Formation of the Moon's Orientale multi-ring basin

Abstract:

We model the formation of the Moon’s Orientale basin from impact to crater collapse using the iSALE hydrocode. Although we use GRAIL and LOLA data to constrain our models, we show that the modeled basin structure reveals more detail than inverse models can predict. Orientale exhibits three concentric topographic rings; the Outer Rook (R=330 km) and Cordillera (R=430 km), which are morphologically consistent with normal fault scarps; and the Inner Rook Ring (R=230 km), which is not. Our numerical models allow us to see, for the first time, how these ring structures form in real time. Shortly after ejecta emplacement, as the transient crater collapses, the Outer Rook fault forms followed by the Cordillera fault, with both faults cutting all the way through the crust. Subsequently, the topographic expression of the Inner Rook Ring is produced in part by the collapse of a central uplift, consistent with previous interpretation that the formation of Inner Rook Ring is analogous to the formation of peak-rings seen in smaller basins. The Inner Rook Ring is then modified by a secondary phase of inward collapse. The resultant basin structure is generally in good agreement with GRAIL derived crustal thickness models. Our modeling reveals that that two inflection points in the GRAIL derived crustal thickness are associated with Moho offsets of a few kilometers, which are the result of slip along the Cordillera and Inner Rook faults. In the future this association may be used to determine where faults are located in older mare filled basins. Our models also show that dilatant bulking is enhanced near the faults. Despite GRAIL’s incredible sensitivity and resolution, localized Moho offsets and density changes associated with faults cannot be resolved by inverse modeling. A direct comparison of the gravity anomalies produced by these localized density features and present-day gravity could constrain the geometry of proposed ring dikes in the Orientale basin.