Characterization of secondary mantle convection from geophysical data and models

Abstract

Small-scale convection or secondary convection has been studied using the geophysical data and models such as seismic tomography models and geoid anomalies. We used three seismic tomography models, SEMUCB-WM1, SAVANI and S40RTS to infer the lithosphere/asthenosphere boundary, and the destabilization of the lithosphere, induced by secondary mantle convection. Former studies, based on numerical and experimental models report that secondary convection would occur as sub-lithospheric longitudinal rolls, induced by the shear associated with the lithospheric plate motion, or as helices, formed when sinking cold plumes, falling from the base of the lithosphere are sheared away by the large-scale flow. Our study shows that the secondary convection under the Pacific plate occurs in the form of 3D plumes and helices. The lithospheric drips observed in the tomography models first appear as small wavelength anomalies beneath a 30-40 M.y. old and 75-90 km thick lithosphere. They are observed as longer elongated patterns beneath 105 M.y. old and 138 km thick lithosphere. Most of the lithospheric anomalies are associated with negative geoid anomalies of order and degrees 8-40. No clear correlation between lithospheric destabilization and volcanism is observed.