What mechanism could have formed the Moon's Oceanus Procellarum rift-like gravity anomalies? According to the NASA JPL web-report, Gravity Gradients Frame Oceanus Procellarum, a rift-like ring structure surrounding the Moon's Oceanus Procellarum has been detected as a gravity anomaly by the GRAIL scientific mission (as can be as the dark blue features on the image below).



Image credit: NASA/Colorado School of Mines/MIT/GSFC/Scientific Visualization Studio

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!