AN ALGORITHM TO COMBINE GRAVITY, TOPOGRAPHY, SEISMIC VELOCITY, AND HEAT FLOW APPLIED TO UNDERSTAND THE PROTEROZOIC EVOLUTION AND MODERN STRUCTURE OF THE MIDCONTINENT RIFT

Gravity and topography are inherently non-unique functions of three-dimensional density structure; constraining density models with complementary information such as seismic velocity and heat flow yields more robust results. In this regard the North American Midcontinent Rift (MCR) poses a special challenge: A prominent gravity high sits within thermally homogenous and low-relief terrain, and shear-velocity models from surface wave dispersion suggest anomalously slow—rather than fast—mid-upper crust. Using seismic velocity models, heat flow data, and empirical relations between velocity and density as functions of temperature and composition, I generate an initial estimate of the 3-D density structure of the crust and upper mantle. Then, a random-walk Monte Carlo algorithm iteratively refines the density structure until it reproduces gravity and flexurally modulated topography to within 5 mGal and 50 meters across the region. Embracing the non-uniqueness of the solution, 1000 individual acceptable 3-D structures are derived. Similar to many other focused studies, this modeling reveal dense, mafic material throughout the crustal column beneath the MCR, including eclogitic lowermost crust to depths of ~55 km, consistent with pure-shear thickening during rift inversion. A secondary finding, however, is that the mantle lithosphere beneath the MCR is not anomalously buoyant (i.e., melt-depleted), suggesting that the primary source of rift-related melts was sublithospheric and that lithospheric partial melting played only a small role in magma generation. As such, there is reason to question the hypothesis that a mantle plume is necessary for the onset of rifting. Instead, I suggest that rifting began due to far-field stress and that a positive feedback between passive upwelling and retrogressive phase changes in the underlying asthenosphere focused extension and volcanism. Subsequent rift inversion exploited not only warm, recently faulted crust but also a gravitational potential energy low beneath the low-elevation, sediment-filled rift graben. This conception suggests that intraplate deformation can be focused by gravitational body forces, and this new methodology provides a means by which to quantify these stresses.