• 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.
    • The effect of target lithology on the products of impact melting

      Osinski, G. R.; Grieve, R. A. F.; Collins, G. S.; Marion, C.; Sylvester, P. (The Meteoritical Society, 2008-01-01)
      Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or sheets are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impactgenerated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater-fill impactites) for impacts into sedimentary-bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.