The subsurface structure of oblique impact craters Dissertation vorgelegt von Dipl.-Geol. Michael H. Poelchau vom Fachbereich Geowissenschaften der Freien Universität Berlin zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) Berlin, 2010 The subsurface structure of oblique impact craters Dissertation vorgelegt von Dipl.-Geol. Michael H. Poelchau vom Fachbereich Geowissenschaften der Freien Universität Berlin zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) Berlin, 2010 Gutachter: 1. PD Dr. Thomas Kenkmann 2. Prof. Wolf-Uwe Reimold Tag der Disputation: 23.02.2010 Statement regarding the contributions of the author and others to this thesis This thesis is comprised of three published, peer-reviewed articles and one submitted manuscript, which each form separate chapters within this thesis. The chapters “Introduction” and “General Conclusions” were written especially for this thesis. The PhD candidate is the first author of two of these articles, and the second author of the third article. The PhD candidate is also the first author of a manuscript currently submitted to Earth and Planetary Science Letters. Therefore, these four chapters have their own introduction, methodology, discussion, conclusions and references. The articles and manuscripts used in this thesis are the following: Poelchau, M. H., and T. Kenkmann, 2008. Asymmetric signatures in simple craters as an indicator for an oblique impact direction, Meteoritcal and Planetary Science, 43, 2059-2072. Poelchau M. H., Kenkmann T. and Kring D. A., 2009. Rim uplift and crater shape in Meteor Crater: the effects of target heterogeneities and trajectory obliquity. Journal of Geophysical Research, 114, E01006, doi:10.1029/2008JE003235. Kenkmann, T. and Poelchau, M. H., 2009. Low-angle collision with Earth: The elliptical impact crater Matt Wilson, Northern Territory, Australia. Geology 37, 459-462. Poelchau, M.H., and T. Kenkmann, 2010. Feather Features: A Low Shock Pressure Indicator in Quartz. Submitted to Earth and Planetary Science Letters. Hans Knöfler and Ulli Raschke (MfN Berlin) prepared the thin sections used in this thesis. Sebastian Quart and Dirk Scherler (formerly MfN Berlin) acquired samples and data points from Wolfe Creek Crater, Australia. David A. Kring (LPI Houston) assisted in field work in Meteor Crater, Arizona. Alex Kurta (MfN Berlin) assisted in the generation of the map of Martin Crater, Mars. Thomas Kenkmann (MfN Berlin) created the map of Matt Wilson crater, Australia. All field work was performed by the candidate under the supervision and assistance of his supervisor, Thomas Kenkmann. All other maps, diagrams, microscope and u-stage measurements were generated by the candidate unless noted otherwise. This thesis was funded by the DFG, project KE 732/11-1. SUMMARY ........................................................................................................................................................... 3 1. INTRODUCTION............................................................................................................................................. 7 THE IMPORTANCE OF IMPACT CRATERS............................................................................................................... 7 THE IMPACT CRATERING PROCESS....................................................................................................................... 8 OBLIQUE IMPACTS ............................................................................................................................................ 10 REFERENCES ..................................................................................................................................................... 12 2. ASYMMETRIC SIGNATURES IN SIMPLE CRATERS AS AN INDICATOR FOR AN OBLIQUE IMPACT DIRECTION ...................................................................................................................................... 15 ABSTRACT ........................................................................................................................................................ 15 INTRODUCTION ................................................................................................................................................. 15 PROPOSED “TWO CORNERS” PATTERN............................................................................................................... 19 GEOLOGICAL SETTING OF WOLFE CREEK CRATER ........................................................................................... 22 METHODOLOGY ................................................................................................................................................ 23 FURTHER ANALYTICAL METHODS ..................................................................................................................... 25 RESULTS ........................................................................................................................................................... 28 Tooting crater.............................................................................................................................................. 29 Wolfe Creek Ejecta...................................................................................................................................... 29 Wolfe Creek Autochthonous Rim................................................................................................................. 30 DISCUSSION ...................................................................................................................................................... 30 CONCLUSIONS................................................................................................................................................... 34 ACKNOWLEDGEMENTS...................................................................................................................................... 34 LITERATURE...................................................................................................................................................... 35 3. RIM UPLIFT AND CRATER SHAPE IN METEOR CRATER: THE EFFECTS OF TARGET HETEROGENEITIES AND TRAJECTORY OBLIQUITY.......................................................................... 39 ABSTRACT ........................................................................................................................................................ 39 INTRODUCTION ................................................................................................................................................. 39 Meteor Crater.............................................................................................................................................. 41 Previous structural research in Meteor Crater ........................................................................................... 41 CAN THE IMPACT VECTOR BE DERIVED IN SIMPLE CRATERS? - A WORKING HYPOTHESIS.................................. 42 METHODS.......................................................................................................................................................... 44 1. Measurement of bedding planes .............................................................................................................. 44 2. Conversion from a geographic to an azimuthal reference scheme – the concept of “concentric deviation”.................................................................................................................................................... 46 3. Statistical treatment – the “overlapping bins” method ........................................................................... 46 RESULTS I: STRATA ORIENTATION, JOINTS, FAULTS AND UPLIFT IN METEOR CRATER ...................................... 47 1. Bedding data............................................................................................................................................ 48 2. Concentric deviation ............................................................................................................................... 49 3. Joints and radial faults in the crater corners .......................................................................................... 49 4. Interthrust wedges ................................................................................................................................... 50 5. Differential uplift..................................................................................................................................... 52 RESULTS II: DATA RELEVANT TO DETERMINING AN IMPACT DIRECTION ........................................................... 54 1. Deviation of strike from concentric orientation ...................................................................................... 54 2. Bedding dip and differential uplift........................................................................................................... 55 3. Canyon Diablo Meteorite distribution..................................................................................................... 55 4. Ejecta blanket .......................................................................................................................................... 56 5. Discussion of impact direction ................................................................................................................ 56 SYNTHESIS