STRUCTURAL CHARACTERIZATION AND GEOCHRONOLOGY OF THE FAULT NETWORKS IN THE MONTREAL REGION (SOUTHERN QUEBEC, CANADA): A PRELIMINARY CALCITE U-PB DATING STUDY

The Montreal region is located within the St. Lawrence Platform, a sequence of Cambrian and Ordovician siliciclastic to carbonate rocks that represent the transition from a rift basin to a passive continental margin, itself overlain by the Appalachian foreland deposits of the Trenton, Utica and Lorraine shale formations. These rocks are not only punctuated by the presence of bentonites (an altered volcanic ash with very low cohesion), but also by numerous faults, making geotechnical engineering, which is crucial for urban development of the city of Montreal, highly complex with developers unable to predict which precise geological units their excavations will intercept at depth. To solve these problems, we recently started collaboration between multiple partners including universities, provincial and city agencies and private engineering companies. Here we present the structural part of this project, which combines both fieldwork and U-Pb dating on calcite, and aims to characterize the network of faults in the Montreal area and determine their relationship with local and global tectonic events. Recent mapping and data compilations show that brittle faults in the Montreal area are mainly normal faults, and intersect the St Lawrence platform sedimentary rocks in two main orientations, E-W and NW-SE. N-S and NE-SW faults also exist but these are more discrete and are mostly found south of the Montreal area. Most of the individual faults, have small fault throws (cm to dm), but once the fault networks are taken into account, hundreds of metres of stratigraphic offset can be determined. Most fault planes also contain calcite veneer associated with hydrothermal fluid circulation. Preliminary results of U-Pb dating of this calcite show a range of ages from ca. 110 Ma down to ca. 20 Ma suggesting multiple relatively recent periods of fault activity. These ages are much younger than the opening of the Atlantic rift, long thought to be the cause of these faults. Our new ages also post-date the plume-related magmatism found in the area suggesting that they may correspond to episodes of crustal uplift and in response to changes in mantle magmatic circulation at least during the Cretaceous, or possibly fluid migration in the basin. More sampling and dating are yet to come and will help to more precisely determine their origin.