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Mare basalts cover much of the Earth-facing side of the Moon. The underlying cause for this distribution has been attributed to an ancient nearside megabasin, asymmetric accretion, and differential tidal effects. While each hypothesis is plausible, the hypothesis for a megabasin also accounts for a subconcentric and radial system of graben and ridges that centers on a region southwest of Imbrium basin. Moreover, such a nearside megabasin could account for the distribution of nearside geochemical anomalies related to localized igneous intrusions. The farside South Pole–Aitken basin, however, is a well-established impact megabasin exceeding 2200 km in diameter. Here, we propose an oblique collision scenario for this basin on the farside that would have created the initial conditions for localized deep-seated and long-lasting weaknesses on the nearside. Laboratory and computational experiments demonstrate that a large oblique collision generates asymmetric shock waves that converge in a region offset from the basin-center antipode. The resulting damage would have provided pathways for deep magma to reach shallow reservoirs.

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