Analysis of lineaments observed in Mariner 6 imagery of the chaotic terrain in Pyrrhae Regio suggests that the lineaments are tectonic fractures related to stresses within the crust of Mars rather than the effects of meteorite impact or removal of permafrost. Apparently the chaotic terrain was created by slumping of blocks up to 15 km in length from a plateau down into the lower lying chaotic terrain. These blocks seem to have been isolated by tectonic fractures prior to the slump movement. The tectonic fractures follow a consistent orientation-frequency pattern across both the chaotic terrain and the surrounding cratered plateaus over an area of about 1.5 million km2. This orientation-frequency pattern is consistent with a conjugate shear fracture pattern. If this interpretation is correct, the observed fracture pattern apparently indicates a local tectonic extension of the Martian crust as the mechanism for producing the chaotic terrain.

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