Structural asymmetry in martian impact craters as an indicator for an impact trajectory

Impact crater formation is a highly dynamic and complex geological phenomenon. Methods of structural geology are capable of giving insights into deformation processes that occur during cratering. While most deformation observed in craters exhibits a generally radial symmetry, recent discoveries of non-radial structural elements in the central uplifts of terrestrial craters have led to the suggestion that obliquely impacting projectiles can cause non-radial deformation in the central subsurface area of the crater. The lack of ejecta blankets as accepted indicators of an oblique impact on Earth make it difficult to correlate the observed non-radial structures with the impact direction. Therefore, in this study three martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift. The central uplifts were structurally mapped using High Resolution Imaging Science Experiment (HiRISE) imagery and additional high-resolution digital terrain models (DTMs). Structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact: (1) The strike of upturned bedrock layers is on average perpendicular to the impact trajectory and (2) the majority of faults show a preferred trend parallel to the impact trajectory. A fourth martian crater was examined as a counterexample, which had a radial ejecta blanket and thus no indication of an oblique trajectory. Strike in this crater showed no preferred orientation. These results will help to solidify the suggested correlation between non-radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target. (C) 2012 Elsevier Inc. All rights reserved.