Roughness of the Global Oceanic 220-km Discontinuity and Implications for Spreading Rate Dependent Small-Scale Convections

A global discontinuity at a depth of 220 km was first introduced in PREM in the 1980s as the lower boundary of the asthenosphere. Later studies suggested that this interface may not be a global feature as it is rarely observed in the oceanic regions. In this study, we process twelve years of SS precursors recorded at the global seismological network. We observe SS precursors reflected off this boundary across the global oceanic areas. The amplitudes of the SS precursors require about 6% of velocity increase across the 220-km discontinuity, roughly consistent with that in PREM. We apply finite-frequency tomography to obtain depth perturbations of the discontinuity in the global oceans. We show that the depth perturbations roughly follow the age contours, however, the average depth of the 220-km discontinuity is independent of seafloor age. The most interesting observation on the topography of the 220-km discontinuity is that the roughness of the discontinuity increases with seafloor spreading rate. The geodynamical processes responsible for this positive correlation is unclear but it indicates small-scale convections in the oceanic asthenosphere may be controlled by seafloor thermal structures formed during the initial melting processes at spreading centers. In addition, the increase in boundary roughness is distinctly different for slow spreading seafloors and fast spreading seafloors. The Atlantic Ocean is dominated by slow-spreading seafloors and the roughness of the 220-km discontinuity increases rapidly with spreading rate, while in the Pacific Ocean and the Indian Ocean, the increase in roughness is much slower. The differences in the rate of increase in roughness indicate that seafloors may have "permanent memory" of their birth places, the (different) melting processes at the spreading centers may have been imprinted in their chemical compositions and physical properties (e.g. porosity and thermal properties) millions of years after they have moved away from the spreading centers.