Burial, Exhumation, Metamorphism and Other Dastardly Deeds Exposed at the Hesperian/Noachian Boundary in the Southern Nili Fossae Region
Introduction: The best place on Mars to examine the boundary between Noachian and Hesperian surfaces is at the southern boundary of the Nili Fossae region. Here younger Hesperian Syrtis Major lavas have been emplaced conformably over the older Noachian terrain. In this region this important boundary in the geological history of Mars is relatively free of young aeolian deposits, exposing dramatic mineralogical and physical distinctions between mafic-toultramafic Syrtis lavas and the phyllosilicate-rich Nili Fossae terrain that document the distinction between an earlier "wet" Mars and the drier Mars that followed. The richness of secondary minerals exhibited by this region of Nili Fossae has been well-documented and is under increasing observation [e.g. 1]. What the exposed Nili surface represents in geological terms is less well understood; is it a relatively unmodified surface reflecting Noachian processes, or has it been significantly altered by later interactions with Syrtis lavas? To address this question we have conducted a preliminary survey of the Hesperian /Noachian border between 70° and 80° E longitude and 15° and 20° latitude. Our results suggest that Syrtis lavas buried a significant portion of southeast Nili Fossae and that these flows were later removed, providing interesting implications for what the local mineralogy and geomorphology actually records. Syrtis Lavas: Syrtis Major Planum is one of several 1000-km-scale volcanic constructs in the circumHellas region of Mars [e.g. 2]. At the border with Nili Fossae individual flows are long, thick and low-slope, with prominent straight and unroofed channels, suggesting low viscosity, high effusivity and ultramafic to mafic compositions [e.g. 3]. The top surfaces of flows show scarce evidence of fluvial erosion and flow margins are generally rounded with modest landsliding and talus development. Burial: One flow in particular (hereafter called I80) offers significant insight into the relationship between Syrtis flows and the pre-existing eastern Nili surface (Fig 1). I-80 originates within a broad lava plain that fed flows into the largest of the Nili "Fos-