The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism, as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the Moon. Here, we report on ages derived from crater size-frequency distribution measurements for exposed mare basalt units on the lunar nearside hemisphere. Crater size-frequency distribution measurements provide a statistically robust and accurate method to derive absolute model ages of unsampled regions on the Moon and other planetary surfaces. This paper summarizes and synthesizes results from our crater-counting efforts over more than 10 yr. We have dated basalts in Oceanus Procellarum, Imbrium, Serenitatis, Tranquillitatis, Humboldtianum, Australe, Humorum, Nubium, Cognitum, Nectaris, Frigoris, and numerous smaller occurrences like impact craters and sinus and lacus areas. Our investigations show that (1) in the investigated basins, lunar volcanism was active for almost 3 b.y., starting at ~3.9–4.0 b.y. ago and ceasing at ~1.2 b.y. ago, (2) most basalts erupted during the late Imbrian Period at ~3.6–3.8 b.y. ago, (3) significantly fewer basalts were emplaced during the Eratosthenian Period, and (4) basalts of possible Copernican age have been found only in limited areas in Oceanus Procellarum. Our results confirm and extend the general distribution of ages of mare basalt volcanism and further underscore the predominance of older mare basalt ages in the eastern and southern nearside and in patches of maria peripheral to the larger maria, in contrast to the younger basalt ages on the western nearside, i.e., in Oceanus Procellarum. New data from the recent international armada of lunar spacecraft will provide mineralogical, geochemical, morphological, topographic, and age data that will further refine our understanding of the flux of lunar mare basalts and their relation to petrogenetic trends and lunar thermal evolution.

Abstract A systematic survey was undertaken and an investigation carried out into the geomorphological characteristics of lobate debris aprons in the Tempe Terra region of Mars. Based on the most recent high-resolution (sub 15 m per pixel) imagery and on new topography data, this study endeavoured to raise and discuss questions regarding their formation (emplacement) and modification (deformation sequence), as well as the role of a mantling deposit found at mid-latitude locations on Mars. Furthermore, a model for the formation of debris aprons in the Tempe Terra–Mareotis Fossae settings is proposed. Image survey, in combination with basic morphometric observations within a geomorphological context, provided additional insights into the source, emplacement and modification of hillslope debris material. Our results imply that lobate debris aprons are not mainly relicts of remnant degradation but are substantially composed of mantling material probably deposited episodically in the course of planetary obliquity changes and over a long timespan, as derived erosion rates suggest. Crater-size frequency statistics and the derivation of absolute ages show ages of sub-recent modification and document earlier resurfacing events.

Abstract The circum north-polar Rupes Tenuis unit forms the polar-proximal basal stratigraphical and morphological units that delineate the north polar cap between 180° and 300°E. In the region of the mouth of the Chasma Boreale re-entrant, the Rupes Tenuis unit is likely to extend further southwards into the northern plains. This is suggested by the occurrence of isolated remnants that have been interpreted as basaltic shield volcanoes, maar craters or mud volcanoes in the past. As key elements of this study, we assessed the quantitative characteristics of this unit using layer attitudes derived from high-resolution images and terrain-model data, and by performing cross-correlations of prominent layers whose outcrops are observed at eight cone-like remnants. The identification and unambiguous correlation of characteristic layers across the study area provided a reasonable basis for introducing at least three additional stratigraphical subunits of the Rupes Tenuis unit. Extrapolation of altitude data indicates a gentle southward dip of remnant layers, suggesting that the unit had a much larger areal extent in Martian history. The palaeo-layer contact between two subunits of the Rupes Tenuis unit correlates well with elevation values of the Hyperborea Lingula surface. Both results disagree with an interpretation of a volcanic origin for isolated mesas but underpin that they are erosional relicts of the Rupes Tenuis unit. Average erosion rates of 2.5×10 −4 ±4×10 −5 mm year −1 are relatively high when compared to Amazonian rates but are not exceptional for areas undergoing deflation. They also corroborate the idea of aeolian denudation of the Rupes Tenuis unit.