The Decale Desert in Tren is accessible after you have passed through Flash Canyon, however you cannot pass through the desert until after receiving the Decale Desert Map key item. The map is received after clearing the events at Palisade Prison.
Path through the Desert Edit
Go north until you reach the oasis. From there go:
West, North, North, East, North
This will take you to the desert exit area. Follow the single path northward until you reach the exit to the World Map.