Abstract
AbstractThe 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 wave 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 wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW‐SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed 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 wave speed 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.
Highlights
Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec
The decrease in seismic wave speed 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
Model for Grenville Province Development we propose a new model for Grenville Province development (Figure 8) to reconcile our seismological observations (Figures 5 and 6) with geological constraints [Hanmer et al, 2000; Ludden and Hynes, 2000; Rivers and Corrigan, 2000; Chiarenzelli et al, 2010; Hynes and Rivers, 2010] and studies of tectonic processes at cratonic margins [e.g., Nolet and Zielhuis, 1994; Rondenay et al, 2000; Snyder, 2002; Pedersen et al, 2013]
Summary
Overview Cratons, the ancient cores of the continents, are the longest-lived tectonic features on Earth. They are thought to have resisted thermomechanical erosion for ≥1 Ga over multiple Wilson cycles [e.g., Hoffman, 1990; Griffin et al, 2009; Lee et al, 2011]. Key to their survival are their thick (∼250 km) lithospheric keels [e.g., Lebedev and Van Der Hilst, 2008; Lekicand Romanowicz, 2011] that possess a range of attributes that promote longevity, buoyancy, high strength and viscosity [e.g., Lenardic and Moresi, 1999; Sleep, 2003], low heat flow [Mareschal and Jaupart, 2004], and extensive chemical depletion in heavier elements such as Fe, Al, and Ca [e.g., Jordan, 1988; Griffin et al, 2003]. Thick keels often extend beneath both Archean and (less depleted) Proterozoic terranes (e.g., Australia and North America [Simons et al, 2002; Yuan et al, 2011; Schaeffer and Lebedev, 2014]), but some cratonic regions show thick keels to be restricted to the highly depleted Archean provinces (e.g., southern Africa [James et al, 2001])
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