ORIGIN OF A THRUST-RELATED FOLD - GEOMETRIC VS KINEMATIC TESTS

Geometric tests to determine the origin of fault-related folds are com mon, but as is typical in structural geology, more than one fold origi n 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 necessar y data collection requires much more work than for a geometric test. I n this study, the thrust-related Barclay anticline is analyzed both ge ometrically and kinematically to determine which test is more effectiv e. Geometric teals, using angular relationships, indicate three possib le origins: fault-bend, fault-arrest, and break-thrust. For the kinema tic test, predicted deformations for interlayer and flexural slip, fle xural flow, simple shear, and bending strains are compared to micro- a nd mesostructural distributions, solution strain, and finite strain fr om the anticline. Strain measurements indicate that microscale deforma tion is uniformly distributed through the structure and is lithificati on-dominated. The microscale deformation does not match kinematic pred ictions, and did not accommodate fold formation. Fold growth was achie ved primarily through layer-parallel slip restricted mostly to the for elimb and absent in the hinge, which eliminates fault-bend and fault-a rrest origins that require material transport from forelimb to backlim b. The Barclay anticline is therefore interpreted to be a break-thrust structure. Interestingly, a suite of contraction faults in the foreli mb and hinge indicates material transport from forelimb to backlimb. S uch transport has been discounted for break-thrust folds. The most imp ortant result of this study is that a kinematic test was a necessary s tep for distinguishing fold origin. Geometric testing alone was insuff icient. Given that the Barclay anticline has geometric characteristics typical of many thrust-related folds, kinematic testing appears neces sary to determine their structural origin. (C) 1997 Elsevier Science L td.