Publication Date:
2016-06-19
Description:
The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (∼250 km), cold, chemically-depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P- and S-wave relative arrival-time tomography to constrain upper-mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wavespeeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, to the Archean Superior craton in central Québec. We also recover the NW-SE-trending track of the Great Meteor hotspot that cross-cuts the major tectonic domains. The decrease in seismic wavespeed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian-age rocks up to ∼300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wavespeed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less-depleted Proterozoic composition at formation.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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