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 load s are generally considered to produce broadly distributed bending or f lexure. The same processes of weakening or yielding that lead to local ization of deformation on faults during lithospheric stretching or sho rtening can produce highly localized faulting during lithospheric bend ing. 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 t o bending in the ''snapping'' mode while thicker layers respond in a m ore 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 be nding. The average horizontal stress on the layer also affects the mod e 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 w ay. Lithosphere at many mid-ocean ridges is thought to be only a few k ilometers thick and also may be a site of vertical loading. The locali zed snapping mode of bending may produce abyssal hills at fast-spreadi ng mid-ocean ridges.