Origin of a thrust-related fold: geometric vs kinematic tests

Geometric tests to determine the origin of fault-related folds are common, but as is typical in structural geology, more than one fold origin may yield the final natural geometry. Thus, the results of geometric tests are usually non-unique. In contrast, kinematic tests of origin, which employ both geometry and data about deformation, commonly yield more constrained, if not unique, results. Unfortunately, the necessary data collection requires much more work than for a geometric test. In this study, the thrust-related Barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective. Geometric tests, using angular relationships, indicate three possible origins: fault-bend, fault-arrest, and break-thrust. For the kinematic test, predicted deformations for interlayer and flexural slip, flexural flow, simple shear, and bending strains are compared to micro- and mesostructural distributions, solution strain, and finite strain from the anticline. Strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification-dominated. The microscale deformation does not match kinematic predictions, and did not accommodate fold formation. Fold growth was achieved primarily through layer-parallel slip restricted mostly to the forelimb and absent in the hinge, which eliminates fault-bend and fault-arrest origins that require material transport from forelimb to backlimb. The Barclay anticline is therefore interpreted to be a break-thrust structure. Interestingly, a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb. Such transport has been discounted for break-thrust folds. The most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin. Geometric testing alone was insufficient. Given that the Barclay anticline has geometric characteristics typical of many thrust-related folds, kinematic testing appears necessary to determine their structural origin