Deep-Seated Reverse Faults in Mare Crisium, the Moon

Wednesday, 17 December 2014: 4:45 PM
Paul K Byrne1,2, Christian Klimczak1,3, Patrick Joseph McGovern Jr2, Erwan Mazarico4, Peter B. James5, Gregory A Neumann4, Maria T Zuber6 and Sean C Solomon1,5, (1)Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC, United States, (2)Universities Space Research Association, Lunar and Planetary Institute, Houston, TX, United States, (3)University of Georgia, Department of Geology, Athens, GA, United States, (4)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (5)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, United States, (6)Massachusetts Inst Tech, Cambridge, MA, United States
Mare Crisium partially fills a Nectarian basin 556×455 km in diameter on the lunar nearside, one of several such basins associated with a mass concentration or “mascon.” The basin's interior topography is dominated by an elevated, circumferential bench that extends inward from the perimeter by ~20% of the basin's radius. A set of wrinkle ridges, landforms that are interpreted as folds over reverse faults that may be blind or surface breaking, lies along the inner edge of this bench. With the elastic dislocation program COULOMB we matched model solutions for surface displacements to topographic profiles across five of these wrinkle ridges. We find that the faults underlying the ridges each accumulated substantial along-slip displacement (c. 0.5–1.5 km) and, despite differences in geometry (some faults are planar whereas others are listric), they all penetrate the lunar lithosphere to depths of 18–20 km. Notably, the wrinkle ridges that follow the inner edge of the elevated bench are spatially coincident with the outer boundary of the highest free-air gravity anomaly values for the Crisium mascon returned by the Gravity Recovery and Interior Laboratory (GRAIL) mission. Further, a GRAIL-derived crustal thickness model of the basin indicates that the subsurface geometry of the deep-seated faults bears a strong resemblance to the shape of the crust–mantle boundary beneath Crisium. The basin's mascon, therefore, appears to be structurally bound by a set of individual features that together define a shallowly and outward-dipping reverse ring-fault system. TEKTON finite-element models of lithospheric loading within the basin suggest that the combined action of a subsiding superisostatic mantle plug and a rising subisostatic collar of thickened crust produce a stress state consistent with the orientation of, and sense of displacement along, these ring faults. Importantly, Crisium is not the only lunar mare-filled basin that hosts both a mascon and a topographic bench. Humorum, Imbrium, Nectaris, Orientale, and Serenitatis also feature wrinkle-ridge-bounded, arcuate portions of elevated topography within their perimeters—and so they, too, may be underlain by deep-seated reverse faults.