WHAT LATENT STRESSES ARE ASSOCIATED WITH FAILED RIFTS? A CASE STUDY OF THE MIDCONTINENT RIFT, NORTHCENTRAL US

Failed rifts and passive margins are commonly sites of historical seismicity, leading to the suggestion that ancient structures represent persistent weak zones that localize strain in a uniform stress field. Alternatively, the density variations of geologic structures associated with rifting (e.g., rift pillows, intrusions, thinned crust, mafic underplates) may augment tectonic stress via lithospheric flexural stress or lateral variations in lithostatic pressure. The western arm of the Mesoproterozoic Midcontinent Rift (MCR) extends northeast from Kansas through eastern Minnesota and is one of the most spectacular examples of a failed intraplate rift in the world, with a ~80 mGal gravity anomaly that requires large variations in crustal density. Nevertheless, the region surrounding the rift is nearly aseismic, with only one felt event (M_w 3.1, Feb. 2, 1994 near Willmar, MN) in the past 40 years. Using a high-precision, 3D lithospheric density model derived jointly from gravity, seismic, topographic, and heat flow data, I compute internally-derived (i.e., gravity-induced) stress tensors in the region. Magnitudes of crustal deviatoric stress are generally small, but are highest in the rift itself, contributing ~5 MPa of NE-SW compressive stress, subparallel to the ambient North American stress field. Since these lateral variations in internally-derived stress are greater than variations expected in stress transferred from distant plate boundaries, generated by post-glacial rebound, or related to pore-fluid pressure, it is plausible that density-derived stresses encourage or subdue strain localization in continental interiors. Work is currently underway to systematically categorize internally-derived stress across the continental United States with the goal of better understanding the origin of intraplate seismicity.