Investigation of an Argyre basin ring structure using Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars

The Argyre basin, a large impact basin in the southern highlands of Mars, excavates early Noachian geologic units and thus is an ideal probe of the compositional layering of ancient highland crust. A ring structure defined by circumferential graben is believed to represent the outermost topographic scarp of the basin. Two distinct layers in the fluvially dissected highland plains unit Npld have been exposed by the ring graben in northwest Argyre: a lower unit enriched in high-calcium pyroxene and an upper unit showing laterally extensive occurrences of phyllosilicates, including chlorite and iron-magnesium smectite. These phyllosilicates closely follow the unit contact between unit Npld and Hpl(3) in this region; the Hesperian-aged Hpl(3) caprock directly covers unit Npld. Two nearby knobs also show phyllosilicate-bearing materials over a high-Ca pyroxene-enriched unit, although the knobs are located in regions mapped as Npl(1) and Nplh. From these observations, we suggest that an extensive layer of phyllosilicates underlying a layer of high-Ca pyroxene-bearing rock existed in the region prior to the Argyre impact. These materials were thrown up by the impact event and deposited as an inverted flap. This inverted flap of ejecta was then cut by the circumferential graben that formed as the transient Argyre crater collapsed, exposing the materials.