LUNAR SINUOUS RILLES ORIGINATING IN HIGHLAND SETTINGS: REVELATIONS FROM LRO

Several prominent lunar rilles appear to originate in non-mare environments such as highland rises and plateaus including Vallis Schröteri, Rimae Plato III, and Vallis Alpes. Though sinuous rille formation has been attributed to thermal erosion of the substrate by turbulent flow of low-viscosity lava, for those originating in highland sites, the influence of the different substrate material, pre-existing topography, and structural controls on the emplacement and morphology must be considered. Here, we focus on the morphology of three rilles using high resolution images (0.5 m/pixel) from the Lunar Reconnaissance Orbiter Camera (LROC) and topographic data from the Lunar Orbiter Laser Altimeter (LOLA). Vallis Schröteri originates on Aristarchus Plateau, a topographic high comprised of a complex stratigraphy of units interpreted to be anorthositic and/or granitic, basaltic, pyroclastic, and impact. The path of the 155-km long primary rille (<10 km across) is influenced by pre-existing topography. Apparent layering in outcrops observed in LROC images in both upper wall of the primary rille and the inner rille are potentially related to lava flows. Volcanic mounds on the Cobra Head and a flow margin at the end of the primary rille are also observed. The 140-km long Rimae Plato III originates along a fault scarp near the crater rim and has a broad primary rille (~3 km across) with a flat, lava-filled floor and inner rille (~500 m across) that does not exhibit tight meanders. The walls of the primary rille grade out towards the base of the ejecta blanket, influenced by the thickness of the deposit. LROC images show enigmatic low albedo material associated with higher albedo rock exposures along the upper wall of the primary rille and potentially layered units in the inner rille wall related to the lava flows. LROC also provides details of a potential overflow deposit from the inner rille that occurs in Mare Imbrium. Vallis Alpes is a 150-km long rille confined down the center of a narrow, lava-filled valley that stretches from the rim of the Imbrium Basin to Mare Frigoris. No distinct vent is visible, but LROC images show a remnant lava pond, breached dam, and an island confined between mountainous terrain. LRO data are being used to constrain dimensions, morphology, and geologic context to interpret the emplacement of these unique rilles.