Two Holocene impact craters at Emmerting, Germany: deformation, fracturing, and their relationships to the melting and decarbonization

In two craters near Emmerting, three major processes which variably affected the original pebbles are documented in the following order: 1. Deposition of hot material which solidified to glass (usually thin and transparent) or reacted with carbonate to form expanded “pumice” on the surface of pebbles. 2. Ductile deformation of variable intensity (with limited fragile deformation but intense fracturing of mineral grains), using older as well as newly formed discontinuities; in some cases this deformation is consistent with extreme strain, rendering a human-induced origin highly improbable. The largely ductile character of deformation points to a high temperature, but it was not necessarily accompanied by melting. 3. Solidification of melts generated within pebbles or derived from secondary projectiles. These disequilibrium melts were hot enough to have very low viscosity (in some cases, they may have also been injected by high pressure/strain, or sucked in), which enabled them to fill even thin fractures in individual mineral grains; gas expansion also formed extrusions resembling miniature volcanic features on the surface of some pebbles. In some zircon grains baddeleyite was observed, probably formed by shock metamorphism. However, no additional evidence was found to suggest pressures exceeding the threshold typically required for shock-induced melting (~8 Gpa or more). Nevertheless, the energy transformed during repeated mutual collisions may have heated the interior of pebbles sufficiently. Origin of the depression at Grabenstätt-Kaltenbach is unclear, the disequilibrium melting and decarbonization may also be explained by anthropogenic processes.