RAMPS AND SLIDES: THE CONTROL OF UPPER-CRUSTAL ARCHITECTURE ON LANDSLIDE SUSCEPTIBILITY IN THE SOUTHERN APPALACHIAN FOLD AND THRUST BELT

In the Appalachian foothills of Alabama, landslide susceptibility and occurrence are relatively low compared to elsewhere in the Appalachian Mountains. Several tightly clustered landslide and rock fall prone areas have been recently documented along northern slopes of Colvin Mountain, east-central Alabama. We present new geologic mapping and joint-orientations from the Ordovician Colvin Mountain Sandstone to investigate the relationship between preexisting fractures, Appalachian crustal deformation, and Quaternary mass wasting susceptibility.

The Colvin Mountain ridge is nearly continuous for ~75 km and lies in the hanging wall of the Helena thrust fault. Along the northeast section of the Helena fault, the Colvin Mountain ridge bends ~40° from NE regional Appalachian strike to an E-W orientation before terminating into the eastern Coosa thrust fault. This change in orientation likely represents a northwest-striking, southwest-dipping lateral ramp in the underlying Helena fault, with mass-wasting prone areas isolated along the tightest turn in the structural bend. Structural analysis reveals a dominant, syntectonic joint set (075°-255°) along the Colvin Mountain ridge. Structural bending is responsible for a zone where joint and strike orientations are nearly parallel at ~075°. Large landslides and numerous rock falls were observed downslope from this zone with little mass wasting observed elsewhere, thereby linking along-strike structural bending controlled by an upper-crustal lateral ramp to an increased landslide susceptibility where structural strike parallels preexisting joint set orientations. Here, understanding upper crustal structure and the local and regional deformation histories provides insight when assessing potential controls on mass wasting.