CALCITE TWINNING AROUND THE NÖRDLINGEN RIES IMPACT CRATER (25KM DIAMETER)

The 14.7 Ma Ries crater of southern Germany is located 120 km north of the alpine thrust front. The impacting body penetrated to an estimated depth of 1.4 km into crystalline basement across 500 m of cover rocks. The final crater has a outer diameter of 25 km within Upper Jurassic limestones and an inner ring of 11 km made of uplifted metamorphica and granites. An ejecta blanket is well preserved around the southern half of the crater. A chaotic melange of heavily fractured sedimentary rocks are found in a lower unit of "Bunte Breckzie" topped by a second layer of "Suevite" breccias, composed mainly of partially molten rocks of crystalline origin including small angular fragments of sedimentary rocks. Shock metamorphism is well documented within these ejecta layers through the presence of ultra-high pressure minerals diamond and coesite, shock lamellae in quartz, diaplectic glasses, rare shatter cones and fractured belemnites.

We examined sparry calcites for twinning in flat lying, macroscopically undeformed carbonate country rocks at distances of 25 to 50 km from the center of impact. Twinning strain was measured using the technique of Groshong (1972) and the twin's morphology was examined in ultra-thin sections and in SEM. Shortening directions are found to be layer-parallel with two distinct azimuths. A set of NNW-SSE oriented shortening directions, perpendicular to the Alpine front, is interpreted as belonging to the Neogene Western European stress field. Another set of twinning strain axes, found in samples taken from the eastern and western side of the crater, indicate a radial pattern of E-W oriented shortening, strongly suggesting a causal relationship with the impact. Very high twin intensities of up to 300 twins/mm, an abundance of double and triple twin sets and abnormally high twin widths (up to 2 ?m) are observed in rare isolated grains. Similar observations were made around Steinheim at 40 km to the WSW, where a much smaller "twin crater" of identical age, with a diameter of 3 km is present.

Coarse grained calcite within the stiff Upper Jurassic limestones around Nördlingen and Steinheim is sensitive to impact shock metamorphism at distances of up to two crater diameters from the center of impact, much further than shock induced macroscopic fractures.