• 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.