Marcia, you made me think here. Others will call this idea absurd, but I don't find it so. I like simplistic theories that link things on different scales.

I have looked closely at some of the Malin pictures of the very bottom of the VM and other Chasms. What I saw was not some fluvial channel like I expected, but instead, deep pits and cracks. I see VM as just the end case of the more common and much smaller pit chains (which it is connected to and grades in to). The pit chains also seem to have formed by tensional forces. Their typical linearity and lack of effluent argues against volcanic or fluvial processes of formation. This idea relates to the "Mars as the core of a gas giant" idea. I don't really mean gas giant, but a water rich world with thick atmosphere and 10,000 feet or more of various salt deposits planet wide. In the later bone dry epochs, these salts could change hydration states, releasing water vapor, and shrinking. This would create tensional forces, and once the cracks started, deeper layers would become exposed to dessication.

These hydration changes can cause a great deal of reduction in volume of the minerals, such as sodium sulfate decahydrate, Na2(SO4)(H2O)10 > sodium sulfate. This destroys crystals and creates small fragments that could then blow away as dust. Thus, once some overlying protecting layer (such as a flood basalt or permafrost) has been fractured or pierced, the underlying salt layers could slowly contract and disintegrate to produce the "wasted away" pits zones such as Nick's Sysiphi Cavi region. This area was explained as quote "localised thawing of CO2 or H2O permafrost is one possibility" Why was that thawing localized, and 1 kilometer deep, yet shows no evidence of volcanism? That permafrost must have been 1 kilometer thick, and sure holds the edges up pretty steeply. The region certainly has the appearance of an underlying layer being wasted away, but permafrost would be near the surface. I propose an alternative possibility, that 10,000 feet of hydrated salts were exposed to dehydration by cracks or holes in the relatively thin permafrost, and then lost 30% of their volume.

Thus, this one theory explains the Valle Marineris and other chasms, pit chains, Cavi type regions, Chaoses (spelling?), and patterned ground in one shot. Just one possibility, I know, but I wouldn't have gotton here without some help (beer, hypnosis, meds, etc)!