The Tharsis region is conventionally interpreted as an uplifted volcanic region active from the earliest period of Martian history to very nearly the present.

The canyons of Valles Marineris show that the region is finely layered, and the many landslides in the canyons show that the material is not well consolidated. In fact, it tends to collapse and flow like a viscous fluid.

Speaking of collapse: the whole Tharsis region seems to be riddled with subsurface collapse, which is a primary cause of some of the channels referenced in your post. Take a close look at Olympica Fossae as an example. Something -- quite possibly water -- has flowed beneath the surface, and that flow is expressed at the surface as channels that seem to have formed through the collapse of surface materials into the subsurface channels, as well as flows of volatiles at the surface (though perhaps in explosive gasseous flows rather than liquid flows.)

It is certainly possible that there are subsurface flows in Tharsis right now -- and they could certainly be flows of liquid water.

Tharsis fine layering resembles nothing so much as polar layering on Mars. Well... perhaps that's what it is. Perhaps much of Tharsis, except the very top layers, IS a polar deposit. And if it is, then massive water content would be expected. The floods emanating north, east and west from Tharsis would make perfect sense if this formerly polar region were penetrated by volcanism.

It could even be that the volcanism in Tharsis has been -- at least partially -- a "mud volcanism" rather than lava. I don't think anybody really knows. It may not be possible to tell remotely.

Just some things to think about...

Cheers, BFelix