Bending thin lithosphere causes localized ''snapping'' and not distributed ''crunching'': Implications for abyssal hill formation

Horizontal loads (i.e. pushes or pulls) on the lithosphere are thought to produce significant, localized, dip-slip faults, but vertical loads are generally considered to produce broadly distributed bending or flexure. The same processes of weakening or yielding that lead to localization of deformation on faults during lithospheric stretching or shortening can produce highly localized faulting during lithospheric bending. Localized bending, or ''snapping'' should occur when the bending moment begins to decrease with increasing plate curvature. If yielding causes a reduction in cohesion then thin brittle layers may respond to bending in the ''snapping'' mode while thicker layers respond in a more distributed ''crunching'' mode. For a Mohr-Coulomb layer floating on an invicid substrate it is the ratio of the cohesion to the average shear stress needed to overcome friction that controls the mode of bending. The average horizontal stress on the layer also affects the mode of bending. For estimated rock properties, a brittle layer has to be less than a few, up to perhaps 10 km thick, to break in a localized way. Lithosphere at many mid-ocean ridges is thought to be only a few kilometers thick and also may be a site of vertical loading. The localized snapping mode of bending may produce abyssal hills at fast-spreading mid-ocean ridges.