Seismic Imaging of the North American Midcontinent Rift Using S-to-P Receiver Functions.

Ben Chichester,Nicholas Harmon,Andrew Frederiksen,Suzan Van Der Lee,Catherine Rychert,Hao Zhang

doi:10.1029/2018jb015771

Abstract

North America's ~1.1‐Ga failed Midcontinent Rift (MCR) is a striking feature of gravity and magnetic anomaly maps across the continent. However, how the rift affected the underlying lithosphere is not well understood. With data from the Superior Province Rifting Earthscope Experiment and the USArray Transportable Array, we constrain three‐dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the MCR using S‐to‐P receiver functions. We image a velocity increase with depth associated with the Moho at depths of 33–40 ± 4 km, generally deepening toward the east. The Moho amplitude decreases beneath the rift axis in Minnesota and Wisconsin, where the velocity gradient is more gradual, possibly due to crustal underplating. We see hints of a deeper velocity increase at 61 ± 4‐km depth that may be the base of underplating. Beneath the rift axis further south in Iowa, we image two distinct positive phases at 34–39 ± 4 and 62–65 ± 4 km likely related to an altered Moho and an underplated layer. We image velocity decreases with depth at depths of 90–190 ± 7 km in some locations that do not geographically correlate with the rift. These include a discontinuity at depths of 90–120 ± 7 km with a northerly dip in the south that abruptly deepens to 150–190 ± 7 km across the Spirit Lake Tectonic Zone provincial suture. The negative phases may represent a patchy, frozen‐in midlithosphere discontinuity feature that likely predates the MCR and/or be related to lithospheric thickness.

Highlights

The shallow, dense igneous rocks of North America’s 1.1-Ga failed Midcontinent Rift (MCR) System create a striking magnetic anomaly and gravity high that demarcate the rift axis (Chase & Gilmer, 1973; Hinze et al, 1992; King & Zietz, 1971)
It was proposed that the rifting event instead occurred during a lapse in local compression related to the Grenville Orogeny and ceased when motion was taken up by seafloor spreading between the supercontinents Amazonia and Laurentia (Stein et al, 2014)
The Superior Province Rifting Earthscope Experiment (SPREE) stations were mainly located over the rift axis, with 66 of the stations distributed along the rift axis and in two riftperpendicular lines (Figure 1)—the other 16 stations are located in Ontario north of Lake Superior

Summary

Introduction

The shallow, dense igneous rocks of North America’s 1.1-Ga failed Midcontinent Rift (MCR) System create a striking magnetic anomaly and gravity high that demarcate the rift axis (Chase & Gilmer, 1973; Hinze et al, 1992; King & Zietz, 1971). Active rifting attributes impingement of anomalous large-scale hot mantle upwelling and magmatism as the cause, driving thermal erosion and lithospheric weakening that leads to isostatic uplift, which causes tensional stresses (White & McKenzie, 1989). Seismic profiling of Lake Superior and the rift’s arms and extensive geochemical and isotopic studies of exposed and well-preserved Lake Superior rocks have already constrained a complex history of the MCR rifting events, including crustal extension (Green et al, 1989; Hinze et al, 1992), volcanism that filled the rift basin with large volumes of flood basalts (Paces & Miller, 1993; Vervoort et al, 2007), and thermal subsidence and sedimentation (Cannon, 1992). Crustal shortening lasted ~20 Myr and amounted to a shortening of ~30 km in the southwestern arm of the rift, based on seismic reflection profiling that recognizes marker horizons between flood-basalt sequences and overlying sedimentary rocks (Cannon, 1994)

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