re: Valles Marineris - a gigantic dehydration crack?
Interesting.
Any reason not to make it both freezing and evaporation / sublimation?
As Mars moves into a cooler orbital pattern (plus the loss of the magnetic core), the water both freezes (which expands the saturated surface materials) and sublimates into space (resulting in the internal shrinkage). Both of these will cause tensional structures.
More geologically recent orbital variations result in melting and freezing process cycles; continuing the developement of these structures.
Impacts (as has been suggested) cause local melting and dewatering.