Upper Imbrian
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In the Lunar geologic timescale, the Upper Imbrian epoch occurred 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 occurred 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.
It was preceded by the Lower Imbrian period and followed by the Eratosthenian.