THE ROLE OF JOINT-SET ORIENTATIONS, UPPER-CRUSTAL ARCHITECTURE, AND PALEOSEISMICITY IN MEGA-LANDSLIDE EVENTS IN THE NORTHEASTERN ALABAMA, SOUTHERN APPALACHIAN FOLD AND THRUST BELT

The development of landslides in the eastern United States is generally controlled by oversaturation of high-relief slopes. Three recently investigated landslides, isolated along the base of Colvin Mountain, a low-relief ridge in the southern Appalachian Valley and Ridge of Alabama, provide the opportunity to examine parameters that influence landslide development. We present structure and stratigraphic field data from the Colvin Mountain thrust sheet to evaluate the relationship between landslide susceptibility and upper-crustal architecture.

The Colvin Mountain ridge formed during the Alleghanian orogeny (315-250 Ma) by the northwestward translation and exhumation of the Helena thrust sheet. Presently, the ridge trends ~045-055° (NE) for 100 km, before turning ~40° to the southeast for 18 km and ultimately terminating underneath the Western Coosa fault. The ridge is capped by the Colvin Mountain Sandstone, a ~20 m thick quartz arenite, with less resistant mudstone, limestone, and siltstone forming the north and east facing slopes. Three landslides (24,275,000 m³; 4,500,000 m³; and 23,400,000 m³ in size from east to west) detach along the northern scarp slope of the Colvin Mountain ridge. Fracture measurements (N=27; n = 1,061) in the Colvin Mountain Sandstone demonstrate a prominent 070° (NE) joint trend. At the oroclinal bend where the strike of the mountain shifts from NE to SE the ridge is oriented ~070° along a short segment upslope of the large landslides. Therefore, we propose that landslide susceptibility along the Colvin Mountain ridge as well as other ridges in the Southern Appalachian Mountains are increased when the strike/trend of the structural features parallels prominent fracture trend. We propose a paleoseismic trigger for the landslides along Colvin Mountain, presumably associated to activity along deep-seated basement faults (i.e., the East Tennessee seismic zone). With the identification of large landslides in a low relief area we also explore: are other enigmatic structural features in the Southern Appalachians landslide features?