STRUCTURAL CHARACTERIZATION OF BRITTLE FAULTS OF THE ST. LAWRENCE PLATFORM IN THE MONTREAL AREA, CANADA

In this study, structural data from the surface and the compilation of underground mapping data (subway and urban infrastructures), in combination with the mapping of K-bentonite beds as stratigraphic markers, are use to better constraint the nature of regional faults, the true offset values, and to build a precise 3D bedrock geological map for the city of Montreal, in order to improve the planning of infrastructures development.

Regional brittle faults crosscutting the Cambrian-Ordovician sedimentary sequence of the St. Lawrence platform (SLP) in the Montreal area show two main orientations, EW and NW-SE. The EW-trending faults are, from north to south, the Bas-Sainte-Rose (BSR), Rapide-du-Cheval-Blanc (RCB), Ile-Bizard and Sainte-Anne-de-Bellevue faults. These structures are subvertical, with an apparent normal-sense fault throw toward the North or the South. Fault offsets range from a few tens to several hundred of meters, up to 435 meters for the RCB fault. Some of these faults extends continuously for ≥30 to 40 km and seem to merge westward into the Ottawa graben. The main NW-SE faults are the Saint-Vincent-de-Paul and Duvernay faults. They occur between the BSR and RCB faults and extend along-strike for ≤10 km. They are subvertical or steeply SW-dipping, and show down-dip normal-sense offsets less than a few tens of meters.

In the field, all faults are poorly-exposed, except for some quarries and a few outcrops on Bizard Island. When visible, they are underline by a series of closely-space, discrete faults and fractures within 10 to 20 meters-wide zones, and are in sharp contacts with the hosting carbonate rocks. Fault surfaces are frequently mark by calcite coating, indicative of fluids circulation during faulting. When present, fault striations are slightly to moderately plunging toward the West for the EW faults and toward the NE or the SW for NW-SE trending faults. The calcite coating of the Ile-Bizard fault has been sample for U-Pb isotopic dating. Three of the four samples that were analyze yield a nearly perfect isochron age at 112 ± 2 Ma, whereas a fourth sample yield a less robust age of 102 ± 16 Ma. Such a late Early Cretaceous age is consistent with field observation suggesting that some of these EW trending structures cut across the Mt Royal intrusion, which belongs to the ca. 125 Ma Monteregian Hills igneous Suite.