THE KINEMATICS OF BREAK-THRUST FOLDS

The kinematics of asymmetric, open to close, often overturned thrust-related folds are commonly explained with fault-propagation or detachment fold models. Field observations of thrust-related folds exposed in Tennessee, Virginia, Wyoming and Montana that exhibit this geometry indicate that models of fault-propagation and detachment folds do not adequately describe the kinematics of these structures. Existing models of fault-propagation and detachment folding employ migrating, kink-shaped hinges and relate fold geometry entirely to fault geometry, slip and to the thickness of the basal detachment layers. The models exclude the relation between fold shape and amplitude, and they do not predict a correlation between competent-layer thickness and fold wavelength as expected from buckling theory. Thrust-related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration. Furthermore, the ratios of dominant member thickness to fold wavelength agree with those expected for folds controlled by a single competent layer and they are within the expected range for folds with multiple, evenly spaced, harmonically deforming layers. Our analyses suggest these folds evolved from an initial, brief stage of sinusoidal buckling to a later stage of fixed-hinge kinking and thrusting. Thrust-related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break-thrust folds.