Olympus Mons is a shield volcano, analogous to what we see in Hawaii.
I can't conceive (argument from ignorance?) of an impact that could create a bulge on the other side of a planet like Mars; the rocks on Mars, first of all, would be elastic enough
not to transmit the shock to form a
bulge on the other side, and any impact that great would do a heck of a lot more than create a bulge - more like reduce most of Mars to outright rubble.
Edited to add:I was going to edit this message to say "melt most of Mars" instead of "reduce to rubble", but I see that Crashfrog has already seen information about Olympus Mons - I'm a bit late, I guess.
[This message has been edited by Chiroptera, 01-13-2004]