ALBERT

Description: How freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere, which includes information regarding the magnitude of driving/resisting forces of plate tectonics. We investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes (OCEQs) and numerical simulations. The stress release due to OCEQs in young oceanic lithosphere (5–15 Ma) shows significant differences between the spreading-directional component and the ridge-parallel component. The extensional stress release of the ridge-parallel component is six times as large as that of the spreading-directional component, while the compressional stress release of the ridge-parallel component is one-seventh that of the spreading-directional component. We conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge-parallel direction can explain the observed anisotropic stress release. The result indicates that young oceanic lithosphere (5–15 Ma) barely contracts in the ridge-parallel direction (only 0–30% of the spreading-directional contraction rate), while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere. From the results, we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0.44 MPa.