• Constraints on central uplift structure from the Manicouagan impact crater

      Spray, J. G.; Thompson, L. M. (The Meteoritical Society, 2008-01-01)
      Recent drilling operations at the 90 km diameter, late Triassic Manicouagan impact crater of Quebec, Canada, have provided new insight into the internal structure of a complex craters central region. Previous work had indicated that the impact event generated a ~55 km diameter sheet of molten rock of relatively consistent (originally ~400 m) thickness (Floran et al. 1978). The drilling data reveals melt sheet thicknesses of up to ~1500 m, with kilometer-scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet. The thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe-shaped uplift of Precambrian basement. The uplift constitutes a modified central peak structure, at least part of which breached the melt sheet. Mineralogical and compositional segregation (differentiation) of the thicker melt sheet section, coupled with a lack of fractionation in the thinner units, shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification. Marked lateral changes in sub-melt sheet (basement) relief support the existence of a castellated footwall that was created by high-angle, impact-related offsets of 100s to 1000s of meters. This indicates that deformation during the modification stage of the cratering process was primarily facilitated by large-displacement fault systems. This work suggests that Manicouagan is a central peak basin with rings, which does not appear to fit with current complex crater classification schemes.
    • Mid-sized complex crater formation in mixed crystalline-sedimentary targets: Insight from modeling and observation

      Collins, G. S.; Kenkmann, T.; Osinski, G. R.; Wünnemann, K. (The Meteoritical Society, 2008-01-01)
      Large impact crater formation is an important geologic process that is not fully understood. The current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets. Real targets are rarely uniform; for example, the majority of Earths surface is covered by sedimentary rocks and/or a water layer. The ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process. To advance understanding of the mechanics of crater collapse, and the effect of variations in target properties on crater formation, the first Bridging the Gap workshop recommended that geological observation and numerical modeling focussed on mid-sized (15-30 km diameter) craters on Earth. These are large enough to be complex; small enough to be mapped, surveyed and modelled at high resolution; and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables. In this paper, we compare observations and numerical models of three 18-26 km diameter craters formed in different target lithology: Ries, Germany; Haughton, Canada; and El'gygytgyn, Russia. Based on the first-order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid-sized complex crater formation in a subaerial, mixed crystalline-sedimentary target. We compared our results with interpreted geological profiles of Ries and Haughton, based on detailed new and published geological mapping and published geophysical surveys. Our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre-impact sedimentary cover in each case. We conclude that the presence of an inner ring at Ries, and not at Haughton, is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post-impact erosion. For constant impact energy, transient and final crater diameters increase with increasing sediment thickness.