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

摘要

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.