Seismic evidence of the lithosphere-asthenosphere boundary in the young Juan de Fuca Plate

The lithosphere-asthenosphere boundary (LAB) is the most prevalent plate boundary on earth, but its definition and nature remains a matter of debate. Using wide-angle ocean bottom seismic (OBS) data, here we present the very first reflection image of the LAB over young oceanic lithosphere. The OBS data were acquired in 1996 to image the crustal structure, but advanced processing of these data show the presence of two wide-angle reflections originating from within the mantle. We first perform forward modeling to exclude the possibility of multiples and show these events are real. Based on tomographic velocity model, we perform migration and find that these reflections are continuous over 25-30 km distance and originate at 14 km and 19 km below the sea surface for ~2.6 Ma Oceanic Lithosphere. These seismic reflections lie several kilometers below the Moho, and the seismic forward modeling requires these reflections to originate from velocity discontinuities, which could relate to the presence of melts in the mantle that create the low velocity anomalies, therefore we suggest that they may define the LAB. These observations provide constrain of the depth, distribution and physical properties of the LAB transition zone, combined with the recent study of Mehouachi and Singh (2018), suggest that the LAB could consists of melt rich layers.