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Nearside Megabasin: The Largest Basin from the Moon

Ambrose, William A.1
1 Bureau of Economic Geology, Austin, TX.

The Nearside Megabasin, 3,200 km wide and centered on the western half of the Lunar nearside, is inferred to have formed from a giant impact ~4.3 billion years ago. Evidence for the basin includes: radial graben, remnant ring structures, aligned igneous features such as domes and dome fields along remnant rings, a thin (<20-km) depressed crust, and anomalous volatile deposits along zones of structural weakness. The Nearside Megabasin is also associated with the Moon’s greatest concentration of thorium and KREEP (Potassium, Rare-Earth Elements and Phosphorus) lavas that formed from late-stage partial melts, possibly in response to decompression melting following deep excavation of the lower crust and upper mantle from the Procellarum impact event. These thorium deposits and related volatiles are important resources for sustaining future human settlement on the Moon. The Nearside Megabasin contains the largest continuous extent of lunar basalts on the Moon and its upper fill is a complex of at least four different flow units, recognized on the basis of albedo and spectral reflectivity. Individually, these flow units are only a few hundreds of meters thick, but may be underlain by 2-4 km thick basin-filling basaltic units. In contrast to many other lunar basins, the Nearside Megabasin lacks a surrounding mountain rim and underlying mascon, features commonly associated with other nearside lunar basins such as Mare Tranquillitatis, Serenitatis, and Crisium. However, the absence of these features may be due to the basin having formed so early that the lunar crust may have not been sufficiently rigid to support rim material and excess masses of thick basin-filling units.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009