MARTIAN PARENT CRATERS FOR THE SNC METEORITES

The young ages (approximately 1.3 Ga) and the basaltic to ultramafic compositions of the shergottites, nakhlites, and chassignites meteorites severely restrict their potential source regions on Mars. We have used this age and compositional information, together with geologic data derived from Viking Orbiter images, to identify 25 candidate impact craters in the Tharsis region of Mars that could be the source crater for these meteorites. None of these craters are close to the size (approximately 100 km diameter) implied by the dynamical study of SNC ejection developed by Vickery and Melosh (1987). The craters in our study were selected because they are >10 km in diameter, have morphologies indicative of young craters, and satisfy both the petrologic criteria of the SNCs and the proposed 1.3 Ga crystallization ages. Of these 25 craters, only nine are found on geologic units believed to be young (crater density is less than 570 craters of greater than 1 km diameter per 10(6) km2). No crater exists to satisfy well the criteria of sampling both a 1.3 Ga surface (nakhlites and Chassigny) and a 180 Ma surface (shergottites) without at the same time imposing significant constraints on the chronology of Mars as inferred from the cumulative crater curves. The relatively young age (based on their inferred position in the stratigraphic column of Tharsis (Scott et al., 1981)) of the SNCs implies that volcanic activity on the plains of the Tharsis region extended well past 1.3 Ga.