Global finite-frequency tomography of the 220-km discontinuity

Abstract

The asthenosphere is a weak layer in the upper maare available from the publicntle that supports the movement of the overriding tectonic plates and facilitates mantle convection. In this study, we compile a global data set of SS precursors reflected at the base of the asthenosphere, also known as the 220-km discontinuity. The global data set includes the oceanic SS precursors from Sun & Zhou and new measurements with bounce points in continental regions. Similar to the oceanic data set, the continental SS precursors are observed on about 45 per cent of the SS waves, with bounce points distributed across all tectonic regions—from orogeny belts to stable cratons. We image the depth of the discontinuity at a global scale using finite-frequency tomography. In oceanic regions, the depth of the 220-km discontinuity agree well with the previous study, with discontinuity depth structure characterized by alternating linear bands of shallow and deep anomalies that roughly follow seafloor age contours. In continental regions, the variations are not spatially oscillatory but are instead much broader, with prominent perturbations associated with convergent plate boundaries. The base of the asthenosphere is shallow along the southern border of the Eurasian plate, from the Mediterranean region to Southeast Asia. Shallow discontinuity anomalies are also observed in the continental interiors—in Eurasia, from the northern Tian Shan through Mongolia to eastern Siberia, and in North America east of the Rocky Mountains. These anomalies form a linear structure roughly parallel to the Pacific subduction zones. The average depth of the discontinuity, as well as the velocity contrast across the interface, is globally consistent across both oceans and continents, with an average depth of approximately 251 km and a velocity increase of about 7 per cent. Given that the continental lithosphere has been cooling for much longer than the oceanic lithosphere, the observed consistency in the average depth of the discontinuity implies that secular cooling does not significantly impact the thermal structure at the base of the asthenosphere.