Structural record of an oblique impact: the central uplift of the Upheaval Dome impact structure, Utah, USA


Most asteroids strike their target at an oblique angle (Pierazzo & Melosh 2000). The common criterion for identifying craters formed by an oblique impact is the pattern of the ejecta blanket. On Earth, however, ejecta blankets are rarely preserved and morphological, structural, geophysical as well as depositional criteria were used to infer an oblique impact (e.g. for Chicxulub, Schultz & D’Hondt 1996, Ries- Steinheim, Stöffler et al. 2003, Mjölnir & Tsikalas 2005). However, the significance of such criteria in predicting impact angle or direction is a matter of debate (c.f. Schultz & Anderson, 1996, Ekholm & Melosh 2001). Particularly, it is not yet known whether there is an influence of the impact angle 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 diameter (ca. 2–5 km on Earth), simple bowl shaped craters are getting gravitationally 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 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.
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