A Reference Model of Crustal Thickness and Vp/Vs of Western Canada

Abstract

The Canadian Cordillera marks a transition region from the current plate boundary through the Phanerozoic Cordilleran orogen to the Precambrian cratons. Knowledge of the subsurface structure of western Canada has been greatly advanced by seismological investigations during the past two decades, pioneered by the Lithoprobe project and, more recently, by regional passive seismic arrays. In this study, we construct a new model (WCANM22) of crustal thickness and P- to S-wave velocity ratio, or V_p/V_s, by compiling receiver function data from 473 stations and existing constraints from over 2,600-km long active source experiments. Our model covers a broad swath (about 1/4) of the land area of North America (105°–140°W, 48°–72°N) and shows an overall flat Moho beneath the Cordillera with an average depth of ∼36 km and a standard deviation of 3 km across orogenic belts. This study provides a comprehensive catalog of V_p/V_s in western Canada and reveals a moderate correlation between Poisson's ratio and the age of crustal domains. The average V_p/V_s values are 1.72, 1.79, and 1.82 for the Phanerozoic Cordillera, Proterozoic Cratons, and Archean-aged Medicine Hat Block, respectively, suggesting continued modifications to crustal composition through episodic tectonothermal events. This distinct trend in western Canada sheds new light on the debated role of secular changes in the composition of continental crust.