TECTONIC DOMAINS IN THE UPPER CRUST OF THE CRATONIC PLATFORM, MIDCONTINENT UNITED STATES

The cratonic platform consists of stabilized Precambrian basement overlain by a cover of Phanerozoic strata. This region can be divided into domains, based on structural relief displayed on a digital elevation model (DEM) of the Great Unconformity (Domrois et al., 2015). In the continental interior, two domains stand out. The Midcontinent Domain is a region of low structural relief with very gentle slopes between epeirogenic basins and domes or arches. It is bounded abruptly on the west by the Rocky Mountain front, and is overlapped on the east by Appalachian thrust sheets. In this domain, the Great Unconformity has been offset noticeably along the border faults of the Midcontinent Rift and the Nemaha Ridge. Lesser faults, dominantly in two trends (W to NW; N to NE), divide the upper crust of the Domain into fault-bounded blocks. The bounding faults, some of which root in Proterozoic rift faults, have been reactivated multiple times, in response to strain, and have accumulated tens to hundreds of meters of transpressional or transtensional displacement during the Phanerozoic. NE-SW trending sutures and tectonic fabrics, associated with accretion of the Yavapai and Mazatzal belts, do not appear to influence fabrics in the Midcontinent. Evidently, syn-accretionary metamorphic fabrics formed at mid-crustal levels are not weak under upper-crustal conditions. Stitching by plutons of the eastern granite-rhyolite province, and associated cratonization due to transfer of quartz from the plastic lower crust to the brittle upper crust, may have further strengthened accretion-related contacts. Rift-related faults evidently remain weaker, and perhaps more susceptible to developing overpressures that trigger slip. The Bordering Basins Domain, which defines the southern and eastern border of the Midcontinent Domain, includes a series of deep elongate basins, most of which formed over rifts, and some of which deepened due to flexural loading by adjacent fold-thrust belts. Structural relief along the boundary can exceed 7.5 km. Seismicity of the continental interior concentrates in the Bordering Basins Domain, suggesting that faults in this domain, which formed or reactivated during late Precambrian rifting, remain weaker than faults in the Midcontinent Domain, and/or that the domain acts as a stress concentrator.