Perhaps the awesome fissure was caused by an awesome rock that slammed into the region tearing through the earths crust.
This is like the association between the K/T impact and the formation of the Deccan Traps (Asia) at approximately the same time and again an impact has been indicated as a possible cause of the formation of Basalts in SE Oregon and Idaho about 17mya (starting point of Yellowstone hot spot).
Direct impacts causing volcanism "Impact Volcanism" would probably mask the impact craters in basalt and 250my can hide a lot of stuff.
A direct impact may not be necessary to cause volcanism, but the shock waves of a large impact may reinforce each other at a position globally opposite the impact site (antipode) which, taking into account the positions of the continents 65mya, seems to have happened at the K/T boundary.