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Structural record of an central uplift, that may or may not be oblique impact: the central visible as a central peak, or, when the uplift of the Upheaval Dome peak in turn collapses, as a peak ring at impact structure, Utah, USA yet larger diameters. Vortrag Results and Discussion Dirk Scherler1 Thomas Kenkmann2 We present structural details from the Andreas Jahn2 central uplift of the Upheaval Dome im- pact structure, in SE Utah, that are di- Introduction agnostic of the kinematics during crater Most asteroids strike their target at collapse and central uplift formation. an oblique angle (Pierazzo & Melosh A characteristic imbrication of thrust 2000). The common criterion for iden- slices towards the southeast (see Fig. 1), tifying craters formed by an oblique im- the pattern of strata orientation within pact is the pattern of the ejecta blan- the central uplift, dominant radial faults ket. On Earth, however, ejecta blan- that accommodated NW–SE shortening kets are rarely preserved and morpho- and an elliptical bedding outline indi- logical, structural, geophysical as well cate, that the displacement field dur- as depositional criteria were used to in- ing crater collapse has not been axial fer an oblique impact (e.g. for Chicx- symmetric. Instead, an additional lat- ulub, Schultz & D’Hondt 1996, Ries- eral component, roughly towards the Steinheim, Stöffler et al. 2003, Mjöl- southeast, is preserved in the internal nir & Tsikalas 2005). However, the sig- structure of the central uplift. The nificance of such criteria in predicting structural asymmetries are largest in the impact angle or direction is a matter core of the central uplift and disappear of debate (c.f. Schultz & Anderson, outwards, thereby preserving the large- 1996, Ekholm & Melosh 2001). Par- scale circular shape of the main struc- ticularly, it is not yet known whether tural elements (rim monocline, ring syn- there is an influence of the impact an- cline). We propose, that this lateral gle on the displacement field during the component reflects a shift in the on- collapse of large transient cavities, and set of crater collapse and the migration thus, the final crater. For most impact of the uplifting crater floor downrange angles, the shape of the final crater is (c.f. Kenkmann et al. 2005). Compar- controlled by its size. At a critical di- ison with numerical models of oblique ameter (ca. 2–5 km on Earth), simple impacts supports this view (Shuvalov bowl shaped craters are getting gravi- 2003, Shuvalov & Dypvik 2004) and fur- tationally unstable and collapse to form ther suggests, that the asymmetric dis- complex craters, with a flat floor and placement fades in the later stages of a terraced rim (Melosh 1989). During central uplift formation, which provides collapse, the crater floor rises to form a an explanation for the largely circular appearance of complex impact craters. 1 Institut für Geowissenschaften, Univer- sität Potsdam, Karl-Liebknecht-Straße 24/25, Fault patterns, that are strikingly sim- D-14476 Golm 2 Institut für Mineralogie, ilar to that in the innermost part of Museum für Naturkunde, Humboldt Univer- Upheaval Dome, can be identified in sität zu Berlin, Invalidenstraße 43, D-10115 other impact structures (Fig. 2) and Berlin

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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- Kenkmann T, Jahn A, Scherler D & Ivanov BA tifying the impact direction of deeply- (2005) Structure and formation of a central eroded impact structures. uplift: A case study at the Upheaval Dome impact crater, Utah. Geol Soc Am Spec Pap 384:85–115 References Melosh HJ (1989) Impact Cratering — A Geo- Ekholm AG & Melosh HJ (2001) Crater fea- logic Process. New York: pp 145 tures diagnostic of oblique impacts: The size Milton DJ, Glikson AY & Brett R (1996) and position of the central peak. Geoph Res Gosses Bluff — a latest Jurassic impact Lett 28:623–626 structure, central Australia. Part 1: Ge- Gault D & Wedekind JA (1978) Experimen- ological structure, stratigraphy, and origin. tal studies of oblique impacts. Proc Lunar AGSO Journal of Australian Geology & Geo- Planet Sci Conf 9:3843–3875 physics 16, 453–486

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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. 5.3 km, (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- Shoemaker EM & Shoemaker CS (1996) ing Oblique Impacts from Experiments, Ob- The proterozoic impact record of Australia. servations, and Modeling. Ann Rev Earth AGSO Journal of Australian Geology & Geo- Planet Sci 28, 141–167 physics 16:379-398. Schultz PH & Anderson RR (1996) Asymmetry Shuvalov VV (2003) Cratering process after of the Manson impact structure: Evidence oblique impacts, Third International Confer- for impact angle and direction, Geol Soc Am ence on Large Meteorite Impacts, Nördlin- Spec Pap 302, 397–417 gen, Contribution #4130 Schultz PH & D’Hondt S (1996) Cretaceous- Shuvalov VV & Dypvik H (2004) Ejecta forma- Tertiary (Chicxulub) impact angle and its tion and crater development of the Mjölnir consequences. Geology 24, 963–967 impact, Meteoritics Planet Sci 39, 467–479

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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

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