A LATE TRIASSIC AGE FOR THE ROCHECHOUART IMPACT STRUCTURE, FRANCE

Argon-40/Argon-39 laser spot fusion dating of pseudotachylyte from the similar to 25 km diameter Roche-chouart impact structure of western-c entral France yields a matrix age of 214 +/- 8 Ma (2 sigma). Field evi dence indicates that the pseudotachylyte was generated during the modi fication stage of the impact process, probably during transient cavity collapse. This new age is considerably older than the previously acce pted age of 186 +/- 8 Ma for this structure, which was obtained from h ydrothermally-altered melt sheet samples. The new age is in accordance with earlier paleomagnetic and fission track data, which indicated th at Rochechouart was formed during the late Triassic. Moreover, the new determination is in agreement with the regional geological setting an d field relations of the structure. The new age of 214 +/- 8 Ma falls within the Norian stage of the Triassic system.