INSIGHTS FROM THE FAILED MIDCONTINENT RIFT INTO THE EVOLUTION OF CONTINENTAL RIFTS, PASSIVE CONTINENTAL MARGINS, AND OTHER FAILED RIFTS

A decade of recent studies of North America's 1.1 Ga failed Midcontinent Rift (MCR), catalyzed by the EarthScope program, yield new insight into the rift’s formation and evolution, the processes of continental rifting and passive continental margin formation, and other major failed rifts in central North America. Structural modeling constrained by surface exposures, seismic, gravity, and other data reveal a history of extension, volcanism, sedimentation, subsidence, and inversion giving rise to a thick, dense, and highly magnetized volcanic section underlain by thinned and underplated crust. These features are analogous to those at volcanic passive margins, which are characterized by thick sequences of volcanic rocks yielding magnetic anomalies landward of and sometimes larger than the oldest spreading anomalies. The MCR thus preserves a snapshot of the process by which actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, evolve either into failed and often inverted rifts without upwelling mantle and positive gravity anomalies or into passive continental margins. It also gives insight into two other major failed rifts, the Southern Oklahoma Aulacogen and Reelfoot Rift, which formed in similar tectonic settings and followed similar evolutionary paths. All three formed during rifting associated with Laurentia’s interactions within the supercontinent of Rodinia, via formation of microplates between the diverging major plates analogous to those observed in present-day rifting. These rifting events are recorded by directional changes in apparent polar wander (APW) paths, as often observed in association with plate boundary reorganizations such as continental rifting or major collisional orogenies.