A photogeologic and physical modeling study indicates that Hebes Chasma, Mars, formed by collapse of the megaregolith. Local heating facilitated drainage of ~105 km3 of brines and entrained particulates through fractures in the chasma floor and into a regional aquifer. A megaregolith rich in salts and water is implied by massive, low-gradient allochthonous flows that terminate in deep pits and troughs, by emergent diapirs, and by arching of Hebes Mensa. These structures are consistent with plastic and viscous deformation but inconsistent with collapse of basalt flows and/or tephra. Spectral measurements confirm that hydrated sulfate salts are spatially associated with allochthonous flows from depth and light-toned deposits. Collapse features and flows are present in many other chasmata in Valles Marineris, suggesting that widespread salt tectonics and dissolution may have shaped the region.