LEARNING FROM FAILURE: NEW INSIGHTS INTO CONTINENTAL RIFTING FROM NORTH AMERICA'S FAILED MID-CONTINENT RIFT

Engineers realize that much can be learned about how complicated systems like aircraft or nuclear reactors really - as opposed to ideally - work by studying their failures. The same is likely to be true for the rifting phase of the Wilson cycle. Most of our knowledge about ancient rifting comes from studying continental rifting occurring today or passive continental margins remaining from successful rifts. An alternative is to study failed rifts preserved in continental interiors, such as North America's Mid-Continent Rift System (MCRS). The 2000-km-long MCRS, which is comparable in length to the present East African and Baikal rifts, has two major arms identified using the large gravity and magnetic anomalies resulting from dense and highly magnetic mafic igneous rocks. This system evolved at around 1.1 Ga during a rifting event recorded by igneous and sedimentary rocks. Petrologic and geochemical models favor the MCRS having formed by active rifting over a mantle plume. In such scenarios, the two arms are analogous to today's East African rift - Red Sea - Gulf of Aden system that is splitting Africa into three plates. Alternatively, many tectonic models view the rift as having formed as part of the Grenville orogeny, the series of 1.3-0.9 Ga tectonic events to the east associated with the assembly of Rodinia. In such interpretations, northwest-directed convergence at the southern margin of Laurentia (North America) caused extension and magmatism to the northwest, including formation of the MCRS. This scenario could be similar to the way the Baikal rift results from the Himalayan collision. In either scenario, the rift arms were borders of a microplate that existed until rifting failed, presumably because changing far-field stresses as the Grenville orogeny progressed caused compression that slowed and stopped the extension. The geometry of rifting influences the yet unresolved issue of how far south the arms extend and what happens there. New insight into these processes will be forthcoming from the SPREE (Superior Province Rifting Earthscope Experiment) project that uses Earthscope’s broadband Flexarray seismometers. Records of distant earthquakes will be analyzed to provide three-dimensional images of the structure of the crust and mantle beneath the MCRS, that will be integrated with the gravity and magnetic data.