TSK 11 Göttingen 2006 Scherler et al. Structural record of an oblique impact: the central uplift of the Upheaval Dome impact structure, Utah, USA Vortrag Dirk Scherler1 Thomas Kenkmann2 Andreas Jahn2 Introduction Most asteroids strike their target at an oblique angle (Pierazzo & Melosh 2000). The common criterion for iden- tifying craters formed by an oblique im- pact is the pattern of the ejecta blan- ket. On Earth, however, ejecta blan- kets are rarely preserved and morpho- logical, structural, geophysical as well as depositional criteria were used to in- fer an oblique impact (e.g. for Chicx- ulub, Schultz & D’Hondt 1996, Ries- Steinheim, Stöffler et al. 2003, Mjöl- nir & Tsikalas 2005). However, the sig- nificance of such criteria in predicting impact angle or direction is a matter of debate (c.f. Schultz & Anderson, 1996, Ekholm & Melosh 2001). Par- ticularly, it is not yet known whether there is an influence of the impact an- gle on the displacement field during the collapse of large transient cavities, and thus, the final crater. For most impact angles, the shape of the final crater is controlled by its size. At a critical di- ameter (ca. 2–5 km on Earth), simple bowl shaped craters are getting gravi- tationally unstable and collapse to form complex craters, with a flat floor and a terraced rim (Melosh 1989). During collapse, the crater floor rises to form a 1 Institut für Geowissenschaften, Univer- sität Potsdam, Karl-Liebknecht-StraSSe 24/25, D-14476 Golm 2 Institut für Mineralogie, Museum für Naturkunde, Humboldt Univer- sität zu Berlin, InvalidenstraSSe 43, D-10115 Berlin central uplift, that may or may not be visible as a central peak, or, when the peak in turn collapses, as a peak ring at yet larger diameters. Results and Discussion We present structural details from the central uplift of the Upheaval Dome im- pact structure, in SE Utah, that are di- agnostic of the kinematics during crater collapse and central uplift formation. A characteristic imbrication of thrust slices towards the southeast (see Fig. 1), the pattern of strata orientation within the central uplift, dominant radial faults that accommodated NW–SE shortening and an elliptical bedding outline indi- cate, that the displacement field dur- ing crater collapse has not been axial symmetric. Instead, an additional lat- eral component, roughly towards the southeast, is preserved in the internal structure of the central uplift. The structural asymmetries are largest in the core of the central uplift and disappear outwards, thereby preserving the large- scale circular shape of the main struc- tural elements (rim monocline, ring syn- cline). We propose, that this lateral component reflects a shift in the on- set of crater collapse and the migration of the uplifting crater floor downrange (c.f. Kenkmann et al. 2005). Compar- ison with numerical models of oblique impacts supports this view (Shuvalov 2003, Shuvalov & Dypvik 2004) and fur- ther suggests, that the asymmetric dis- placement fades in the later stages of central uplift formation, which provides an explanation for the largely circular appearance of complex impact craters. Fault patterns, that are strikingly sim- ilar to that in the innermost part of Upheaval Dome, can be identified in other impact structures (Fig. 2) and 1 Scherler et al. TSK 11 Göttingen 2006 Figure 1: (A) Geological map of the central topographic depression of the Upheaval Dome impact structure. (B) Circular schematic cross section, trace as given in A. Variable thickness of the units mainly due to dip of strata out of the plane of intersection. may serve as general criteria for iden- tifying the impact direction of deeply- eroded impact structures. References Ekholm AG & Melosh HJ (2001) Crater fea- tures diagnostic of oblique impacts: The size and position of the central peak. Geoph Res Lett 28:623–626 Gault D & Wedekind JA (1978) Experimen- tal studies of oblique impacts. Proc Lunar Planet Sci Conf 9:3843–3875 Kenkmann T, Jahn A, Scherler D & Ivanov BA (2005) Structure and formation of a central uplift: A case study at the Upheaval Dome impact crater, Utah. Geol Soc Am Spec Pap 384:85–115 Melosh HJ (1989) Impact Cratering — A Geo- logic Process. New York: pp 145 Milton DJ, Glikson AY & Brett R (1996) Gosses Bluff — a latest Jurassic impact structure, central Australia. Part 1: Ge- ological structure, stratigraphy, and origin. AGSO Journal of Australian Geology & Geo- physics 16, 453–486 2 TSK 11 Göttingen 2006 Scherler et al. Figure 2: Simplified sketches of faults and contacts from the innermost part of the central uplifts of eroded complex craters in sedimentary target rocks: (A) Upheaval Dome, United States, D ca. 5.3km, (B) Spider, Australia, D ca. 12 km (after Shoemaker & Shoemaker 1996), (C) Gosses Bluff, Australia, D ca. 24 km (after Milton et al. 1996). Pierazzo E & Melosh HJ (2000) Understand- ing Oblique Impacts from Experiments, Ob- servations, and Modeling. Ann Rev Earth Planet Sci 28, 141–167 Schultz PH & Anderson RR (1996) Asymmetry of the Manson impact structure: Evidence for impact angle and direction, Geol Soc Am Spec Pap 302, 397–417 Schultz PH & D’Hondt S (1996) Cretaceous- Tertiary (Chicxulub) impact angle and its consequences. Geology 24, 963–967 Shoemaker EM & Shoemaker CS (1996) The proterozoic impact record of Australia. AGSO Journal of Australian Geology & Geo- physics 16:379-398. Shuvalov VV (2003) Cratering process after oblique impacts, Third International Confer- ence on Large Meteorite Impacts, Nördlin- gen, Contribution #4130 Shuvalov VV & Dypvik H (2004) Ejecta forma- tion and crater development of the Mjölnir impact, Meteoritics Planet Sci 39, 467–479 3 Scherler et al. TSK 11 Göttingen 2006 Stöffler D, Artemieva NA & Pierazzo E (2002) Modeling the Ries-Steinheim impact event and the moldavite strewn field, Meteoritics Planet Sci 37, 1893–1907 Tsikalas F (2005) Mjölnir crater as a result of oblique impact: Asymmetry evidence con- strains impact direction and angle. In: C Koeberl & H Henkel (eds) Impact Tectonics, Berlin, 285–306 4