What mechanism could have formed the Moon's Oceanus Procellarum rift-like gravity anomalies?
Source: JPL Article

"The gravity anomalies along the border structures are interpreted as ancient, solidified, lava-flooded rifts that are now buried beneath the surface of the dark volcanic plains, or maria, on the near side of the moon."

Because of the lava under Oceanus Procellarum, it's heavier/massive so it has a stronger gravitational pull than the area surrounding it.

Hope that helps!