Lunar floor-fractured craters are a class of ~170 lunar craters with anomalously shallow and fractured floors, divided into eight morphologic subclasses. Recent morphologic studies have supported the hypothesis that floor-fractured craters were formed by the intrusion and evolution of a shallow magmatic body beneath the crater. Further support for this hypothesis comes from the association of volcanic morphologies such as pyroclastic deposits and mare deposits with the fractures of floor-fractured craters. Here we examine the formation of floor-fractured craters to give insight into both global and regional lunar structure, and how the formation and evolution of the magmatic intrusions gives insight into lunar volcanic processes. It has long been recognized that lunar floor-fractured craters are predominately located around the edges of large lunar basins and in the vicinity of mare deposits. We suggest that this distribution reflects a formation time consistent with the end of the main phase of mare volcanism. Flexural models indicate that loading of the lunar maria on the lithosphere would inhibit further dike propagation beneath the basin, and create extensional zones at the edges of the loading which would enhance dike propagation. Thus the distribution of lunar floor-fractured craters reflects both the general timing of their formation and the regional stress state at the time of emplacement.
In addition to using the distribution of floor-fractured craters to explore the timing of their emplacement, we have used detailed mapping of the surface morphologies to assess the formation process of floor-fractured craters. The surface morphologic variations of floor-fractured craters are well documented, and thus far the origin of these differences has been unexplored. We have used detailed mapping of several case study floor-fractured craters, including Humboldt, Alphonsus, Vitello, and Bohnenberger, to explore the detailed process of intrusion formation. Our results support the hypothesis that all magmatic intrusions undergo the same initial formation process, and the final intrusion morphology is determined by the diameter of the host crater—smaller host craters maintain bell-shaped intrusion morphologies and larger host craters transition to tabular sill intrusion morphologies. The tabular sill morphology observed beneath larger floor-fractured craters also creates a stress state favorable to the propagation of dikes from the bending edge of the sill; these dikes serve as sources for the volcanic morphologies observed in lunar floor-fractured craters.