ASTROBLEME FORMATION AFTER THE MOVEMENT OF ROCK MASSES: CHARLEVOIX ET LE RIES

Abstract

One can determine the movement of rock masses in the formation of an astrobleme by ascertaining the displacements of stratigraphic reference levels before and during the impact. A discontinuity in the zone of shock metamorphism in the Charlevoix (Quebec, Canada) structure corresponds with a major fault separating an irregularly uplifted crown area from a depressed inner zone. The displacement along this circular fault is inverse to the normal displacement of the layers of this structure, that is to say, a broad collapsed area of terraces and grabens on the outside, and a gradual central build-up in the inner zone. This phenomenon can be explained as a consequence of the formation of a crater of excavation through impact, where a force of expansion tends to raise the edges of the crater, as insertion continues to the bottom of the crater. An essentially similar model for small and large craters is inferred in the first phase of formation. The readjustment that follows depends on the size of the crater and the material impacted. Started by decompression, the readjustment tends to continue until balance occurs along the fracture slip surfaces that initially formed on impact. The fractures are filled by impact fault breccias, particularly by mylolisthenites. This readjustment also depends on the speed of the meteorite at impact, as suggested by the central prominences found in the Charlevoix and the bassin form for the Ries (Germany). © 2007 CNRC Canada.