THE MIDCONTINENT RIFT: A RIFT-LIP HYBRID

While flood basalts, a class of large igneous provinces (LIPs), are typically not associated with significant crustal extension and faulting, North America’s Midcontinent Rift (MCR) combines these two phenomena. Its 3000-km length formed during the 1.1 Ga rifting of Amazonia from Laurentia, but it contains an enormous volume of igneous rocks that are mostly flood basalts. The Keweenawan volcanics that fill the MCR are significantly thicker than other flood basalts due to their deposition in a narrow rift rather than across a broad region, therefore giving the MCR a rifts geometry but a LIPs magma volume. Structural modeling of seismic-reflection data shows that LIP volcanics were deposited during two phases—an initial rift phase where flood basalts filled a fault-controlled extending basin and a postrift phase where LIP volcanics and sediments were deposited in a thermally subsiding sag basin without associated faulting. Seismic imaging reveals a thickened crust and a high velocity lower crustal body along the rift today, but significant lithospheric thinning occurred during the initial rifting phase when extensional processes dominated. Preliminary gravity modeling at successive stages of rift evolution shows that the MCR may have had a negative gravity anomaly during rifting, and its unusually high gravity anomaly likely required the massive inversion that followed long after rift failure. The restriction of extension to a single normal fault in alternating half-grabens, steeply inward-dipping bounding rift shoulders, and persistence of volcanism after rifting and thinning ended gave rise to a deep flood basalt-filled geometry. The MCR also exhibits many key features of volcanic passive margins, characterized by seaward dipping reflectors, volcanic rocks yielding magnetic anomalies landward of the oldest spreading anomalies, and a high-velocity lower crustal body. The coincidence of a rift and LIP that is well preserved in the MCR provides useful insight into continental extension and its possible end-member evolution to volcanic passive margins.