The Crustal Structure of Orientale and Implications for Basin

Introduction. The topography [1] and gravity [2] of the Moon are dominated by the signatures of impact basins and the mascon gravity anomalies contained within. A method of generating higher resolution crustal thickness models by utilizing the symmetry of the basins is here presented, providing a refined view of the subsurface structure of the basins, with a focus on Orientale. The crustal structure is compared with results from CTH numerical models of basin formation [3]. The implications of the crustal structure for the origin of the super-isostatic mantle plugs observed beneath a number of lunar basins are then considered. These basins possess mascon gravity anomalies in excess of that which can be accounted for by the mare loading in the basin interiors [4]. It is shown that the super-isostatic state of the basin floors is a result of the flexural uplift of an annulus of sub-isostatic thickened crust surrounding the basins. Basin crustal structure: Methodology. Previous studies used gravity and topography data to generate global crustal thickness models for the Moon [5], which were then analyzed to investigate the structure of individual basins [6]. These crustal thickness models were limited by the amplification of the shortwavelength noise in the gravity data during the downward continuation to the Moho depth, requiring aggressive tapering for a stable global solution. However, when analyzing a feature with a natural symmetry axis, such as an impact basin with radial symmetry, the uncertainty in the mean profiles (represented by the standard error on the mean) is greatly reduced by averaging around the symmetry axis. This results in mean gravity and topography profiles of improved accuracy. The averaged gravity and topography profiles can be used to generate crustal thickness models for the basins using the standard technique [5], at higher resolution than can reliably be done on a global basis. Both the quality of the gravity data and the departures from radial symmetry vary greatly from basin to basin, thus it is necessary to choose the optimal spherical filter to apply in the downward continuation. This filter is chosen using the a priori geologic expectation that the uplifted mantle plug below the basin should have a flat roof, resembling the flat floor of the basins themselves. A simple cos taper of constant width is applied for a series of models, while the degree at which filtering commences is shifted to higher degrees for successive models. The optimal filter is that which minimizes the RMS gradient of the Moho beneath the basin floor. Basin crustal structure: Results. The azimuthally averaged gravity and topography over Orientale was Figure 1. Azimuthally averaged profiles of the topography (top) and gravity (middle) over the Orientale basin. The crustal structure model (bottom) includes the mare basalt (black), anorthositic crust (gray), and mantle (red). The surface relief in the bottom panel is stretched by a factor of 5 relative to the Moho relief.