Seesaw ridge propagation and the strength of the lithosphere at slow-spreading ridges

Abstract:

A unique style of ridge propagation has been documented along slow- to intermediate-spreading ridges throughout the ocean basins: small-offset (< 2.5 Ma) discontinuities often propagate along the ridge axis at a varying rate and reverse propagation direction—we call these features seesaw propagators (SSPs). The reasons for this style of propagation are unknown, but we can imagine forces driving propagation and forces resisting propagation. We posit that where juxtaposed old and young crustal blocks at oceanic transform faults resists forces of propagation, the relatively weak crust at non-transform offsets deforms—a similar transition in deformation style at slow-spreading ridge offsets was suggested by Grindlay et al. 1991. To estimate this resistive force, we develop yield strength envelope (YSE) models for cooling oceanic lithosphere having a wet olivine rheology in the crust and dry olivine rheology in the mantle. For ages less than about 2 Ma, the lower crust is weak relative to the upper crust resulting in a “jelly sandwich” type YSE, and the integrated strength of the lithosphere increases rapidly. The weak lower crust decouples crustal and mantle stresses. Using a variety of cooling models, we will investigate how this abrupt change in rheology effects ridge propagation as well as ridge and transform fault morphology.