Constraints of Flexural and Seismic Observations on Lithospheric Rheology at Plate Interior and Plate Boundary Settings

An essential feature of plate tectonics is that lithospheric deformation is localized at plate boundaries with substantially larger magnitude than that in plate interiors, suggesting that lithospheric rheology is weaker at plate boundaries than in plate interiors. Numerous mantle convection modeling studies that approximate this empirically derived lithospheric rheology using different formulations or proxies (e.g., pre-existing weak zones, faults, reduced coefficient of friction or yield stress, …) have largely reproduced the observed features of lithospheric deformation. While the rheological formulations in theoretical modeling studies have become increasingly more sophisticated often with an expressed goal to understand the cause of plate tectonics and initiation of subduction, it is important to place constraints on lithospheric rheology using in-situ observations including flexural (i.e., vertical motion) and seismic response to different forcings. Laboratory studies indicate that lithospheric deformation is controlled by frictional sliding, low-temperature plasticity (LTP) and high-temperature creep with increasing temperature. Observations of lithospheric flexure and seismicity at Hawaiian Islands (i.e., plate interior setting) in response to volcanic construction suggest that internal frictional coefficient µ_f is 0.25, while LTP is significantly weaker than that derived from laboratory studies [e.g., Mei et al., 2010], based on modeling studies of loading response of Hawaiian lithosphere with realistic elasto-frictional-plastic-viscous rheology [Zhong and Watts, 2013]. Further studies [Bellas and Zhong, 2021; Bellas et al., 2020; 2022] showed that µ_f is 0.3 and activation energy of LTP needs to be reduced from laboratory derived value of 320 KJ/mol to 190 KJ/mol to fit the flexural and seismic deformation at Hawaii, and that the same rheological parameters reproduce the observed elastic thickness at other oceanic islands and seamounts on lithosphere of different ages. The Japan subduction zone shows characteristic features of subducting lithosphere with its outer rise and trench topography and transition from shallow normal/extensional faulting to deep reverse/compressional faulting seismic deformation (i.e., neutral plane) [e.g., Craig et al., 2014]. Dynamic deformation models of subduction have been formulated, using realistic slab buoyancy force and elasto-frictional-plastic-viscous rheology, to interpret the observations of trench-outer rise topography and neutral planes [Han et al., 2022]. The modeling indicated that the observed neutral plane in the Japan subduction zone is consistent with the rheology for subducting lithosphere with LTP activation energy of ~220 KJ/mol and µ_f~0.3, which are similar to that inferred for the plate interior at Hawaii. The modeling also found that µ_f<0.1 that is required to generate mobile-lid or plate tectonic convection in mantle convection models [e.g., Moresi and Solomatov, 1998] would not generate the extensional to compressional stress transition (i.e., neutral plane) in the Japan subducting lithosphere, further suggesting the importance of in-situ observational constraint on lithospheric rheology and dynamics of plate tectonics.