WHERE DID CHAMBERLIN GO WRONG? DATA, MECHANICAL INTERPRETATION AND WHAT CONSITITUTES PROOF

Key assumptions in Chamberlin’s analysis of the ‘depth of folding’ in the Appalachians persisted in the literature for over 50 years, despite contradictory evidence dating from before his pioneering 1910 paper. These assumptions are, 1) material only goes up, 2) large horizontal motions didn’t take place, and the related idea that, 3) there is no regional depth of detachment. The main reason for the resilience of this model was lack of subsurface information. Publication of data from hydrocarbon exploration showed that assumptions 2) and 3) could not be supported. Assumption 1) was the last to be challenged.

Another reason that the model persisted was the mechanical argument offered in Chamberlin’s paper. He was struck by the geometrical similarity between cm-scale rock mechanics experiments and his reconstruction of the depth of folding in km-scale Appalachian folds. Problems with scale, material properties, body forces and other factors were discussed but dismissed. These 'embarrassments' were judged less important than the strikingly similar angles between conjugate fractures in the experiments and his depth of folding plot. Reconstructing the arguments used by Chamberlin and workers who followed provides insight into where mechanical interpretations can go wrong in service of reconstructing incomplete subsurface information. The cautionary tale is that calling on a mechanical argument must involve an in depth understanding of the processes and scales that lead to the structures in both experiment and nature.