Abstract:
The lithosphere-asthenosphere boundary (LAB), defined seismologically as the interface separating the high-velocity lithosphere from the low-velocity asthenosphere, records key information about the distinct physical and chemical properties of these two layers and their dynamic interactions. Plate tectonic theory holds that rigid lithospheric plates move horizontally over the weaker, ductile asthenosphere, with observed global plate velocities ranging from 1 to 10 cm per year. This substantial variability is widely considered a direct manifestation of lithosphere-asthenosphere interactions. However, whether systematic relationships exist between LAB structural characteristics and plate motion rates remains an open question that is fundamental to understanding plate-driving forces.To address this question, this study focuses on oceanic regions and oceanic-continental transition zones, where lithospheric structure is relatively simple compared to continental interiors. Using teleseismic S-wave receiver functions from 107 globally distributed broadband seismic stations, we applied a consistent grid-search inversion methodology to systematically extract LAB structural parameters—including depth, shear-wave velocity drop magnitude, and transition zone thickness—beneath each station. This uniform processing approach minimizes biases inherent in compiling disparate results from previous studies. The obtained LAB parameters were then analyzed for correlation with absolute plate motion rates from the NUVEL-1A model. Our results reveal three principal findings. First, LAB depth in mature oceanic regions closely follows the
1100℃ isotherm predicted by the plate cooling model, suggesting that the seismically defined LAB may correspond to a rheological boundary where thermally activated creep becomes dominant. Second, and most significantly, both the magnitude and the gradient of the shear-wave velocity drop across the LAB exhibit strong positive correlations with plate velocity, with Pearson correlation coefficients of
r = 0.686 and
r = 0.650, respectively. We interpret this as evidence for widespread low-viscosity melt or volatiles within the asthenosphere. Greater velocity drops imply higher melt fractions, which substantially reduce asthenospheric viscosity and promote mechanical decoupling between the lithosphere and underlying mantle. This decoupling effectively lowers the basal shear resistance acting on the plate, enabling higher plate velocities. Third, the thickness of the LAB transition zone, consistently measured at less than 30 km across all stations, shows no statistically significant correlation with plate velocity (
r = −0.141), indicating that interface sharpness is not a primary control on plate kinematics.This study provides new seismological constraints on lithosphere-asthenosphere interactions and offers observational support for models in which plate motion is modulated by mantle viscosity and coupling conditions at the LAB. However, several limitations must be acknowledged. The current dataset has uneven global coverage, and the vertical resolution of S-wave receiver functions (typically 5-10 km) imposes limits on parameter precision. Furthermore, our grid-search approach, while systematic, may not fully capture the full complexity of LAB structure. Therefore, the observed relationships, particularly the null correlation with thickness, warrant validation through future studies incorporating denser station coverage, multi-frequency receiver function analysis, and joint inversion with complementary datasets such as surface wave dispersion or body wave tomography. Despite these limitations, our findings provide a step toward quantitatively linking plate-scale dynamics with the seismic structure of the upper mantle.