Xianzhi Cao1,2, Nicolas Flament3, R. Dietmar Müller2
1Frontiers Science Center for Deep Ocean Multispheres and Earth System; Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education; College of Marine Geosciences, Ocean University of China, Qingdao 266100, China; 2EarthByte Group, School of Geosciences, The University of Sydney, Sydney, New South Wales, Australia; 3GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Northfields Avenue, NSW 2522 Australia
The relationships between plate motions and mantle flow remain poorly understood over the life cycle of supercontinents and superoceans. Contrasting models propose that the structure of the deep Earth may have remained stable over time, or that it could be linked to the aggregation and dispersal of supercontinents. Here we investigate the evolution of mantle flow driven by synthetic end-member plate tectonic models extending back one billion years. We implement a tectonic scenario in which supercontinent breakup and reassembly occurs by introversion, and consider three distinct references frames that result in different net rotation of the lithosphere with respect to the mantle. Our flow models predict a dominant degree-2 mantle structure most of the time, which is similar to the present-day structure of the lower mantle that is dominated by two antipodal Large Low Shear Velocity Provinces (LLSVPs). We analyse the relationship between imposed tectonic velocities and deep mantle flow, and find, as expected, that at spherical harmonic degree 2, the maxima of lower mantle poloidal flow and temperature follow the motion path of the maxima of surface divergence. We show that a time lag of up to 250 Myr can occur between major changes surface kinematics and the motion of long-wavelength basal mantle structures when (1) the lower mantle is reorganised by sinking slabs sinking onto basal thermochemical structures, and/or (2) slabs stagnate in the transition zone, for instance due to fast trench retreat. Basal thermochemical structures move at less than 0.6 degree/Myr in our models, with a temporal average of 0.16 degree/Myr when net lithospheric rotation is removed from the reconstruction, and between 0.20-0.23 degree/Myr when net lithospheric rotation exists and partial introduced into the lower mantle. Our results suggest that basal thermochemical structures are not stationary, but rather linked to global plate motions, indicating that the lithosphere and the entire mantle constitute a co-evolving dynamic system.
Xianzhi Cao is a postdoc now working on plate tectonics and geodynamics in the Earthbyte Group, the University of Sydney.
Nicolas Flament is a senior lecturer in geophysics at the University of Wollongong.
Dietmar Müller is a professor of geophysics at the School of Geosciences, the University of Sydney.