THE EFFECTS OF SEISMIC SHAKING ON THE CRATERING RECORD AROUND LOBATE THRUST FAULT SCARPS (Invited Presentation)

Like Earth, our Moon is covered with an array of tectonic landforms, both compressional and extensional. Lunar orbiter images have revealed the fresh morphology of very young lobate scarps, which are the surface expression of thrust faults formed by contraction of the crust due to long-term interior cooling and tidal deformation [1,2]. First age estimates from [3] using crater degradation measurements on craters cut or superposed by the scarps revealed that they formed within the Copernican period (<1 Ga). Thousands of new lobate scarps have been identified across the lunar surface with high-resolution images from the Lunar Reconnaissance Orbiter Camera [4], enabling the lobate scarps to be studied in more detail and globally across the lunar surface. Using traditional crater size-frequency distribution (CSFD) measurements [5] on 34 lobate scarps, absolute model ages (AMAs) determined for surfaces proximal and distal to the trace of the faults revealed that they formed in the late Copernican period (< 400 Ma) and some as recent as 29 Ma ago [6]. Globally, the timing of scarp formation is random, which is in agreement with thrust faults being mostly formed under isotropic stress conditions from global contraction due to long-term interior cooling of the Moon. Occasionally, the CSFDs allow two crater populations to be fit, possibly indicating multiple fault events. Distal locations approximately 3-4 kilometers away from the scarp typically have older ages than the proximal locations, likely due to the decreased amount of seismic shaking. This reduction in seismic shaking causes the crater population to not be completely reset as it preferentially erases smaller craters and partially resurfaces larger craters, thus reducing the amount of craters to determine a meaningful AMA. Over the last ~250 Ma, the range of crater diameters used to determine AMAs has shrunk, possibly indicating that seismic activity (duration and/or magnitude) has been decreasing, where smaller quakes would erase a smaller size range of craters.

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