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Lunar mare basalts represent melting of mantle material, buoyant ascent in dikes, and eruption onto <20% of the surface. Global mare distribution is distinctly asymmetrical, with a paucity on the farside, plausibly interpreted to be related to thicker farside low‐density crust inhibiting buoyant magma rise to the surface. Challenging this hypothesis is the presence of the huge, ancient farside South Pole‐Aitken (SPA) basin, site of the thinnest crust and deepest depression observed on the Moon. We hypothesize that an oblique impact stripped the farside crust within the SPA basin, permitting early mare basalt emplacement as cryptomaria due to thin/absent crust. However, removal of the SPA thermally insulating megaregolith/crust accelerated lithosphere thickening beneath the basin. This deepening rheological barrier inhibited buoyant rise of mantle diapirs below SPA, resulting in early abatement of mare basalt extrusions compared to the nearside, and retention of the deep, underfilled SPA impact basin observed today. Plain Language Summary: The cause of the observed asymmetry in nearside/farside mare basalt distribution (nearside ∼15% of the surface, farside ∼1%) has been attributed to differences in crustal thickness and the magma buoyancy at the base of the thicker crust, but this single factor is challenged by the presence of the ancient very deep SPA basin (average ∼5 km), significantly underfilled by mare basalts compared to the nearside. We examine the effects of the formation of the giant (∼2,300 km) SPA basin in removing the insulating megaregolith/crust, and show that it could first result in rapid cryptomaria emplacement due to near‐absent crust, accompanied by enhanced heat loss within the basin, and formation of anomalously early thick sub‐SPA lithosphere, inhibiting its mare filling due to a rheological barrier to basaltic magma eruptions, analogous to that which occurs later in lunar history on the lunar nearside. Key Points: The nearside‐farside asymmetry in mare basalt volcanic deposits has been attributed to thicker farside crust inhibiting eruptionChallenging this hypothesis is the very deep, thin‐crust ancient SPA impact basin, significantly underfilled by mare basaltsAfter SPA impact‐removal of insulating crust, sub‐SPA lithosphere thickens anomalously early, inhibiting mare fill following cryptomaria [ABSTRACT FROM AUTHOR]