QUATERNARY DISPLACEMENT RATES ON THE MEEMAN-SHELBY FAULT AND JOINER RIDGE HORST, EASTERN ARKANSAS: EVIDENCE FOR MIGRATING STRAIN IN THE REELFOOT RIFT

Many faults in and adjacent to the New Madrid seismic zone do not have detailed Quaternary fault displacement histories. This research used coring and OSL dating of deeply buried, displaced Quaternary alluvium to determine late Quaternary vertical displacement rates for the Meeman-Shelby fault and the Joiner ridge horst, two structures in northeastern Arkansas that have no modern seismicity associated with them. We drilled continuous cores of the entire Quaternary section in the hanging wall of each structure and obtained 3 OSL ages (1 σ) from each core. The Meeman-Shelby core consists of 36 m of 4.3 ka to 5.2 ka Holocene alluvium overlying 4 m of 14.3 ka alluvium that in turn overlies Eocene Claiborne Group sediment at 41 m. Seismic reflection indicates that basal Quaternary alluvium at the Meeman-Shelby fault, identified by OSL dating as 14.3 ka Kennett alluvium, is displaced vertically ~28 m, resulting in a minimum time-averaged displacement rate of 2.0 mm/yr for the last 14.3 ka. The Joiner ridge core consists in descending order of 11 m of 6.3 ka Holocene alluvium, 14 m of 11.5 ka Morehouse alluvium, a paleosol, 6 m of Kennett alluvium, and 4 m of 20.3 ka Sikeston alluvium overlying Eocene Claiborne Group sediment at 36 m. Lignite exploration drilling conducted in the 1970s indicates that basal (Sikeston) alluvium on the Joiner Ridge horst is displaced vertically ~20 m, which produces a minimum time-averaged displacement rate of ~1 mm/yr for the last 20.3 ka. These late Quaternary displacement rates are comparable to time -averaged displacement rates of structures that lie within the zone of active New Madrid seismicity, suggesting that sites of local strain accumulation are not constant and can move to different structures within the Reelfoot Rift over time. Our calculated rates assume the basal alluvium on both sides of the faults are the same ages, and this assumption needs to be tested by coring and dating alluvium in the footwalls of each fault.