essentially on the equator... crossing, or rather sitting on top of the dark material covering the leading hemisphere of Iapetus... hmmm

Does it seem possible that the mountain chain, if that's actually what it is, formed at a crack in the surface, and that fluid/semi-fluid material from the interior was pushed upwards, freezing of course at the surface? A process analogous to what is thought to go on at Europa even as we speak. Only of course at the distance of Iapetus/Saturn, temperatures are colder, surfaces more rigid, etc. Ergo, a crack might offer an outlet for interior material -- ices of some sort -- but cold conditions might preclude any substantial resurfacing. Just a chain of mountains. Be interesting to see if they continue for any distance on the dark side. (Another potential analogy is the aligned volcanoes on the Tharsis Bulge of Mars, one of which is smack on the equator, the others at an angle to it, ~30 degrees iirc.)

Of course the black stuff remains mysterious. A thought: if a carbonaceous asteroid disintegrated in the orbital path of Iapetus, it could form a cloud of dust. Iapetus would then essentially sweep it up as it passed through. Phoebe would not necessarily have anything to do with it.

Cheers, BRFelix