Stratigraphy and Structural Evolution of Southern Mare Serenitatis' A Reinterpretation Based on Apollo Lunar Sounder Experiment Data

The Apollo Lunar Sounder Experiment (ALSE) detected two subsurface reflecting horizons in southern Mare Serenitatis. These horizons appear to be regolith layers, >2 m thick, which correlate with major stratigraphic boundaries in southeastern Mare Serenitatis. Our analysis differs from previous interpretations and implies that the lower horizon represents the interface between the earliest mare unit (unit I; •-3.8 b.y.) and the modified Serenitatis basin material below. The upper horizon represents the regolith developed by sustained impact bombardment of the unit I surface prior to the emplacement of the intermediate basalts (unit II; ---3.5 b.y.). The latest volcanic episode (unit III; ---3.2 b.y.) resulted in a very thin infilling (<400 m deep) in the southern portion of the basin and was undetected by ALSE. Profiles of the stratigraphic surfaces represented by the ALSE reflecting horizons are reconstructed for three stages in the basin filling history. These reconstructions permit the timing and magnitude of various volcanic and tectonic events within the Serenitatis basin to be assessed. On the basis of comparison with Orientale ring topography, the large subsurface arch is interpreted to be the peak-ring-related topography of the Serenitatis basin. The thickness of mare basalts across the ALSE ground track is highly variable: basalts are <400 m thick above the peak ring, while they reach a maximum thickness of---2.5 km in the depression between the first and second basin rings. Comparison with Orientale topography suggests that a major increase in basalt thickness of-6 km may occur ---50 km inside the peak ring. This region was unsampled by the ALSE traverse. Extensive load-induced deformation occurred after the emplacement of unit I (---1.2 km thick, maximum). This loading of the lithosphere resulted in graben formation along the margins of the basin, major downwarping and tilting of the unit I surface, and production of discrete zones of faulting, buckling, and subsidence. Pre-mare topography exerted a strong control over the tectonic patterns in the basin: Topographic highs such as ring structures underwent <400 m of subsidence, whereas intraring depressions underwent as much as 1.4 km of downwarping. Although the lithosphere was tectonically most active between the emplacement of units I and III, it continued to respond to the load of units I and II after the emplacement of unit III, as evidenced by ---600 m of postunit III structural relief. The association of all mare ridges transected by the ALSE ground track with distinct offsets or depth changes in the subsurface layering indicates that these surface features are tectonic in origin.