Shock metamorphism as a fundamental process in the evolution of planetary bodies: Information from meteorites

Shock effects in meteorites comprise two major phenomena: (1) Shock metamorphism defined as the mechanical deformation and transformation of rocks below or above the solidus by shock compression and (2) breccia formation which involves ballistic or non-ballistic transport and the relative movement of rock fragments and melts by displacement from the primary location in the parent target bodies. Various collision scenarios lead to specific combinations of shock metamorphism and breccia formation if the relative sizes and velocities of the colliding bodies and the specific impact energy are freely variable above a certain threshold value of the impact velocity. In the low velocity regime accretionary breccias can be formed by catastrophic disruption and reaccretion of the fragmented bodies. These breccias may lack distinct shock-induced features of their constituents. In the impact cratering regime (impact velocities >0.5 to 1 km/s) shocked rocks and impact melts are formed and incorporated into crater deposits or in the crater basement of asteroidal surface-subsurface units in which various types of impact breccias can be recognized: monomict breccias and polymict breccias such as regolith breccias, fragmental breccias, impact melt breccias and granulitic breccias