Lunar graben formation due to near-surface deformation accompanying dike emplacement

Linear and arcuate rilles (simple graben) represent some of the most distinctive tectonic features on the Moon. They have been interpreted to be linked to impact basin structure (Mason et al., 1976), and to the emplacement of mare deposits in the basins via flexural deformation related to the mare deposit load (Solomon and Head, 1980). Linear rilles are generally thought to represent deformation confined to the upper layers of the crust (Golombek, 1979) where stresses are concentrated as rocks at deeper levels adjust viscously to long-term applied loads (Melosh, 1976). There is evidence, however, that several linear rilles have volcanic (commonly pyroclastic) deposits associated with them over at least parts of their horizontal extents (Schultz, 1972 ; Head, 1976). This has often been explained as being due to volcanic materials utilizing deep-seated zones of weakness, or shallow zones of weakness associated with the bounding faults of the graben. These graben-related faults probably only extend to depths of a few kilometres (Golombek, 1979). This depth is such a small fraction of the total rise distance of magma ascending from any plausible source [at least many tens of kilo-] that it is very unlikely that the location and orientation of the dike providing a pathway for the magma would be significantly influenced by the surface topography and near-surface structure represented by the graben. Thus, it is unlikely that emplace-ment of magma from these depths would coincide directly with a shallow graben structure formed in a separate, earlier event. A more likely situation is that graben with associated volcanic deposits are themselves produced simultaneously as the surface result of stress fields generated by dikes approaching the surface from great depths (Pol-lard et al. At least one example of such a shallow dike intrusion (Rima Sirsalis) is suspected on the basis of Apollo mission magnetic measurements (Srnka et al., 1979). The surface manifestation of dike emplacement in the crust may be quite different depending on the depth below 720 J. W. Head III and L. Wilson : Lunar graben formation the surface to which the dike penetrates as it rises. In general, if the dike propagates to the surface and erupts, then the stress field immediately surrounding the propagating dike will be compressional all the way to the surface and the surface manifestation will be a fracture. If the dike does not propagate to the vicinity of the surface (perhaps stalling at depths greater …