PLANETARY WRINKLE RIDGES: A TALE OF SCALE

Wrinkle ridges are one of the most common tectonic landforms on the terrestrial planets. It is generally agreed that wrinkle ridges are the result of compressional stresses and crustal shortening and are the surface expression of folding and thrust faulting. One of the most debated aspects of wrinkle ridge formation is the depth of faulting. It has been proposed that wrinkle ridge thrust faults are either deeply rooted (thick-skinned deformation) or shallow (thin-skinned deformation). Models of wrinkle ridge formation are based on their morphology, the regional topography of ridged plains, and ridge spacing. One of the most striking features of wrinkle ridges on plains units is their regular spacing. This regular or periodic spacing is strongly suggestive of dominant or critical wavelength deformation. Ridge spacing on scales to tens of kilometers can be accounted for with buckling and localization instability models where the lithosphere is free to deform. Larger scales of folding and faulting on the order of hundreds of kilometers can also be accounted for. Conversely, a buckling instability where the lithosphere is assumed to be rigid and decoupled from the deforming ridged plains material can also account for the ridge spacing. The apparent “stair step” topography of martian ridged plains have been attributed to a stacked group of thrust faults with depth-scales of tens of kilometers, possibly cutting the entire elastic and seismogenic lithosphere. Recent modeling has attempted to define the geometry and number of thrust faults needed to account for the major morphologic elements of martian wrinkle ridges. These models require a deeply rooted primary thrust fault and one or two secondary backthrusts to account for the typical broad low relief arch and the superimposed ridge. An alternative model accounts for the morphology of wrinkles ridges with a single listric thrust fault that flattens into a mechanically weak horizon. Thus, the question of the maximum depth of wrinkle ridge thrust faults remains unanswered.