Analysis of plate spin motion and its implications for strength of plate boundary

Abstract

In this study, we investigate the driving forces of plate motion, especially those of plate spin motion, that are related to the toroidal components of the global plate velocity field. In previous works, numerical simulations of mantle convection have been used to examine the extent to which toroidal velocity components are naturally generated on the surface, by varying key parameters, notably the rheological properties of plates and plate boundaries. Here, we take the reverse approach and perform analyses of observed plate motions, which show an increase in the toroidal/ poloidal ratio at high degrees of spherical harmonic expansion, as well as a rapid change in the plate spin rate and the estimated driving stress around a critical plate size of approximately 1000 km. This quantitative relationship constrains the strength at plate boundaries to 3-75 MPa, which is consistent with several seismological observations, including those from the NE Japan arc associated with the 2011 Tohoku earthquake.

Long‐Term Stability of Plate‐Like Behavior Caused by Hydrous Mantle Convection and Water Absorption in the Deep Mantle

“…Elevated pore fluid pressure can significantly reduce the frictional strength along the subducting slab that undergoes dehydration (e.g., Hirth & Beeler, ; Kelemen & Hirth, ; Scholz, ). A low differential stress (several tens of MPa) along the plate boundaries is consistent with the weak strength of oceanic lithosphere, which is suggested based on the analysis of earthquakes before and after the M 9 2011 Tohoku‐oki earthquake (Hasegawa et al, ) and based on the plate strength analysis using spin analysis of rigid plate motion (Matsuyama & Iwamori, ). The burst of mantle water content is found to occur with mobile lid behavior but not with long‐term episodic lid behavior within 4.6 Gyr because the heat transfer caused by vigorous plate‐like behavior associated with the mobile lid efficiently cools down the mantle. This would allow stored water to be transported by subducted slabs in the mantle transition zone.…”

“…Elevated pore fluid pressure can significantly reduce the frictional strength along the subducting slab that undergoes dehydration (e.g., Hirth & Beeler, ; Kelemen & Hirth, ; Scholz, ). A low differential stress (several tens of MPa) along the plate boundaries is consistent with the weak strength of oceanic lithosphere, which is suggested based on the analysis of earthquakes before and after the M 9 2011 Tohoku‐oki earthquake (Hasegawa et al, ) and based on the plate strength analysis using spin analysis of rigid plate motion (Matsuyama & Iwamori, ).…”

“…An additional factor that may affect the generation of tectonic plates is the effect of water on the rheology of nominally anhydrous minerals; in general, minerals become weaker with a higher water fugacity in deformation experiments at high pressure and temperature (Karato & Wu, ; Korenaga & Karato, ; Mei & Kohlstedt, ). In addition to rheological weakening in the deep mantle, the water may also lead to weakening of the oceanic lithosphere, which requires the small differential stress (approximately a few tens of megapascal) along the plate boundaries associated with a spin motion of surface plates (Matsuyama & Iwamori, ). Regarding physical explanations for water weakening of oceanic lithosphere, Hirth and Beeler () suggest that the dependence of the strength of the oceanic lithosphere on fluid pore pressure may cause such weakening, and this effect may lead to a one‐sided subduction (Gerya et al, ).…”