Timing and Origin of Compressional Tectonism in Mare Tranquillitatis

Frueh T.; Hiesinger H.; van der Bogert C.H.; Clark J.D.; Watters T.R.; Schmedemann N.

Abstract

The lithosphere of the Moon has been deformed by tectonic processes for at least 4 billion years, resulting in a variety of tectonic surface features. Extensional large lunar graben formed during an early phase of net thermal expansion before 3.6 Ga. With the emplacement of mare basalts at ∼3.9–4.0 Ga, faulting and folding of the mare basalts initiated, and wrinkle ridges formed. Lunar wrinkle ridges exclusively occur within the lunar Maria and are thought to be the result of superisostatic loading by dense mare basalts. Since 3.6 Ga, the Moon is in a thermal state of net contraction, which led to the global formation of small lobate thrust faults called lobate scarps. Hence, lunar tectonism recorded changes in the global and regional stress fields and is therefore an important archive for the thermal evolution of the Moon. Here, we mapped tectonic features in the non-mascon basin Mare Tranquillitatis and classified these features according to their respective erosional states. This classification aims to provide new insights into the timing of lunar tectonism and the associated stress fields. We found a wide time range of tectonic activity, ranging from ancient to recent (3.8 Ga to <50 Ma). Early wrinkle ridge formation seems to be closely related to subsidence and flexure. For the recent and ongoing growth of wrinkle ridges and lobate scarps, global contraction with a combination of recession stresses and diurnal tidal stresses, as well as with a combination of South Pole-Aitken ejecta loading and true polar wander are likely.