Abstract Title: Shallow Mantle Enrichment Beneath the Midcontinental Rift from Seismo-petrological Interpretation

Abstract Submitted to: TECTONOPHYSICS

Abstract Text:

Continental rifting is the process by which two continents are separated by the thinning and rupture of the lithosphere. The Midcontinent Rift (MCR) is a 1.1 Gyr old-billion-year U-shaped failed continental rift over 3000 km long located in the central United States from Lake Superior to central Oklahoma. The extent of the MCR is defined by magnetic and gravitational anomalies. Recent seismic models found up to 1% km/sec variation in shallow (60 km) mantle wave speed (Vs) and up to a 0.5% Vp/Vs ratio beneath the MCR compared to surrounding areas. At greater depths, these heterogeneities disappear. While temperature is the strongest control on mantle wave speed, temperature variation alone is unlikely to explain this seismic wave speed variation as thermal heterogeneities are unlikely after 1.1 Gyr. To better understand continental rifting, we investigate recent seismic models with Whole-rock Interpretative Seismic Toolbox for Ultramafic Lithologies (WISTFUL), a new MATLAB toolbox that interprets mantle seismic wave speed into best-fit temperature, density, and composition with uncertainty by comparing observed mantle wave speeds with forward calculations of wave speed of 4485 ultramafic whole-rock compositions. WISTFUL predicts temperatures ranging from 600–750°C at 60 km, in agreement with estimates using steady state geotherms and regional heat flow. Assuming 600°C at 60km (1–2° from the edge of the MCR), the shallow mantle beneath the MCR is estimated to be less enriched than its surroundings (Mg#= 88.6 within compared to 90.3 surrounding, error= ±1.0). Assuming 750°C at 60 km, the MCR is predicted to be similarly more enriched than the surrounding mantle (Mg#= 90.5 within compared to 91.7 surrounding, error= ±1.0). At 80 and 100 km, the mantle is more depleted but shows little compositional variation beneath the MCR and its surroundings due to similar wave speeds (Mg#=90.0 at 80km and Mg#=90.0 at 100km). We interpret the preserved shallow enrichment beneath the MCR compared to the surrounding mantle to indicate that the shallow mantle was enriched by melt during the failed rift.

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