LATE PLEISTOCENE AND HOLOCENE FAULTING SOUTHEAST OF THE NEW MADRID SEISMIC ZONE: THE JOINER RIDGE AND MEEMAN-SHELBY BLIND HORSTS, EASTERN ARKANSAS

The Joiner Ridge horst and Meeman-Shelby horst are blind transpressive structures that underlie the Mississippi River floodplain in northeastern Arkansas. Both structures are southeast of the New Madrid seismic zone and are currently aseismic, yet both have documented Quaternary displacement. Through bore hole drilling, shallow P-wave and SH-wave seismic reflection, and OSL and radiocarbon geochronology, this project revealed that both the east-bounding fault of the Joiner Ridge horst and the east-bounding fault of the Meeman-Shelby horst have undergone Holocene displacement. The hanging wall of the Joiner Ridge fault has a 30-m wide horst that has been vertically displaced 2 m within the last 9.75 ka, indicating that the 30-m wide horst has an average uplift rate of 0.3 mm/yr. Two separate faults have been mapped within the Meeman-Shelby fault zone. The principal fault and fold have 25 m of vertical displacement on the top of the Eocene/base of Quaternary alluvium. Within the overlying Quaternary and Holocene sections, 14.3 ka alluvium has been vertically displaced 12 m, 5.2 ka alluvium by 10 m, and 4.28 ka alluvium by 1 m. These different slip magnitudes indicate multiple faulting events with different slip rates within late Wisconsinan and Holocene time. A subsidiary graben located 0.8 km southeast of the main trace of the Meeman-Shelby fault has 2 m of vertical displacement on 9.68 ka alluvium, thus indicating an average slip rate of 0.21 mm/yr over the past 9.68 ka.