Cratonic modification and subsequent asthenospheric interaction: Evidence from absolute P-wave tomography

Cratons are Earth's primordial geological features. They are thought to have withstood multiple Wilson cycles over their >2.5Ga history, in part, due to their thick lithospheric keels that are buoyant, strong and chemically depleted. Recent studies have shown that cratons may not be entirely indestructible however and often display post-formation modification signatures, particularly at their margins. The Superior craton of SE Canada is an ideal locality for study as within a few hundred kilometers, one encounters geology that spans three quarters of Earth's history. Whilst lithospheric keels are characterized by high seismic wavespeeds, younger tectonic processes may also manifest in present-day wavespeed variations and thus are detectable seismically. Here we combine the latest USArray dataset with data from permanent stations in SE Canada by using a uniquely developed data processing tool, to reliably convert relative to absolute arrival-times. Our data is set within the high-resolution continental portion of the global tomographic inversion of Burdick et. al., (2014). This affords a well-refined mantle model beneath SE Canada, a region that has traditionally lacked station coverage. Our tomographic model shows a sharp craton edge beneath the Grenville Front, suggesting that the North American craton was modified by subduction zone interaction. Consequently, post formation modification rather than variation in iron depletion levels at the time of formation may better-explain variations in wavespeed between Archean and Proterozoic terranes worldwide. Global constraints provide improved resolution in the deeper mantle, thus we are able to reliably map the extent of Farallon slab beneath SE Canada for the first time. We also may image the SW shift in the base of the North American craton, supporting the view that cratons are not entirely stagnant features over geological time and that present day asthenospheric flow can impact the evolution of cratonic keels.