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St. John's 2001
Technical Programme

SS16:  The Architecture, Origin and Evolution of the Paleozoic Continental Margin of Laurentia 
Organizers / Organisateurs:  Dennis Lavoie
Room / Salle:  A1046

Date:  29/05/2001
Time:  3:00 PM
Presenter:  Yvon Lemieux


Geometry and origin of supracrustal faults of the Charlevoix area, Quebec

LEMIEUX, Y., ylemieux@nrcan.gc.ca, TREMBLAY, A., INRS-Géoressources, P.O. Box 7500, Quebec, QC, G1V 4C7, and LAVOIE, D., Geological Survey of Canada, P.O. Box 7500, Quebec, QC, G1V 4C7

The Charlevoix area, which is host to an impact crater of inferred Devonian age (ca. 350 Ma), is located 100 km NE of Quebec City.  Many structural and microstructural features supporting impact cratering (i.e. shattercones, shocked quartz and feldspar grains) are found in the region.  The area also features well-developed supracrustal fault systems that have been historically attributed to the impact event.  Few studies have been focused, however, on the establishment of genetic relationships between the various sets of supracrustal faults of the Charlevoix area in regards to its tectonic evolution since the Lower Paleozoic.

The supracrustal faults in the study area mostly consist of normal faults that are commonly marked by cataclastic and gouge breccias and pseudotachylyte.  They vary from a few centimetres to a few metres in thickness, and crosscut both the Precambrian crystalline basement and the Cambrian-Ordovician sedimentary sequence of the St.Lawrence Platform.  Two major fault systems striking NE-SW and NW-SE are found in the Charlevoix area.  These fault systems show mutual crosscutting relationships and are, respectively, subparallel to the St.Lawrence Valley rift fault systems and to the Saguenay and Ottawa-Bonnechère grabens.  Detailled mapping of these supracrustal faults both within and outside the limit of the Charlevoix impact crater suggests that brittle faulting occurred before and after impact cratering.  Syn-sedimentary faults and breccias, locally found in Ordovician strata of the St. Lawrence Lowlands are consistent with a pre-impact origin.  Polymictic fault breccias, occurring in some supracrustal faults, are the only clear evidence for impact-related fault rocks in the Charlevoix area.  Brittle faults of various orientations are host to these polymictic breccias.  These faults are commonly crosscut by the NE-SW and NW-SE trending fault systems.  The St. Lawrence fault, striking NE-SW, represents a major structure of the area and is basically interpreted as related to rifting of the Iapetan ocean and the establishment of the St. Lawrence Platform.  However, the St. Lawrence fault also crosses the limit of the impact crater without any major trend deflection, clearly indicating post-impact fault reactivation.  Structural observations of this study and the current knowledge of the isotopic signature of fluids which have percolated through the NE-SW and NW-SE supracrustal faults suggest that they represent inherited and neoformed post-impact structures, most probably related to Atlantic rifting in Mesozoic time.