The Charlevoix impact structure and seismic zone





The Charlevoix impact structure and seismic zone

Léopold Nadeau (Geological Survey of Canada (retired)) and Gordon Osinski (University of Western Ontario)

Leaders and affiliations : Léopold Nadeau (Geological Survey of Canada (retired)), Gordon Osinski (University of Western Ontario)

Contact : [email protected]

Description : Located less than 125 km east of Quebec City, the Charlevoix impact structure is among the largest, ~54 km in diameter, and the most accessible meteorite impact structure in eastern North America. The impact structure, at the heart of the Charlevoix Seismic Zone, overprints Iapetus rift faults and Logan’s Line, which marks the edge of the Appalachian Orogen. The Charlevoix Seismic Zone is the locus of the highest seismic hazard in continental eastern Canada.

The Charlevoix impact structure gives the region its singular landscape. The ~40 km diameter peripheral ring trough forms a prominent open valley that locally reaches an altitude of 250 m. The highest point in the valley is nearly 850 metres below the ~1100 m mean elevation of the external Laurentian plateau. The highest point is also 550 metres below the central uplift, “Mont-des-Éboulements,” which stands at 780 m above sea level. The overall morphology of the Charlevoix impact structure matches that of complex impact craters. Shatter cones, injections breccias and shock-related planar deformation microstructures in quartz and feldspar crystals are widespread and provide compelling evidence for the extent of shock metamorphism. The age of the impact is poorly constrained. Recently acquired Ar/Ar data from impact melt rock and pseudotachylite gives a Late Ordovician age, which is in better agreement with field relationships than the previously reported Devonian K-Ar age.

Based on historical and current earthquake rates, the Charlevoix Seismic Zone is a region of high seismic hazard. Since the arrivals of the first Europeans in the early 1600s, it has been subject to five earthquakes of magnitude 6 or larger: in 1663 (M~7); 1791 (M ~6); 1860 (M ~6); 1870 (M ~6 ½); and 1925 (magnitude MS 6.2 ± 0.3). Recently, the magnitude of the 1663 earthquake was reviewed and estimated to be as large as M 7.2 to 7.9.

The field trip provides an opportunity to enjoy panoramic views of the peripheral trough and ring structure and of the central uplift, and to visit key outcrops featuring shock-related features including shatter cones, impact breccias, and related fault zones. The field trip allows revisiting the region’s long fault reactivation history, dating back to Iapetus Ocean rifting. It also includes stops at outcrops of the St. Lawrence Platform Cambro-Ordovician sedimentary cover, with inferences on the structural relationships with Logan’s Line. The area is also known for its landslide sensitivity; one of the stops features a major landslide caused by the 1663 earthquake.

Number of participants (Min/Max) : to be determined

Duration : 1 day

Date : Pre-Congress

Associated special session / short course :  to be determined

Sponsor : GAC Planetary Science Division