Small-scale lunar farside volcanism

Abstract

We identified and mapped 28 mare basalt occurrences, between the Australe and South Pole-Aitken basins on the southern lunar farside, and determined their absolute model ages (AMAs) by performing crater size–frequency distribution (CSFD) measurements. Our study area can be subdivided into seven major mare basalt occurrences in and around Bolyai, Roche V, Rosseland, Pauli, and Roche craters, south of both Rosseland and Coblentz craters, as well as mare patches between Eötvös and Roche craters. The AMAs of the mare basalts in and around Bolyai crater range from 2.1Ga to 3.5Ga (two units), varying drastically within short distances. The mare patches south of Coblentz crater contain nine units that have AMAs ranging from 2.1Ga to 3.8Ga. The mare basalts in Roche V crater show an AMA of 2.2Ga. We mapped seven volcanic units in Rosseland crater and derived AMAs for five of these units. The mare basalts in Rosseland crater show the youngest AMAs found in our study area, ranging from 1.5Ga to 2.9Ga. The mare basalt occurrence south of Rosseland crater shows significantly older ages with an AMA of 3.3Ga. The mare basalts in Pauli and Roche craters show AMAs from 1.7Ga to 3.1Ga. The mare patches between Eötvös and Roche craters show a similar range of AMAs from 1.9Ga to 3.1Ga. The AMAs of the mare basalts in our study show that the lunar farside was volcanically active for nearly as long as the lunar nearside (1.2Ga ago), or at least longer than previously thought (2.5Ga ago) and predicted by models of the ascent and eruption of lunar basalts. In addition, we calculated thicknesses and volumes of the investigated mare basalts. With thicknesses between ∼21m and ∼172m and volumes from ∼0.1km3 to ∼379km3, the mare basalts in our study area show a wide range of dimensions, similar to other mare basalts of the lunar near- and farsides.