QUANTIFICATION OF SHORTENING FOR BALANCED CROSS SECTIONS ACROSS THE CENTRAL APPALACHIAN FOLD-THRUST BELT OF PENNSYLVANIA AND WEST VIRGINIA

The Central Appalachian mountains are a blind fold-thrust belt with few or no emergent thrust faults. In such fold-thrust belts, the total shortening accommodated by folded strata must equal the magnitude of shortening accommodated by subsurface faults. Some previous attempts at constructing cross sections have relied upon unquantified layer parallel shortening (LPS) to account for the discrepancy in the restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Here we quantify bulk finite strain using the normalized-Fry method from 96 oriented samples collected along three transects (through the Susquehanna River Valley the Juniata Culmination, and one in WV) spanning the Valley and Ridge (V&R) and Appalachian Plateau (AP) physiographic provinces. Of the 96 strain samples, 66 were collected from the V&R and 30 from the AP. For each sample, the axial ratio between the X and Y directions obtained from the bedding parallel plane defines the ellipticity ratio (R_s) and directly reflects the LPS of the system, providing the magnitude of material lost in the transport direction. Samples from the two Pennsylvania transects closer to the Juniata Culmination have a mean R_s of 1.20 ± 0.06 while samples collected in West Virginia have a mean of 1.07 ± 0.05. These data sets have ranges of 1.08-1.35 and 1.01-1.25, respectively. The easternmost, Susquehanna transect, is characterized by different mean R_s values between the V&R (1.20 ± 0.06) and 1.13 in the AP. Along the Juniata Culmination transect, mean R_s values do not vary a between the V&R (1.19 ± 0.06) and AP (1.21 ± 0.08). Along the WV transect R_s values are lower but similar magnitudes in the V&R (1.06 ± 0.04) and AP (1.08 ± 0.05). Using the calculated magnitudes of finite strain, we are able to construct geologic cross sections where the magnitude of shortening in the cover sequence balances that in the faulted interval. The magnitude of grain scale finite strain and underlying shorting is greatest at the Juniata Culmination. The central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain.