Lately I've been trying to better understand crystallized water ice, the process of it becoming amorphous and the effect ammonia plays in this process.
According to Jason Cook, the crystallized structure of Charon's surface indicates it's surface ice is less than 100,000 years old.
The smooth nature of the southern hemisphere demonstrates it went through a catastrophic process of resurfacing.
The raised diagonal ridge (Serenity Chasma) along with the 6 mile lower plane to the south (Vulcan Planum (its actually a Planitia, raised planes are planum's, lower planes are planitia's)) separating the Northern Hemisphere from the South indicates the ridge line itself was not the only place subsurface fluid expelled onto the surface.
If fluid had expelled only from this diagonal crack it would mound upwards on both edges as we see on other moons like Enceladus or Europa.