On the Absence of Strike-slip Faults from the Planets and Satellites

Lunar grabens are bounded by two downward converging 60 deg dipping normal faults. These faults intersect below the surface at a major mechanical discontinuity in the shallow crust between the megaregolith (total ejecta built up by repeated cratering) and the underlying in situ fractured rock. The ymmetry and simplicity of lunar grabens demands that the bounding faults initiate at this mechanical discontinuity and propagate up. Wrinkle ridges are though to result from thrust faulting or compressional folding of mare basalts. Identification and analysis of a number of terrestrial analogs that are morphologically similar to lunar and martian examples indicates that wrinkle ridges results from thrust faulting. The location of wrinkle ridges above suggested subsurface basin structures indicates that the trend and location of many wrinkle ridges are inherited from these pre-existing basin structures. This implies that the thrust faults responsible for wrinkle ridges initiate at the ase of the basalt-basin floor contact. This contact almost certainly represents a mechanical discontinuity in the shallow lunar crust because mare basalts are undoubtedly stronger than the underlying fall-back ejecta of the basin floor.