FOLD ROTATION DURING TECTONIC TRANSPORT: SALEM THRUST SHEET, IRONTO QUADRANGLE, VIRGINIA

Detailed 1:24000 geologic mapping has delineated several branches within the Pulaski-Blue Ridge Thrust System. The Salem Branch has been of particular interest because of well-preserved poly-phase folds that appear to have evolved from eastern ramp-parallel folds within a cataclastic broken formation zone near the base of the sheet. Structural analysis of folds in the Salem sheet on Villamont, Montvale, Daleville, Roanoke, Salem, Glenvar, Elliston, Ironto and Blacksburg quadrangles shows early-formed, reclined to recumbent folds delineated by tightly-folded massive Elbrook limestone and dolomite beds. These broken formation zone folds have a consistent NW-SE (325-335) orientation along the leading edge of the thrust sheet adjacent to the partially eroded ramp in Ordovician-Devonian clastic rocks. These folds were assigned an early Alleghanian origin by Bartholomew and others (1994) and were proposed to have formed during the cataclastic event as the base of the Salem Sheet was transported across a 100 km wide flat beneath the NW transported Blue Ridge crystalline thrust sheet . Schultz (1986) proposed rigid body rotation of the earliest fold cores under sub-greenschist to greenschist metamorphic conditions, however, the relict 325 -335 degrees orientation is apparently not well preserved in his field area SW of the Yellow Sulfur Cross fault (SW of termination of the Salem thrust on the Blacksburg. quadrangle).The early – formed fold hinges on Villamont as well as Ironto quadrangle have been rotated up to 90 degrees across broad open NE-trending (060 degrees) folds uplifting Conocochegue and Beekmantown formation carbonate rocks from beneath the Salem thrust and forming the Trinity syncline in the Fincastle Valley and the Christiansburg antiformal window that crosses the Eastern Continental divide between the New River and Roanoke Valleys. Sheath-like early-formed Elbrook limestone fold cores exposed within the cataclastic zone along the Norfolk Southern Railway near Shawsville appear to have rotated some 90 degrees from an 060 initial ramp orientation into the direction of tectonic transport during movement across the broad flat which Bartholomew has reconstructed between Danville and the Roanoke Valley following the Jeffry Model of rigid body rotation during early Alleghanian collision events.