The shallow structure of the Martian lithosphere in the vicinity of the ridged plains

Abstract

Wrinkle ridges in the smooth plains of the Coprates and Lunae Planum regions of Mars form linear or concentric patterns with a regular spacing of 25–50 km. We test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression, that occurred prior to probable ridge‐related faulting, of a strength‐stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement. Results show that a range of models with both rigid and deformable megaregolith‐basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials. In models that incorporate viscous and plastic rheologies, uniform and exponentially varying vertical strength distributions, and the presence or absence of interfacial slip at the base of the surface plains unit, the ridge spacing is primarily controlled by the power law exponent of the lithosphere, the megaregolith/plains unit thickness ratio, and the plains unit/megaregolith strength contrast. Deformable basement models with a viscous rheology (1 ≤ n ≲ 3) can explain the ridge spacing if the megaregolith was thicker than and approximately 1–3 orders of magnitude weaker than the plains unit. Similar models with a perfectly plastic (n → ∞) rheology require plains unit thicknesses greater than 5 km, which exceeds most observational estimates. The dominant wavelength in the plastic models is not sensitive to internal strength contrasts and provides no constraint on the relative competence of the plains and megaregolith. Ridge spacing in the combined viscous and plastic models can be consistent with either a dry, water‐rich, or ice‐rich megaregolith at the time of ridge formation. If the ridge spacing was controlled primarily by the mechanical properties of lithospheric basement with little or no influence from the plains unit, then the minimum megaregolith thickness in the vicinity of the ridges must have been of the order of 0.4 km. The fact that ridges are most commonly found in the smooth plains but ridge spacing can be explained by models without a plains unit suggests that the unit may have facilitated the nucleation of reverse faults that are interpreted to characterize ridge structure but conceivably could have folded passively in response to unstable compression of the basement.