New numerical mid-ocean ridge models for interactions between plate-driving and resistant forces

Evidence for asymmetric plate growth, variable crustal thickness, and non-uniform spreading rates is ubiquitous on the seafloor. However, conventional numerical models for lithospheric growth at mid-ocean ridges usually adopt constant velocity boundary conditions, which are incapable of producing non-uniform growth of oceanic lithosphere. Noting that plate-boundary forces can dynamically determine plate speed by finding a balance against the resistance to extension at ridge axes and at lithosphere-asthenosphere boundary, we introduce plate-boundary forces instead of kinematics to drive plate motions. We construct such models using an open-source version of FLAC, a mature finite element code frequently used for modeling mid-ocean ridge processes. We follow the well-tested parameterization of magmatic accretion in the form of dikes. Three different boundary conditions are implemented: prescribed velocities of 2.5 cm/yr, zero boundary force, and constant non-zero boundary force. All three types of boundary conditions can produce faulting styles consistent with the previous studies but mean plate speeds respond differently to different boundary condition types. Mean plate speed is almost constant at 2.5 cm/yr in kinematic models, and at 2.75 cm/yr in zero boundary force models. The constant non-zero boundary forces make plates move at 1.8 cm/yr of mean speed, but plate speed changes over time by more than 1 cm/yr. These model results suggest that even if far-field plate-driving forces are nearly constant, seafloor can grow non-uniformly over time due to its internal responding forces. However, further investigation is necessary for matching the periods of modeled plate speed variations with relevant observations, reducing numerical noise and extracting mean plate speed with greater confidence.