Extensional structures at lunar lobate scarps

Abstract

The Moon's lithosphere has been in a net compressional stress regime for the past 3.6 billion years, leading to the formation of globally distributed kilometer-scale thrust faults that form lobate scarps at the surface. Lobate scarps are among the youngest features on the lunar surface. A sign of their recent activity is the presence of meter-scale extensional structures in their hanging walls, which are generally either parallel or perpendicular to the scarp-forming thrust fault. However, the origin of these structures and their orientation relative to the primary fault is not well understood. Here, we present detailed tectonic maps of three lobate scarps (Lee-Lincoln, Lilius C-1, and Morozov-1), which reveal sets of fault-perpendicular to fault-parallel extensional structures at all scarps. Using high-resolution digital elevation models, we measure along-strike variations in fault throw and document maximum throws of ∼216 m for Lee-Lincoln, ∼98 m for Lilius C-1, and ∼140 m for Morozov-1. By comparison with terrestrial analogs, we show that fault-parallel extensional structures are mainly formed by flexural bending of the hanging wall, whereas fault-perpendicular features are potentially formed by along-strike variations in thrust orientation, and potential strike-slip components on the thrust fault. We hypothesize that small extensional structures preferably either form near the main displacement, at the most recently active regions of a scarp, or at segment boundaries. Based on degradation-rate estimates, we propose that small graben provide the best evidence for very recent activity of lobate scarp faults, because the graben form during coseismic slip events on the thrust faults.