Layer parallel shortening: A mechanism for determining deformation timing at the junction of the central and southern Appalachians

Near the transition from the southern Appalachians to the central Appalachi ans at the Roanoke recess, major southern Appalachian thrusts (similar to 0 70 degrees-striking) with steep initial ramps (similar to 25 degrees to 30 degrees) are associated with footwall synclines with nearly vertical to ove rturned southeast-limbs. The 170-km long, overturned, Glen Lyn footwall syn cline (to the St. Clair thrust) forms the structural front of the southern Appalachian fold and thrust belt southwestward from the recess. Deformed mu dcracks, oolites, crinoid-stem plates, chert nodules, and bedding-normal bu rrows at 32 localities, mostly in the overturned limb, for 84 km from near Glen Lyn, Virginia (on the New River) to near Richlands, Virginia, primaril y reflect presyncline layer parallel shortening (LPS) strain by intragranul ar mechanisms. The eastern part of the syncline averages 14 percent shorten ing attributable to LPS, assuming constant volume deformation, whereas in t he western part, LPS averages 9 percent, Percent shortening for LPS strain is typically much lower (1-13 percent) in carbonates than it is in nearby e lastic sedimentary rocks (3-21 percent), because strain is largely accommod ated by outcrop-scale pressure solution along stylolites in the carbonates. Restoration of overturned bedding to horizontal, taking into account the tr end and plunge of the segments of the fold axis, yields LPS directions of 2 64 degrees to 342 degrees with an average of 316 degrees and a standard dev iation of 19 degrees. Although 25 percent of the LPS directions are in the range of southern Appalachian trends (330 degrees-345 degrees), most are in the ranges of central Appalachian (44 percent) and transitional (31 percen t) trends. All are oblique to the local trends (067 degrees-081 degrees) of the segments of the axis of the Glen Lyn syncline used for restoration of bedding to horizontal. Hence, 28 LPS strain-ellipses were rotated about a v ertical axis, so that the local segments were aligned with the trend (067 d egrees) of the axis at Glen Lyn. LPS directions then range from 262 degrees to 331 degrees, with an average of 308 degrees and a standard deviation of 17 degrees If anomalous shortening directions obtained for carbonates with very low percent (1 percent) LPS are excluded, then the average LPS direct ion is 311 degrees. This principal LPS direction lies 26 degrees from the p erpendicular to the Glen Lyn syncline (067 degrees) but is more consistent with shortening directions of similar to 300 degrees in the central Appalac hians reported by other workers. Earlier papers concluded that development of central Appalachian structures postdated development of southern Appalac hian structures. However, preservation of central/transitional LPS strain i n the overturned limb of this major southern Appalachian fold indicates tha t central/transitional Appalachian deformation initiated prior to formation of the Glen Lyn syncline, a southern Appalachian fold.