Imaging Pacific lithosphere seismic discontinuities—Insights from SS precursor modeling

Oceanic lithosphere provides an ideal location to decipher the nature of the lithosphere-asthenosphere system which is vital to our understanding of plate tectonics. It is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to the conductive cooling models. Yet this simple realization fails to explain various observations. For example, old oceanic lithosphere does not subside as predicted. Further, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we use SS precursors to image the discontinuity structure across the Pacific Ocean using 24 years of teleseismic data. We image a sharp pervasive velocity discontinuity (3–15% drop over <21 km) at 30–80 km depth that increases in depth with age from the ridge to at least 36 ± 9Myr along the 1100°C conductive cooling isotherm. Beneath seafloor >36Myr, there is no age-depth dependence, and we image the discontinuity at an average depth of 60 ± 1.5 km. The amplitude and sharpness of the boundary suggests that a compositional variation and/or layered carbonatitic melt may be required to explain our observations rather than temperature alone. The strength and pervasiveness of the boundary suggest that it is likely related to the lithosphere-asthenosphere boundary. An additional deeper discontinuity at 80–120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth.