Collaborative Research: Deciphering the Structure and Evolution of North America's Cratonic Core

Intellectual Merit. To solve the puzzle of continental architecture and growth history from the surface to the base of the cratonic lithosphere, a key target of the EarthScope Science Plan, it is essential to integrate the lithologic, structural, and geochemical characteristics of the shallowest crust with geophysical data sets suitable for probing deeper into the crust and the mantle lithosphere. This will reveal the processes governing accretion of the North American crust over geological time that have been obscured by the subsequent reworking of their surface geological expressions. With the integration of these complementary data sets, we seek to synthesize the evolutionary history of a hypothesized ancient convergent margin currently located within the continental core of the central U.S. This is expressed as a cryptic and enigmatic continent-scale mid-Proterozoic geochemical boundary that requires two contrasting lower crustal and upper mantle sources, the Nd model age line, which extends from Texas to southwestern Ontario, and separates magmatic rocks of the same emplacement/eruption age (Granite-Rhyolite provinces, ca. 1.4 Ga). This geochemical boundary coincides nearly perfectly with deep crustal magnetization contrasts that argues strongly for a fundamental lithospheric structure not evident from shallow geology or geophysics. We propose to undertake a coordinated seismic, gravity, and magnetic investigation in the midcontinent of the U.S. to decipher accretionary and rifting processes and associated variations across the Nd model age line. We shall also investigate an enigmatic 400 km long lithospheric boundary (called TIKL) from the edge of the Grenville (1.1 Ga) craton northwestward toward the Paleoproterozoic province. This latter boundary exhibits curious gravity and magnetic field patterns, where presently available information cannot distinguish between possibilities of a Mesoproterozoic rift margin or an accretionary or subduction wedge. The structural and petrologic variations associated with these major tectonic boundaries appear to have caused zones of weakness that have reactivated in Paleozoic times (and, for the Nd model age boundary, potentially a post-Miocene reactivation in the St. Louis area). The present deep crustal magnetization model over the central U.S. suggests that there are different sub-provinces within the Paleoproterozoic and Mesoproterozoic units that are not perceived by isotope geochemistry due to sparse sampling of Precambrian rocks in drillholes. By joint inversion of refined seismic USArray TA and gravity data, we will investigate whether these magnetic sub-provinces have Vp/Vs and density properties that could further elucidate the cryptic Proterozoic evolution of North America. Broader Impacts. This project will provide exceptional opportunities for cross-disciplinary research and learning experiences for undergraduates and graduate students at the University of Kentucky and Utah State University. Students will become educated in seismology and potential fields methods, and will undertake practical fieldwork as well as investigations integrating across these disciplines as well as petrology and geochemistry. We will make available three-dimensional parameters of the lithospheic models we develop during this project (densities in the crust and in the lithosphere, Vp, Vs, Vp/Vs, magnetization, and crustal thickness). Further societal benefits accrue from better understanding the evolution and the processes of Proterozoic accretion, and rifting and isolating potential zones of weakness associated with the tectonic structures we have targeted to resolve.