QUESTION:
I realize the similarity of the Valles Marinaris feature in a gross sense to
a failed triple junction in the earth plate tectonic regime and the
consequent age considerations but I admit I do find it difficult to
comprehend that features millions/billions of years old could still be
preserved with such a relatively fresh aspect (cratering considerations
aside) without continuing geologic activity (eg wall slumping etc).  With
this in mind, is there any evidence of fumarolic activity in the Tharsis
Bulge area?

ANSWER from Jeff Plescia on October 22, 1997:
Valles Marineris is of Early Hesperian age (probably about 3.1-3.5 billion
years).  It post dates most the major erosional processes on Mars; that is
why it is so well preserved.  However, it has been significantly modified. 
The canyons extending into the edges, particularly on the south side suggest
some type of aqueous sapping process.  The floor is covered in many places
with landslide debris. Numerous landslides have occurred off the walls of
the structure.  There is also evidence within the canyon for sediments
filling parts of the canyon, perhaps suggetive of a one time lake.  Volcanic
ash has been suggested to occur along the inner edges of the structure,
localized along the bounding faults.  There is no direct evidence for any
fumerolic activity within the Tharsis region.  However, we have no really
had the appropriate instrument to search for such activity. The Mars Global
Surveyor mission, currently in orbit, has a thermal instrument which might
detect areas of significant surface heat.