P and S Wave Tomography of Southeast Canada: Insights into Mantle Development and Evolution from Archean to Phanerozoic Times
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Whether the tectonic processes seen today at plate boundaries are analogous to those on the young, hotter, more ductile Earth of the early Precambrian is still widely debated. Extensive slab melting and other Archean processes, such as the extraction of komatiitic magmas, caused large-scale lithospheric depletion, resulting in a buoyant cratonic core able to survive multiple Wilson cycles. The cratons are characterized by a thick (>200 km), seismically fast keel, which is readily identified on tomographic images. The Laurentian keel beneath North America does not fit easily within the Archean model of formation; the lithospheric root is thought to persist beneath both the Superior craton and the younger Proterozoic Grenville province in southeast Canada, suggesting that keel formation may not have been restricted to Archean times. In order to address this issue we present a P and S wave relative arrival-time tomographic study using data from seismograph networks in SE Canada and the NE US, stretching from the southern tip of Hudson Bay within the Superior craton to the coastal Phanerozoic Appalachian terranes. The tomographic images display three broad zones of increasing mantle wavespeed from globally “slow” in the Appalachian terranes, to a “fast” Grenville Province and “extremely fast” Superior craton. We observe a linear low-velocity feature resulting from modification of the Laurentian keel by the passage of the Great Meteor hotspot. This feature is progressively offset southwestward with depth, potentially due to viscous coupling with mantle flow. No major plate-scale underthrusting during the Grenville Orogeny is apparent, which contradicts the inferred results from crustal seismic reflection and refraction studies. Our results support the developing consensus that keels form in two stages: a chemically depleted core of Archean age followed by a thermally developed, less-depleted lithosphere during Proterozoic times, highlighted by an abrupt wavespeed contrast in the tomographic images.