The Commerce geophysical lineament (cgl) is a 600-km-long, 5- to 10-km-wide, northeast-trending magnetic and gravity anomaly that extends from northeastern Arkansas to central Indiana. The seismogenic potential of the cgl is poorly known. However, recent geologic and geophysical studies along faults overlying the cgl support the notion that this prominent geophysical anomaly may be a source of future large-magnitude earthquakes in the central United States. Our geomorphic mapping, acquisition of seismic reflection, and ground-penetrating radar data, coupled with paleoseismic trenching and borehole information, collected at the South Holly Ridge site, near Idalia, Missouri, provide evidence of late Pleistocene to early Holocene deformation on the Idalia Hill fault zone. The Idalia Hill fault is a 0.5-km-wide zone of northeast-striking, near-vertical faults that directly overlie the cgl and coincide with the southeastern margin of the Bloomfield Hills. At the South Holly Ridge site, fault-related geomorphic features include a prominent, northeast- trending linear trough bounded on the southeast by a 2- to 3-m-high linear, north- facing escarpment and an apparent right-laterally deflected creek. These features, in turn, are aligned with regional northeast-trending linear valleys, escarpments, linear and deflected drainages, springs and bedrock notches mapped northeast and southwest of the site. Seismic reflection data across the Idalia Hill fault zone image near- vertical faults that displace Tertiary/Cretaceous and Tertiary/Quaternary reflectors and project upward to faults interpreted from ground-penetrating radar (gpr) surveys, and trench exposures. Together, these results suggest that deformation at the site is coincident with the cgl and is of primary tectonic origin.

Trenches excavated across the linear escarpment at the South Holly Ridge site expose faulted and warped Tertiary deposits, Pleistocene gravel (reworked Mounds Gravel), the Sangamon Geosol, late Pleistocene Peoria Loess, and a late Pleistocene to early Holocene paleosol. Analyses of trench, borehole, and gpr data constrain the timing of the most recent event on the Idalia Hill fault to between the latest Pleistocene (about 18 ka) and early Holocene (pre-7.7 ka). Evidence for a penultimate event predates 23 to 18 ka. The timing of these events overlaps with late Pleistocene to early Holocene faulting and paleoliquefaction events evaluated elsewhere on northeast-striking faults overlying the cgl in southeastern Missouri. The earthquake- timing data from these sites in southeast Missouri indicate that the Commerce section of the cgl acted as a seismogenic source into at least the early Holocene and thus should be considered in future seismic-hazard analysis of the central United States.

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