northern and central section of the arc we image the base of the upper plate at 73 +/- 5 km depth beneath the arc and backarc. In the southern arc we image a negative phase that may be related to a more gradually dipping slab at 101 – 117 km. We image a high velocity slab and upper plate, with the slowest anomalies and highest attenuation beneath the backarc.

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