In the Lunar geologic timescale, the Upper Imbrian epoch occured between 3800 million years ago to about 3200 million years ago. It was the period during which the mantle below the Lunar basins partially melted and filled them with basalt. The melting is thought to have occured because the impacts thinned the overlying rock - either causing the mantle to rise because of the reduced pressure on it, bringing molten material closer to the surface; or the top melting as heat flowed upwards through the mantle because of the reduced thermal insulation on top of it.
