Whatever happened here in Kasei happened long after the formation of the valley, long after the glaciers or whatever flowed here to cause these enormous trenches.

Something split the bottom of the channel asunder, and out poured some fluid, volatile, something, probably not lava, which flowed away, in either direction along the bottom of the channel, eroding the surface to some depth, leaving piles of debris along the margins.

See the terraces at the edges of the flow features? Were there several episodes of outflow from this crack in the surface? Did each flow freeze, say, and then sublime? And was each subsequent flow less massive than the last? What would cause something like this to happen billions of years (perhaps) after the creation of the canyon itself? And why would these flows occur at each end of the crack?

What happened here?