Discovery of distinct lithosphere-asthenosphere boundary and the Gutenberg discontinuity in the Atlantic Ocean.

The plate tectonic theory requires a rigid lithosphere floating over a weak asthenosphere, separated by the lithosphere-asthenosphere boundary, which has been sometimes interpreted as the Gutenberg discontinuity. Using a deep seismic reflection technique, we report the presence of two continuous reflections covering 27 Ma to 58 Ma oceanic lithosphere in the Atlantic Ocean. We find that the upper reflection deepens with age and follows the ~1250°C isotherm, whereas the deeper reflection lies at a constant depth of ~75 km. We suggest that the upper reflection represents the thermally controlled lithosphere-asthenosphere boundary, whereas the lower reflection is the Gutenberg discontinuity, a frozen-in dehydration boundary separating the dry mantle melting region above from the hydrated mantle below formed at the ridge axis. We also find that thermal mantle anomalies rejuvenate the lithosphere, uplift the lithosphere-asthenosphere boundary, and destroy the Gutenberg discontinuity.