the clutch appears as though it was stuck open(shifted out) and some serious belt slipping occured causing the belt residue.
not too surprising it was contained to a single sheave, the other one was perhaps out of the line of fire due to being stuck open or an alignment issue.
possible causes would be not enough pretension on the spring and helix, or an improperly wound helix.
had an srx years back that if you slammed the brake on during evasive manuevers, the clutch would hang open and burn the belt. a little more wrap cured the problem.