S receiver function imaging of the lithosphere-asthenosphere boundary (LAB) in western Canada

Abstract:

S receiver function imaging of the lithosphere-asthenosphere boundary (LAB) in western Canada The lithosphere-asthenosphere boundary (LAB) offers critical constraints on craton formation and development. Commonly associated with a negative velocity gradient/contrast from the lithosphere to the asthenosphere, the presence, depth and nature of the LAB vary broadly and are often debated on a global scale. In this study, we investigate the LAB beneath western Canada, where distinct episodes of plate convergence may have played major roles since the early Precambrian. Our data set consists of three regional networks and a subset of USArray, with earthquake recordings from more than 100 stations. The LAB depths are measured using S receiver functions, which is 1) strongly sensitive to lithospheric discontinuities and 2) relatively unaffected by crustal multiples.

We obtain an average LAB depth of 207 km in the Western Canada Sedimentary Basin, which is comparable to the global mean depth. The LAB depth fluctuates from -35 km to 40 km, showing anomalously shallow LAB beneath the Buffalo Head Terrane that deepens towards the Kiskatinaw and Ksituan domaine; deformed lithospheric roots due to mantle upwelling may be responsible. Other areas of elevated LAB include the Proterozoic western Chinchaga domain, the Wabamun domain, and the Archean southern Hearne province. The general patterns of our measured LAB depths near craton margin are well correlated with those of recent mantle seismic tomography at depths greater than 200 km, suggesting potential common origins of these observed anomalies. In particular, the depth of LAB varies strongly along the strike of the Cordilleran front. Covering the western Chinchage, Wabamun, Thorsby, and Rimbey, the LAB thins by as much as 30 km in central Alberta compared with its northern and southern counterparts. We suggest that the edge-driven mantle convection in association with the intrinsic strength of the lithosphere may jointly control the dynamics near the craton margin and modify the structure of the LAB.