Imaging the lithosphere-asthenosphere boundary beneath the Pacific using SS waveform modeling

Oceanic lithosphere constitutes the bulk of Earth's tectonic plates and also likely represents the building blocks of the continental lithosphere. The depth and nature of the oceanic lithosphere-asthenosphere boundary are central to our understanding of the definition of the tectonic plates and lithospheric evolution. Although it is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to conductive cooling models, this relatively simple realization of the tectonic plates is not completely understood. Old (> 70 Ma) ocean depths are shallower than predicted. Furthermore, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we directly map the depth and nature of a seismic discontinuity that is likely the lithosphere-asthenosphere boundary across the Pacific plate using a new method that models variations in the shapes of stacked SS waveforms from 17 years of seismic data. The depth to the discontinuity varies from 25 to 130 km and correlates with distance from the ridge along mantle flow lines. This implies that the depth of the oceanic lithosphere-asthenosphere boundary depends on the temperature of the underlying asthenosphere, defined by a best fitting isotherm at 930 degrees C with a 95% confidence region of 820-1020 degrees C, although the sharpness of the observations in some locations implies a mechanism besides temperature may also be required.