How old are lunar lobate scarps? 2. Distribution in space and time

Abstract

Based on their fresh morphology, lunar lobate scarps are thought to be some of the youngest landforms on the Moon. Age estimates using crater degradation measurements on craters cut or supposed by the scarps revealed that such lobate scarps formed in the last ∼700 Ma. Modern crater size-frequency distribution (CSFD) measurements provide a new method for investigating scarp formation ages, which we used to determine ages for 34 lobate scarps. Our work shows a global distribution of lobate scarp ages that is spatially random and thus consistent with the theory that their formation is largely the result of global contraction from the long-term interior cooling of the Moon. CSFD measurements can provide further information about the scarp such as the possibility of multiple fault reactivation events, the geographical extent and magnitude of shaking associated with scarp activity, or whether additional resurfacing processes affect the crater population. Using a traditional CSFD method, we reveal that the thrust fault scarps have been active in the last 400 Ma – some as recently as 24 Ma. Distal locations approximately 3 – 4 kms away from the scarp trace typically have older ages and larger errors than the proximal locations, likely due to decreasing and/or heterogeneous seismic shaking away from the fault traces. Over the last ∼250 Ma, the range of crater diameters reset by scarp activity has shrunk, possibly indicating that seismic activity (duration and magnitude) has been decreasing, where smaller or more punctuated quakes would erase a smaller size range of craters. Thus, our results show that the Moon has been tectonically active within the late Copernican.

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