SHOCK EFFECTS IN TARGET ROCKS FROM THE CHARLEVOIX IMPACT STRUCTURE, QUEBEC, CANADA

The shock effects generated in target rocks are sensitive to the loading and relaxation conditions during hypervelocity impact and can therefore provide important information on complex cratering processes.

The late Devonian Charlevoix impact structure is located 105 km north of Quebec city (47°32’N; 70°18’W) along the north shore of the St. Lawrence River. The topographic expression of the ~55 km diameter structure is well defined. The target lithologies comprise Grenvillian charnockites and granites as well as Ordovician sedimentary rocks. The Charlevoix region is affected by intense tectonic faulting, pre- and post-dating the impact event.

The shock effects in the crystalline target rocks comprise planar fractures (PFs) and planar deformation features (PDFs) in quartz. Many grains with a high PDF density exhibit a characteristic browning, or "toasting", which represents a post-shock alteration. The abundance of specific PDF sets in quartz is characteristic of the shock pressure. However, at Charlevoix a highly heterogeneous stress field is indicated by a strongly variable PDF density present in grains within single samples and within individual grains. The PDFs are commonly concentrated around inclusions and along grain boundaries, probably due to the contrasting (de)compressibility of the adjacent phases.

PDFs in feldspar at Charlevoix occur mostly parallel to (310), (100), (24-1) and (241) planes. Pyroxenes exhibit mechanical twins, though these may represent high strain rate deformation in shear zones during tectonic faulting. This may also hold true for the highly heterogeneous plastic deformation of quartz recorded in some samples.

Shatter cones are well developed in the Ordovician limestone, and also in the charnockites. White-greyish, vesicular Ca-rich zones occur in the charnockites. As Ca-rich phases are rare in the charnockites, this Ca-metasomatism may be due to the mobilisation of Ca from the Ordovician sedimentary cover rocks as a result of the impact.