Wrinkle ridges of Mars: structural analysis and evidence for shallow deformation controlled by ice-rich décollements

The ridged plains of Mars display large assemblages of wrinkle ridges usually interpreted as shortening features. The formation of these ridges has been explained by (1) deep thrusting, (2) buckling of lava plains or (3) thrusts rooted on shallow décollement levels. These explanations were grounded on observations of Tharsis flank ridged plains where ridges are straight and regularly spaced. This paper shows observations about irregular ridges with curved, circular or en-échelon shapes. These different shapes are due to pre-existing faults and buried craters. Structural evidence shows that such irregular ridges are created by thrusts rooted on shallow décollement levels. allow crust of Mars is composed of a megaregolith sometimes overlain by volcanic stratas. Ice-rich levels are supposed to exist as deduced from observations of fluidized ejecta craters. In order to understand the nature of the décollements a simultaneous study of lavas thicknesses and décollements depths have been done. The results show that décollements are not related to lava thicknesses but to icy levels. The rheology of ground ice favours this last assumption because ductile icy levels can localize décollements. Furthermore fluidized ejecta craters are preferentially localized in ridged plains indicating a correlation between groundice and wrinkle ridges. Finally the origin of the stress creating the deformation is discussed taking into account the occurrence of shallow icy layers and the possible occurrence of deeper deformation.