The Chamberlin 1910 balanced section: Context, contribution, and critical reassessment

Chamberlin (1910) was the first to quantitatively predict the shape and position of a basal detachment from detailed surface observations. His predicted detachment beneath the Appalachian Valley and Ridge fold-thrust belt of central Pennsylvania is substantially deeper and differs in geometry from the current interpretation. The modern profile by Faill and Nickelsen (1999) across the same area shows a much shallower, planar lower detachment and the presence of a duplex below the surface folds, a significant conceptual difference from the Chamberlin profile. Two assumptions prove to be critical in causing the discrepancies, the width of the segments used as the unit of analysis, and the assumption of constant bed length. The segmentation scheme led to an erroneous conjugate-fault detachment geometry. The depth discrepancy is primarily due to ignoring the layer-parallel strain. The duplex is of the coupled-roof style and not responsible for the differences, a result supported by an experimental model. We use the modern profile and the area–depth relationship to quantify the effects of uncertainty on profile geometry and estimates of orogenic shortening. Small differences in the assumptions (inference error) lead to significant differences in displacement, detachment location and layer-parallel strain. Our best area-balanced interpretation is a slightly modified version of the Faill and Nickelsen profile.