Modeling the Ries-Steinheim impact event and the formation of the moldavite strewn field

Abstract

Using detailed geological, petrographic, geochemical, and geographical constraints we have performed numerical modeling studies that relate the Steinheim crater (apparent diameter Da = 3.8 km), the Ries crater (Da = 24 km) in southern Germany, and the moldavite (tektite) strewn field in Bohemia and Moravia (Czech Republic), Lusatia (East Germany), and Lower Austria. The moldavite strewn field extends from ~200 to 450 km from the center of the Ries to the east-northeast forming a fan with an angle of ~57 degrees. An oblique impact of a binary asteroid from a west-southwest direction appears to explain the locations of the craters and the formation and distribution of the moldavites. The impactor must have been a binary asteroid with two widely separated components (some 1.5 and 0.15 km in diameter, respectively). We carried out a series of three-dimensional hydrocode simulations of a Ries-type impact. The results confirm previous results suggesting that impacts around 30-50 degrees (from the horizontal) are the most favorable angles for near-surface melting, and, consequently for the formation of tektites. Finally, modeling of the motion of impact-produced tektite particles through the atmosphere produces, in the downrange direction, a narrow-angle distribution of the moldavites tektites in a fan like field with an angle of ~75 degrees. An additional result of modeling the motion of melt inside and outside the crater is the preferred flow of melt from the main melt zone of the crystalline basement downrange towards the east-northeast rim. This explains perfectly the occurrence of coherent impact melt bodies (some tens of meters in size) in a restricted zone of the downrange rim of the Ries crater. The origin of these melt bodies, which represent chemically a mixture of crystalline basement rocks similar to the main melt mass contained (as melt particles < 0.5 m in size) in the suevite, do not occur at any other portion of the Ries crater rim and remained enigmatic until now. Although the calculated distribution of moldavites still deviates to some degree from the known distribution, our results represent an important step toward a better understanding of the origin and distribution of the high-velocity surface melts and the low-velocity, deep-seated melt resulting from an oblique impact on a stratified target.