Continuous seismic reflection image of the Lithosphere-Asthenosphere Boundary (LAB) from 2-75 Ma on the African plate in the Equatorial Atlantic Ocean

Lithosphere-asthenosphere boundary (LAB) has generally been obtained using surface tomography and receiver function methods, indicating a change in the S-wave velocity. Here, we present the very first seismic reflection image of the LAB starting from 2 Myr near the Mid-Atlantic Ridge continuously to 75 Myr near the margin of Africa in the Equatorial Atlantic Ocean, indicating the presence of sharp P-wave velocity contrasts. We have used a 12-km long multi-sensor streamer and a large well-tuned airgun source along a 1400 km long profile. We find two prominent reflections: The upper reflection varies from ~12 km (3.5 s) below the seafloor at 2 Myr to 78 km (28 s) at 75 Myr. The second reflection is sub-horizontal at 27-28 s (70-78 km) and can be clearly observed for 27 Ma and 55 Ma. There is also evidence of some other reflections between these two reflections, whose depth increase with age. We interpret the uppermost reflection as the base of lithosphere, the top of the LAB, and the lower reflection as the base of the LAB, while the reflections between these as sheared melt sheets. The lower reflection at 70-80 km depth could also be interpreted as the G discontinuity, and might explain the change in anisotropy observed in the oceanic lithosphere. The upper reflection seems to be correlated with the 1180º C isotherm. In this talk, we will discuss different implication of these images and provide comprehensive model of the oceanic LAB, explaining both P and S-wave observations.