LUNAR ORIENTALE BASIN: A COMPREHENSIVE CONCEPTUAL MODEL FOR THE ORIGIN OF BASIN RINGS AND GEOLOGICAL UNITS FROM NEW SPACECRAFT DATA

The ~930 km diameter Orientale basin, the youngest and least modified multi-ring basin, provides key insights into the nature of basin-forming processes in early lunar and planetary history. New altimetry and image data show that the transition from complex craters to peak-ring basins involves morphometric changes that signal the growth and increasing influence of a nested melt cavity; a major manifestation of this is the change from expanding central peak width in complex craters to peak rings that represent the toes of listric wall collapse thrusting inward and upward to the crater floor. Altimetry data permit calculation of Hevelius Formation ejecta thickness decay and volume. Detrended altimetry data reveal buried craters up to the Cordillera ring but not inside the Outer Rook (OR) ring, and ghosts of secondary crater chains that extend up to the OR; these and related data place the approximate final transient cavity rim crest at the 620 km diameter Outer Rook ring, with the Inner Rook representing the peak ring. The knobby Montes Rook Formation (MRF) is interpreted to represent the collapsed transient cavity rim, consisting of the structurally uplifted rim formed from material underthrust from the adjacent transient cavity, and superposed by a huge load of many kilometers of Orientale basin ejecta; during the cavity modification stage, downward and inward translation of the rim at approximately the outer edge of structural uplift formed the Cordillera listric fault ring, causing rotation and translation of the OR Mountains, and the shedding of ejecta outward from the uplifted massifs into the newly formed trough to create the knobby MRF. The inner depression stepdown is interpreted as the surface of a huge cooled impact melt sea, 10-15 km thick and likely to be differentiated. These observations are consistent with a nested melt-cavity basin formation model in which the OR represents the transient cavity, the melt sea represents the collapsed melt cavity, and the peak-ring plateau and MRF represent manifestations of the gravity-driven rotational collapse of the basin rim crest. This model predicts that the nested melt cavity penetrated many tens of km into the mantle, and that basin collapse involved rapid intrusion of a mantle plug into the crust, facilitated by transient shock weakening of deeper crust/mantle material.