FENSTER, KLIPPE, LANDSLIDE, GRABEN, SINKHOLE: WHAT HAPPENED AT BLAND, VIRGINIA?

About 50 years ago, a geologically anomalous area within the Southern Appalachian Saltville thrust sheet was recognized near Bland, Virginia. The feature has been described as a fenster, klippe, sinkhole, graben, and landslide. Recent geologic mapping indicates the feature is a graben. In the study area, there is a sharp 40-60-degree orocline bend in the strike of the Saltville thrust sheet. The graben occurs directly along the apex of the orocline and is 0.5 km wide and 1.5 km long. Middle and Upper Ordovician age rocks are in fault contact with Lower Ordovician and Upper Cambrian age rocks. Within the graben is a structurally complex syncline trending N25-45 West. The bounding border faults of the graben are vertical to steeply dipping. In-board of the border faults is a 100-meter-thick damage zone of thin slices of carbonate rocks separated by faults and/or carbonate dilation breccia, coarse crush breccia, and fine-grained cataclasite. Most contacts between formations in the graben are faulted, and stratigraphic thicknesses are truncated. Cross sections indicate 300-600 meters of down-dip displacement of hanging wall rocks into the graben. The strike of the border faults and long axis of the graben are aligned with a 9-km cross-strike lineament coincident with the apex of the orocline bend. At the northwest end of the apex, a cross-strike fault offsets the leading edge of the Saltville fault in a left-lateral sense. Other possible stratigraphic offsets occur across a water gap that cuts resistant Cambrian rocks northwest of the graben and across a wind gap in Silurian rocks atop Big Walker Mountain, southeast of the graben. A deformation sequence for the Bland graben includes differential movement of the Saltville thrust sheet during emplacement with formation of the 40-60-degree orocline bend. We speculate the Bland graben formed from extension and/or transtension during cross-strike faulting along the apex of the orocline. The Bland graben needs further detailed analysis to understand how it relates to Southern Appalachian thrust tectonics, orocline bends, and cross-strike faults.