Lithospheric Inventory: An Exploration of Quebec, Canada with P-to-S Receiver Functions.

Abstract:

Understanding the structure of the lithosphere is vital to our comprehension of plate tectonic processes. Using a seismograph network in Quebec, Canada, we attempt to identify horizontal discontinuities associated with the crust and lithospheric mantle. We also investigate systematic changes in these discontinuities across the Grenville Front and Appalachian Front, two tectonic boundaries separating the Proterozoic Grenville Province from the Archean Superior craton to the north and the Appalachian orogen to the south.

Using data from long-term seismic stations, we analyzed over 3000 teleseismic earthquakes from the period 2000-2013. For each station we constructed P-to-S receiver function (RF) gathers arranged by backazimuth and epicentral distance, to identify individual phases that we can associate with seismic boundaries within the crust and upper mantle.

At stations across the Superior craton, we see a strong positive phase at ~5 s with a prominent first multiple, suggesting a flat, sharp Moho boundary at ~35 km depth. A phase at ~2 s is also pervasive, likely due to an intra-crustal boundary. We do not see any evidence for a sharp lithosphere-asthenosphere boundary (LAB) in the RF data for this region. A remarkably similar structure is inferred at stations within the Grenville Province.

Around the St. Lawrence River, observations from both sides of the Appalachian Front are broadly similar to each other, but systematically different from those further north. The Moho phase suggests a crustal thickness of ~40-45 km, and we do not observe any pulses that may be interpreted as Moho multiples. The crustal structure is more complex, with significant directional variation in RF signal at individual sites, and noticeable differences in RF gathers between stations separated by tens of km. This complexity likely explains the lack of coherent crustal multiples. At some stations, a negative pulse at ~15-17 s is observed, consistent with a predicted LAB depth of ~150 km.

Our data suggest that the surface tectonic boundaries of the Grenville Province do not correspond to significant changes in lithospheric discontinuity structure at depth. Instead, a major change in lithospheric layering is inferred between the St. Lawrence River and the northern Grenville Province near Lac St-Jean.