Abstract
The formation ages of tectonic structures and their spatial distributions were studied in the northwestern Imbrium and Sinus Iridum regions using images obtained by Terrain Camera and Multiband Imager on board the SELENE spacecraft and the images obtained by Narrow Angle Camera on board LRO. The formation ages of mare ridges are constrained by the depositional ages of mare basalts, which are either deformed or dammed by the ridges. For this purpose, we defined stratigraphic units and determined their depositional ages by crater counting. The degradation levels of craters dislocated by tectonic structures were also used to determine the youngest limits of the ages of the tectonic activities. As a result, it was found that the contractions to form mare ridges lasted long after the deposition of the majority of the mare basalts. There are mare ridges that were tectonically active even in the Copernican Period. Those young structures are inconsistent with the mascon tectonics hypothesis, which attributes tectonic deformations to the subsidence of voluminous basaltic fills. The global cooling or the cooling of the Procellarum KREEP Terrane region seems to be responsible for them. In addition, we found a graben that was active after the Eratosthenian Period. It suggests that the global or regional cooling has a stress level low enough to allow the local extensional tectonics.
Highlights
Mare ridges and straight rilles are map-scale tectonic features on the Moon
Tectonic features and formation ages There are a number of mare ridges, grabens and lobate scarps in the study area, which we describe in the following subsections
We found lobate scarps and mare ridges and grabens that are significantly younger than the majority of the mare basalt fills
Summary
Mare ridges and straight rilles are map-scale tectonic features on the Moon. They are interpreted as compressional and extensional tectonic structures, respectively (e.g., Bryan 1973; Gilbert 1893; Howard and Muehlberger 1973; Maxwell et al 1975). This young activity is consistent with the morphologically crisp and undegraded appearance of the terraces. Graben System B consists of fissures that are 101 and 102 m order in width and length, respectively They are generally straight, parallel to each other and lie along the crest of the mare ridge that turns into the lobate scarps on Mons Chajorra (Fig. 14b), suggesting that they were formed by flexural bending of the thrust sheet. Graben Systems A and C dislocated craters smaller than 100 m in diameter, indicating their formation ages of the Copernican Period (Fig. 15) These young graben formations present with lobate scarps support that the scarps are young tectonic deformations. The fact that there is no graben extending into Unit A suggests that they are probably older than Unit A, which adjoins and is younger than the
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