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THE CRUSTAL STRUCTURE OF ORIENTALE AND IMPLICATIONS FOR BASIN FORMATION

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THE CRUSTAL STRUCTURE OF ORIENTALE AND IMPLICATIONS FOR BASIN FORMATION. J. C. Andrews-Hanna1 and S. T. Stewart2, 1Department of Geophysics, Colorado School of Mines, Golden, CO, [email protected], 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, [email protected] 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 ex-cess 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 mod-els were limited by the amplification of the short-wavelength noise in the gravity data during the down-ward continuation to the Moho depth, requiring ag-gressive 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 un-certainty in the mean profiles (represented by the stan-dard error on the mean) is greatly reduced by averag-ing 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 ra-dial 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 cho-sen 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 cos2 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 topogra-phy (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. used to generate a crustal structure model (Figure 1). Orientale is underlain by a quasi-cylindrical mantle plug, which is surrounded by an annulus of thickened crust. This crustal thickening occurs at the Moho rather than at the surface, resulting in strongly sub-isostatic crust within this annulus. This sub-isostatic crustal thickening is responsible for the pronounced negative gravity ring surrounding many of the mascon basins. A similar crustal thickening is predicted by the CTH nu-merical simulations, resulting from the overturned flap of ejecta (Figure 2). Origin of super-isostatic mantle plugs. The excess positive gravity anomalies beneath many basins indi-cate a super-isostatic uplift of the mantle plug below the basin. Previous studies interpreted this to result from the instantaneous rebound of the basin floor in the moments after the impact, which overshoots and is arrested in a super-isostatic position [6]. However, new CTH models predict that the basin floor should be sub-isostatic rather than super-isostatic in the moments following the impact, as shown by the difference be-tween the topography and that predicted for a state of isostasy (the isostatic anomaly; Figure 2). However, this will rapidly relax to an isostatic state because of the thermal anomaly resulting from shock heating and excavation of the lithosphere (Figure 2). Using nomi-nal rheological parameters [3], the CTH models predict the thickened crust outside the basin to be even more strongly sub-isostatic, though an investigation of a 2194.pdf42nd Lunar and Planetary Science Conference (2011)broad parameter range is needed. This region experi-ences little impact-induced heating, and the lithosphere is thickened as a result of the effect of the ejecta both thickening the crust and increasing the overburden on and therefore the frictional strength of the underlying lithosphere. Thus this sub-isostatic thickened crust will be an enduring feature of the post-impact structure. We propose that the isostatic rise of the annulus of thickened crust outside the basin drives a flexural up-lift of the basin floor, resulting in a super-isostatic ba-sin floor surrounded by a sub-isostatic annulus. We test this hypothesis using spherical harmonic thin shell loading models [7] to represent the loading and flexure of an initially isostatic basin after applying either (Fig-ure 3a-b) a super-isostatic uplift of the mantle, or (Fig-ure 3c) a sub-isostatic annulus of thickened crust. In each case, the initial isostatic basin topography was iterated such that the final topography would match the observed topography of Orientale. For models a and b, the super-isostatic uplift of the basin floor was tuned to match the observed gravity anomaly for lithosphere


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