Extended Data Fig. 10: Mechanics of ridge shoulder unbending.

The yield stress of the lithosphere σ_y is defined as the difference between the vertical and horizontal stresses needed to produce fault slip. a, An idealized plate that was accreted with curvature and no bending stresses. σ₀ is an assumed background stress difference. The simple case shown has no cohesion and parameters A and B depend strongly on the friction coefficient f and assumed pore pressure P_P on the faults, as defined in the Supplementary Information. b, A case with reverse faults that are three times stronger than normal faults. c, Deepening of the neutral depth, D, when the reverse faults are assumed to be weaker than the normal faults. d, Analytical estimate of the neutral depth D, which marks the base of the compressive zone in an unbending ridge shoulder. The ratio of D to the layer thickness, H, is plotted versus the ratio of pore pressures on reverse versus normal faults assuming f = 0.75 and that the pore pressure on normal faults is one-third the lithostatic pressure (blue curve). Assuming a rock density of 3,000 kg m⁻³ and water density of 1,000 kg m⁻³, the left limit is for hydrostatic pore pressure on the reverse faults, whereas the right limit is for lithostatic pore pressure on the reverse faults. Red curve shows the effect on the neutral depth of a regional horizontal extensional stress difference equal to 20% of the extensional yield stress at the base of the layer. See Supplementary Information for details.