NORTH AMERICA’S MIDCONTINENT RIFT: WHEN RIFT MET LIP

Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, which form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. The Midcontinent Rift (MCR) is an unusual combination. Its 3000-km length formed as part of the 1.1 Ga rifting of Amazonia (Precambrian NE South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established, but it contains an enormous volume of igneous rocks. MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, resulting in a rift geometry with a LIP's magma volume. Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting. The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. EarthScope data show two surprising results. Although the MCR's dense volcanic rocks appear clearly on gravity, seismic reflection, and receiver function data, which are sensitive to variations in density, surface wave tomography "sees" primarily low-velocity sediments, with no strong anomaly below. Presumably compared to the surrounding crust, the basalt rift fill is denser but has similar or slightly lower S-wave velocity. Another surprise is that mantle below the MCR shows no significant velocity anomaly or anisotropy, suggesting melt depletion had little effect on seismic velocities and likely reflecting the fact that much of the volcanism occurred after extension ended.