Structural evidence from shock metamorphism in simple and complex impact craters: Linking observations to theory

The structure of Canadian impact craters formed in crystalline rocks is analyzed using shock metamorphism and evidence for movement along shear zones. The analysis is based on an interpretation that, beyond the near field region, shock pressure attenuates down axis as P similar to R-2, in agreement with nuclear test and computed results, and as p similar to R-3 near the surface. In both simple and complex craters, the transient cavity is defined by the limit of fragmentation due to direct and reflected shock waves. The intersection of the transient cavity with hemispheric shock isobars indicates that the transient cavity has a parabolic form. Weakening by dilation during early uplift allows late stage slumping of the walls of simple craters. This is controlled by a spheroidal primary shear of radius r(s) approximate to 2d(t), where d(t) is the depth of the transient crater due to excavation and initial compression. With increasing crater diameter, the size of the transient cavity decreases relative to the shock imprint, suggesting that fragmentation and excavation is limited by progressively earlier collapse of the margins under gravity. Central peak formation in complex craters may be initiated by relaxation of the shock-compressed central parautochthone, so the primary shear, lubricated by friction melting, meets below the crater floor and drives the continuing upward motion.