FAILURE WAS NOT AN OPTION- THE MID-CONTINENT RIFT SYSTEM SUCCEEDED

The 1.1 Ga Mid-Continent Rift (MCR) in North America is often viewed as a failed rift formed by isolated midplate volcanism and extension within the ~1.3-~0.98 Ga Grenville orogeny. An alternative view is suggested by analogy with younger and morphologically similar rift systems, whose plate tectonic settings are more easily understood because their surroundings - including seafloor with magnetic anomalies - have not been deformed or destroyed by subsequent collisions and rifting events.

In this view, the MCR was part of a larger plate boundary rifting event that resulted in a successful episode of seafloor spreading. This view is motivated by various pieces of evidence. The MCR rifting looks much like rigid plate block motion, such as associated with the West Central African Rift systems formed during the Mesozoic breakup of Africa and South America and the ongoing rifting in the East African Rift region with seafloor spreading in the Gulf of Aden and the Red Sea. This view explains the affinities of the Grenville-age rocks in the central and southern Appalachians to Amazonia rather than Canadian Grenville-age Appalachian rocks. The MCR extends farther to the south than traditionally assumed along the East Continental Gravity High (a buried feature from Ohio to Alabama). This failed portion of the rift system connected to the rift successfully separating Laurentia and Amazonia. The seafloor spreading separating Amazonia from Laurentia may explain the former’s relative motion toward Greenland and Baltica. This model is consistent with some of the ~1.1 Ga geological events in Amazonia. A change in the apparent polar wander path for Laurentia during the period of volcanism of the MCR could be attributed to this plate reconfiguration.

The extensional phase on the MCR may have ended because motion was taken up by seafloor spreading between Laurentia and Amazonia rather ending due to another continental collision. Later reverse faulting on the MCR normal faults due to compression, perhaps from collisions around Rodinia’s margins, would not be unexpected because the MCR would be a relatively weak intraplate zone due to higher crustal temperatures and faults.