Evidence for ring-faults around the Orientale basin on the Moon from gravity

Abstract

The Orientale basin is the best-preserved multi-ring impact basin on the Moon, spanning ∼900km in diameter at its outermost ring. It consists of three concentric ring structures known as the Inner Rook, Outer Rook, and Cordillera. Based on geologic and topographic evidence, the Cordillera ring has been interpreted as forming through listric normal faulting from the inward collapse of the interior basin. A similar mechanism may have been responsible for the formation of the Outer Rook, though it has also been interpreted as the rim corresponding to the collapsed excavation cavity. This study uses gravity and topography data to examine the subsurface structure of the Orientale rings. A Markov chain Monte Carlo approach was implemented to find the best-fit fault dip and displacement, as well as the depth to an intra-crustal density interface and the density contrast across that interface. We find that the data supports the presence of a low-angle normal fault beneath the Cordillera ring that offsets a shallow interface separating a lower density upper crust from a higher density lower crust. The best-fit solution for the case of faults at both the Cordillera and Outer Rook has a vertical component of fault displacement of 7.8km, a fault dip of 28°, an intra-crustal interface depth of 10.8km, and a density contrast of 350kg/m3 between the upper and lower crust. However, models with faults only at the Cordillera or with faults that do not cross the crust–mantle interface also provide adequate fits to the data. This analysis presents quantitative evidence for the existence of faults beneath the rings of Orientale and for density stratification of the crust beneath the basin based on gravity data.