Localization of Martian ridges by impact craters: Mechanical and chronological implications

Abstract

The eastern flank of the Tharsis dome (Mars) is affected by compressional ridges. Two conceptual models exist for these structures: (1) the ridges are the expression of a superficial shortening and are rooted on a rheological boundary located in the megaregolith or (2) the ridges result from a global shortening on a lithospheric scale and are rooted on the deep thermal brittle‐ductile transition inside the lithosphere. It is shown that the position of the ridges is strongly influenced by the existence of impact craters. The distribution of crater‐ridge distances has been measured in the Coprates area. This distribution shows that the intersection of ridges and impact craters with a diameter larger than 4 km is four times as frequent as a homogeneous and random distribution of craters relative to what ridges would produce. Very detailed geometric studies of relationships between intersecting ridges and craters show that these craters are older than the ridges. This localization implies that (1) ridges are superficial structures rooted at shallow depths in the megaregolith, and (2) the ridging phase occurred some time after the deposit of the ridged plain unit material, probably during the upper Hesperian. Detailed geometrical studies of ridge edges and graben/ridge width relationships suggest that ridges are bounded by two reverse faults with different offsets, and that the rheological boundary on which ridges and graben are rooted is around 1.5 km deep. Three ways are possible for an impact crater to localize strain in a regional uniaxial compressional stress field: (1) the strength of impacted material is lower than the strength of the surrounding nonaffected rocks, (2) the lithostatic pressure underneath the crater is lower than the lithostatic pressure at the same depth in the surroundings due to the existence of the crater cavity, and (3) the stress is concentrated on the crater borders if craters can be assimilated to holes in a shortened elastic plate. In all three cases, the initiation of the ridges occurs in the area where a low‐strength and/or stress concentration envelope around the crater is nearest to the low‐strength decollement level.