Crustal studies in the La Malbaie Region, Quebec

Abstract

The results of three seismological investigations of the crust in the seismically active La Malbaie Region are reported. First arrivals from a reversed reflection–refraction profile within the Charlevoix structure indicate a uniform Pg velocity of 6.08 ± 0.04 km/s. Secondary arrivals forming discontinuous, sometimes arcuate segments on the record sections are interpreted as subcritical reflections from structural contortions related to impact of the Charlevoix meteorite. A coherent event at 14 s may signal reflection from the M-discontinuity near 45 km depth.Analysis of travel times from calibration shots recorded by a network of stations spanning the St. Lawrence River in terms of time–distance profiles, travel time residuals, and numerical models provides excellent support for a model of the Precambrian–Paleozoic contact striking along the north shore and dipping to the southeast about 20° beneath the wedge of 5.5 km/s sediments.Unreversed profiles obtained by recording timed Thetford Mines blasts across a similar network suggests a comparable deep structure beneath the north and south shores, with average crustal velocities of 6.8 and 6.7 km/s, respectively. A minor mid-crustal discontinuity is suggested, and Moho depth is estimated at 42–43 km.The 6.2 km/s upper crustal velocity found for the crater and immediately surrounding area is anomalously low compared to the 6.4–6.5 km/s reported for the eastern shield region. Structural lineations gleaned from satellite imagery suggest an elongated area of impact-related disruption much larger than previously recognized. This expanded area of crustal weakening encompasses and may partially explain the zone of continuing microseismic activity, as well as the low upper crustal velocity of the La Malbaie Region.