Tremblements de terre et failles de l'astroblème de Charlevoix (Québec)

Abstract

This field guide was first written for the meeting of the Eastern Section of the Seismological Society of America (ES-SSA), held in La Malbaie in 2013. It has been slightly modified, mostly with new figures. The Charlevoix Seismic Zone is the locus of the highest seismic hazard in continental eastern Canada. At the heart of this zone is the ~54 km diameter Charlevoix impact structure. This structure, located less than 125 km east of Quebec City, is one of the most accessible large meteorite impact structures in eastern North America. The Charlevoix impact structure is singled out as it overprints Iapetus rift faults and the Logan's Line marking the edge of the Appalachian Orogen. The Charlevoix impact structure gives the region its singular landscape. The ~5 km wide peripheral ring trough forms a prominent open valley extending from St. Lawrence River (sea level) to a threshold at ~250 m altitude. The highest point in the valley is nearly 850 m below the ~1100 m mean elevation of the external Laurentian plateau. The highest point is also 550 m below the central uplift, 'Mont-des-Éboulements,' which stands 780 m above sea level. The overall morphology of the Charlevoix impact structure matches that of a complex impact crater. Shatter cones, mylolisthenite injections and shock-induced planar deformation microstructures in quartz and feldspar are widespread providing compelling evidence for the extent of shock metamorphism. The age of the impact is poorly constrained. Recently acquired 40Ar/39Ar and U-Pb data from impact melt rock and pseudotachylite give a late-Ordovician age, which appears to be in better agreement with field relationships than the previously reported K-Ar Devonian-age.