Fossil distortion and décollement tectonics of the Appalachian Plateau

Fossil distortion and solution cleavage are associated with low-amplitude folds on the outer fringes of the Appalachian foreland fold belt. Deformed fossils, sampled from Upper Devonian rocks in western New York, correspond to horizontal strain ellipses with axial ratios of as much as 1.5 and with long axes precisely parallel to the Appalachian fold trend. This strain is indicative of at least a 10% layer-parallel shortening normal (north-northwest) to the fold trend. Assuming a rigid basement, 10% layer-parallel shortening requires 5 km of displacement along a decollement thrust on the south-southeast side of any 50-km-wide portion of the affected area.