I believe (as does REB below) that Valles Marineris was formed by tectonic activity and yes they have found volcanic layers in it's makeup see this article of Nature for one. I also agree with their findings that Mars was very volcanically active (obvious from our observations today).

As for Mars having once been a gas planet/gas dwarf(?), it may have been enveloped by noble gases/Hydrogen & Helium during it's creation as most of the bodies in our solar system, but I think and believe that our Gas 'giants' took the bulk of those gases... they may have been formed first (shortly after our Sun) and/or accreted enough mass AND gas to have cleared the way for our terrestrial planets. It is interesting that Jupiter's 'rocky' core has been found to be smaller in mass than that of Saturn, yet it seems to hold more gas.

Paper on The Formation of Giant Planets. No two solar systems are the same.