LUNAR ORIENTALE BASIN: NATURE OF IMPACT MELT AND VOLCANIC FLOODING FROM CHANDRAYAAN-1 (M3, TMC, HySI)

Introduction: Many impact basins on the Moon are filled with extensive mare basalts [1]. This obscures a) the primary structure of fresh basin interiors, including the nature and distribution of impact melts, and b) the nature of the early stages of the filling of basins with mare basalts. In contrast, the interior of the Orientale basin, the youngest and most well-preserved large basin on the Moon, has limited areas covered by mare basalt and the impact melt deposits and pristine ring structures are relatively well preserved [2-5]. In order to evaluate the evolution of selected areas in Orientale using relationships between mineralogy and morphology, we have undertaken a joint analysis of Chandrayaan-1 [6] TMC [7], HySI [8] and M 3 [9] data for a portion of the south-central Orientale basin (near 24S, 95W) where simultaneous data were acquired (Fig. 1). Initial results for the Moon Mineralogy Mapper (M3) analysis of a larger region of the Orientale basin are reported elsewhere [10]. Here we outline the M3 results for the area of joint study, and initial TMC/HySI results for this area are reported in a companion abstract [11]. Orientale Basin Background: Maunder Formation: This unit occurs within the Outer Rook Ring of the Orientale basin [1,2] and lies stratigraphically below the maria [1-3]. It is characterized by two facies [2]: 1) a corrugated and fractured facies that is draped over pre-existing topography, and 2) a smoother, bright plains deposit that occurs adjacent to and below the basin-filling mare deposits. The Maunder Formation has been interpreted as impact melt emplaced during the basin-forming event [1,2], a hypothesis consistent with multispectral image data [4] but as yet unconfirmed by direct measurement of mineralogical relations. The two facies of the Maunder Formation were interpreted to be clast-rich (corrugated) and clast-poor, more pure ponded impact melt (fractured light plains). The impact melt was interpreted to have coated the highly brecciated interior basin floor, and to have cooled and fractured immediately following the collapse of the transient cavity [2]. Cracks, fractures, and graben in the Maunder Formation are attributed to contraction cooling and draping on pre-existing topography of the impact melt facies [12]. Rough, bright, hilly, and mountainous topography in the Maunder Formation was interpreted to be underlying coherent basin floor debris protruding through the impact melt [2]. Orientale Mare Deposits: Mare deposits consist of a thin central deposit (Mare Orientale) and two arcuate occurrences at …