Meteoritics & Planetary Science 43, Nr 10, 1605–1628 (2008) Abstract available online at http://meteoritics.org Polygonal impact craters in Argyre region, Mars: Implications for geology and cratering mechanics T. ÖHMAN1, 2*, M. AITTOLA2, V.-P. KOSTAMA2, J. RAITALA2, and J. KORTENIEMI2, 3 1Department of Geosciences, Division of Geology, University of Oulu, P.O. Box 3000, FI-90014, Finland 2Department of Physical Sciences, Division of Astronomy, University of Oulu, P.O. Box 3000, FI-90014, Finland 3Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany *Corresponding author. E-mail:
[email protected] (Received 20 November 2007; revision accepted 15 April 2008) Abstract–Impact craters are not always circular; sometimes their rims are composed of several straight segments. Such polygonal impact craters (PICs) are controlled by pre-existing target structures, mainly faults or other similar planes of weakness. In the Argyre region, Mars, PICs comprise ∼17% of the total impact crater population (>7 km in diameter), and PICs are relatively more common in older geologic units. Their formation is mainly controlled by radial fractures induced by the Argyre and Ladon impact basins, and to a lesser extent by the basin-concentric fractures. Also basin-induced conjugate shear fractures may play a role. Unlike the PICs, ridges and graben in the Argyre region are mostly controlled by Tharsis-induced tectonism, with the ridges being concentric and graben radial to Tharsis. Therefore, the PICs primarily reflect an old impact basin-centered tectonic pattern, whereas Tharsis-centered tectonism responsible for the graben and the ridges has only minor influence on the PIC rim orientations.