EASTERN EDGE OF THE LAURENTIAN CRATONIC LITHOSPHERE BENEATH SOUTHERN QUEBEC FROM TELESEISMIC P WAVES
Tuesday, 16 December 2014
The eastern margin of Laurentia was deformed by the late-Proterozoic Grenville orogeny, which metamorphosed the original Archean-age cratonic crust and added extensive intrusive rocks. The Laurentian crust clearly extends as far east as the Laurentian Highlands, just north of the St. Laurence River, where Grenville-deformed rocks outcrop. Small outliers of Grenville-deformed rocks amongst west-thrust Paleozoic sediments, 20-50 km east of the Appalachian Front in southern Quebec, suggest that the Laurentian crust extends beneath the shallow thrust sheets of this region, too. On the other hand, Laurentia does not extend east of the Norumbega fault in coastal Maine, for the crust there is derived from the Avalon micro-continent. We search for the eastern edge of Laurentia within this ~250 km wide interval using relative arrival times of teleseismic P waves which ascend sub-vertically through the lithosphere beneath the region. These times are expected to be most sensitive to upper mantle compressional velocity and so to be able to discriminate the cratonic lithosphere on the basis of its faster than average speed. We use signal-correlation techniques to measured delay times for all broadband seismic stations in the region, including the QMIII array, which is especially designed to have high station density near the Appalachian Front. As expected, we observe central Quebec to have anomalously early times and coastal Maine to have anomalously late times, by as much as ±1s, when compared to the predictions of the global AK135 traveltime model. The boundary between the two arrival time regimes is sharp and is collinear with the Appalachian Front, to within the ± 25 km spatial resolution of our study. We hypothesize that it represents the eastern edge of the Laurentian cratonic lithosphere. Tomographic inversion of the data indicates a 0.2 km/s (2.4%) drop in compressional velocity of the shallow (90 km deep) mantle from west to east across the boundary. This is a strong jump in mantle properties, as either a 400 deg C increase in temperature or a 11 decrease in Mg# (or some other as yet unidentified source of heterogeneity) is needed to explain it. Future work will use teleseismic shear waves to further characterize the lithosphere across this boundary.