HOLOCENE FAULTING ALONG THE SOUTHERN MARGIN OF THE NORTH AMERICAN CRATON (ALABAMA-OKLAHOMA TRANSFORM)

Seismic reflection profiles show southeast-striking Triassic rift grabens with > 1 km of structural relief (Saline River fault zone, SRFZ) following the Proterozoic Alabama-Oklahoma Transform margin in southeast Arkansas. Some of these graben faults show post-Triassic through late Cenozoic reactivation with flower structure geometries having both positive and negative elements and up-dip changes in sense of separation (suggesting a strong strike-slip component). Previous excavations of the SRFZ in Monticello, Arkansas show faulting during Eocene, Pleistocene, and Holocene. In this study, five additional sites along the SRFZ were investigated. At Horsehead Island on the southwestern margin of the SRFZ, a surface fault rupture diverted the Saline River 2 km to its floodplain margin circa 1900-1400 yr B.P. Across a graben from the Horsehead Island fault, a trench across a linear fault scarp at Gee’s Landing revealed three paleoearthquakes circa 13,400-7700 yr B.P., 1400-1300 yr B.P., and 1200-1050 yr B.P. Near Rison in the northern SRFZ, a faulted anticline post-dates alluvium that yields 14C ages of 790-550 yr B.P. Thirteen kilometers southeast of the Rison anticline at Vince Bluff, another anticline with no topographic scarp folds alluvium that yields 14C ages of 5893-5804 yr B.P. and 5775-5642 yr B.P. Last, a trench in Mid/Late Holocene alluvium (~6000-3000 yr B.P.) on the southeastern SRFZ at Boydell revealed no faulting within 2 m of the surface, although shallow geophysics and coring indicate near-surface faults in alluvium. These results can accommodate a 70 km rupture along a central fault of the northern graben of the SRFZ circa 6000-5000 yr B.P. Mid-Holocene sand blows in the region may record such a fault rupture, but much more data is required to make meaningful correlations.