ICON: A FUTURE EarthScope PROJECT RESOLVING THE 4D-RECORD OF INCIPIENT CONTINENTAL COLLISION IN THE OUACHITA OROGEN

Nearly 30 years since publication of the first “iconic” 2D images of the Ouachita orogen, the region remains one of North America’s most enigmatic and least understood orogens with respect to timing, tectonic setting, and record of exhumation. Pioneering geophysical transects across the south-central margin of North America were inferred to represent the Mississippian-Pennsylvanian record of incipient continental collision between cratonic North America and an exotic arc terrane. Current deployment of USArray in the Arkansas region has created a timely and urgent opportunity to re-visit these “iconic” transects in light of new EarthScope imaging techniques and recent advances in geochronology. Several “secrets of the North American continent’s structure and evolution” in this area remain unresolved and are the principal foci of the ICON Working Group, including: (1) understanding the role of recently discovered Archean sub-continental lithospheric mantle in the assembly, stabilization, and reactivation of the south-central margin of the North American craton; (2) resolving the 3D geometry and tectonic history of two inferred subducted slabs that occur within ICON’s USArray footprint (related to the Laramide and Ouachita orogenies); and (3) quantifying the timing of orogeny, exhumation, and topographic evolution of a remarkably well-exposed plateau-foreland basin-accretionary wedge system that includes the highest point in the midcontinent US (Mt. Magazine – 839 m) and North America’s only public diamond mine, containing crustal and mantle xenoliths that sampled an intensely reactivated zone of Archean-Mesozoic continental lithosphere. Recent mapping of the lithosphere-asthenosphere boundary (LAB) reveals a pronounced seismic velocity gradient between “stable” and “unstable” cratonic lithosphere beneath the proposed study area. ICON also seeks to investigate the lithosphere-scale structure of the Oklahoma-Alabama transform fault. The transform offers an opportunity to gain new insights into the role of structural reactivation and the control of anisotropy in the mantle during a Wilson cycle of continental rifting, passive margin development, and rift inheritance, culminating with incipient continental collision during the Ouachita orogeny.