Upper crustal investigation of the Gulf of Saint Lawrence region, eastern Canada using ambient noise tomography

We studied the 3-D shear wave velocity (Vs) structure in the Gulf of St. Lawrence (GSL) and adjacent onshore areas to 20 km depth by inverting Rayleigh wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms. The region is divided into three broad zones based on their Vs characteristics. In the northwest, the Grenville Province (i.e., the exposed edge of predominantly Middle-Proterozoic Laurentian crust) is dominated by high Vs, except for well-known anorthosite sites, which are characterized by relatively lower Vs. In contrast, the central segment of the GSL region corresponds to a belt with generally low Vs at upper crustal levels. In the southeastern part of the GSL, prominent low Vs in the uppermost crust are found to coincide with locations of subsidiary basins of the Canadian Maritime Basin, while higher Vs characterize the accreted Appalachian terranes where they are exposed on land. The Grenville Province is wedged out at depth by the Red Indian Line, which is the suture between composite Laurentia and peri-Godwanan Ganderia in the Canadian Appalachians. The geometry and Vs characteristics of the south-easternmost peri-Gondwanan terranes of Avalonia and Meguma suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics, which could be a vestige of the West African craton that was underthrust beneath composite Laurentia during the terminal Alleghenian continent-continent collision. In the middle of the GSL, the 3-D geometry of the Canadian Maritimes sedimentary basins overlying the Appalachian terranes shows that the depth to the top of basement is in excess of 8 km. Plain Language Summary The Gulf of Saint Lawrence region is located on the eastern edge of Canada, within the Appalachian mountain belt. Some important questions in the region are what are the locations and depths of the sedimentary rocks and what is the structure of the different rock units that make up the Appalachian mountain belt at depth. Geophysical methods previously applied in the region provided some insights to these questions, but with large uncertainties. We use a fairly new method, called ambient noise tomography. This method uses seismic noise traveling within the Earth to obtain images of Earth's internal structures. After applying the method, we find that the region is divided into three parts based on the speed of seismic waves. The waves travel faster in the oldest rocks of North America (Grenville Province) and southernmost parts, where there are stronger rocks, but slower within weaker rocks (e.g., sediments) in the middle region. The sedimentary rocks are found to be thicker than 8 km at some places. There are indications that an old West African rock unit might be lying deep beneath Nova Scotia. This study provides important insights on the formation of the Appalachian rocks and the possible locations of hydrocarbon (oil/ gas) resources that form within sedimentary rocks.