QUESTION:
How does the Martian lithosphere behave rheologically? What does the
presence of graben or ridges at the surface imply regarding the behavior of
the lithosphere and its failure? What's the consequence of the absence of
plate tectonics?

ANSWER from Jeff Plescia on 24 June 1998:
The presence of graben and wrinkle ridges (respectively brittle extensional
and compressional features) indicate that the at the surface the crust is
competent enough to develop stresses that ultimately reach the point of
brittle failure.  How these stresses are accommodated at depth is unclear. 
For example, the faults bounding the graben appear to intersect at fairly
shallow depths well above the point at which the temperature would increase
to allow for plastic deformation.  So either the stresses are accommodated
in some other brittle manner or in material with low strength that deforms
throughout (e.g, unconsolidated material).

Because there are no plate tectonics, the Martian crust is not recycled as
it is on the Earth, hence much of the Martian surface (southern hemisphere)
is very old.  For volcanic eruptions, the result is that a single very large
pile of lava is built up (e.g., Olympus Mons) rather than a chain of volanic
constructs (e.g., Hawaiian chain).