Pokorný, J., H. Čížková, A. P. van den Berg, C. R. Bina, E. van der Wiel, D. J. J. van Hinsbergen, and W. Spakman, The effect of phase transitions on slab in the transition zone and lower mantle: Implications for deep earthquakes and oscillations of plate motions, American Geophysical Union Fall Meeting, abstract T41D-0271, 2023.
Abstract
Understanding the Earth's geodynamic processes is vital for comprehending the complexities of plate tectonics, deep earthquakes, and other phenomena. It is known that the combined effects of the phase transitions (PT) and rheology of the crust and mantle are controlling the slab deformation in the transition zone (TZ), where these are usually temporarily stagnant before they penetrate into the lower mantle (LM). By the interplay between trench rollback and the petrological buoyancy associated with the PT, slabs tend to buckle horizontally or vertically in the TZ. This has broad implications for various processes, among which are the periodicity of plate motions and deep earthquakes. Here we employ FEM modelling to further investigate the effects of the PT and rheology both on the stress in the TZ and shallow LM as well as on buckling of the slab in the TZ as a possible driver of the oscillations in plate motion.
Deep earthquakes provide insights into stress distribution in subduction zones. In the Tonga region, ordinary deep (620-680 km) earthquakes exhibit down-dip compressional stresses as expected, but unusually deep (≥680 km) earthquakes show unique focal mechanisms with vertical tension and horizontal compression. We show that the direct buoyancy effects of the endothermic PT at 660 km depth are overprinted by bending-related forces and resistance from the more viscous LM transmitted by a strong slab up-dip. The stress pattern that best fits seismogenic stresses is found for the cold plate (150 Myr old) and a viscosity increase at 1000 km depth. An abrupt change in stress orientations occurs as the slab temporarily deflected by the endothermic PT penetrates the shallow LM while the fold in the flat-lying part tightens.
Plate tectonics is primarily driven by the gravitational pull of slabs. Under stable plate boundary configurations, plate motion changes are thought to occur gradually. Surprisingly, recent high-resolution Indian plate reconstructions have unveiled rapid (2-3 Myr) plate velocity oscillations of ±50%. Our study demonstrates that these oscillations are caused by subhorizontal buckling of slabs in the TZ, primarily driven by petrological buoyancy of PT.