Here's the link to the ESA page with that image and others from Kasei.

http://www.esa.int/SPECIALS/Mars_Express/SEMLF6D3M5E_0.html#subhead3

An "impact plume," eh? Interesting notion. This discussion regarding an impact origin for Tharsis is interesting, but it has little/nothing to do with Kasei, more's the pity.

No, Kasei (here appears to be a massive outflow formation, with some evidence of glacial action long ago. There are some indications of ancient and now buried major impacts in the area. But they don't seem to have anything to do with this feature either.

Kasei Vallis is not a new feature at all; but the feature in the ESA image is much newer. The single visible crater on the northern detritus field ought to give us some means of dating when the flow here happened.

Could the fluid -- whatever it was -- that created and sustained Kasei (word is that there continued to be action in Kasei Valles for eons) have eventually retreated beneath the surface of the valley, there perhaps to freeze for many epochs, only to be heated from below by rising magma that never itself reached the surface?

Do we know how "warm" a fluid must be to be forcefully ejected at the surface? I'm still wondering why the flows from this very large crack in the surface of the valley floor are only at the two ends.