Thrust-breakthrough of asymmetric anticlines: Observational constraints from surveys in the Brooks Range, Alaska

• Field observations constrain thin-skinned foreland fold-and-thrust belt deformation.
• A new surveying technique provides improved resolution of fold profiles.
• Mechanical stratigraphy plays dominant role in fold style in Brooks Range, Alaska.
• Late-stage thrust breakthrough of anticlines accommodated continued shortening.
• Fold tightening, forelimb rotation, and parasitic folding preceded thrust breakthrough.

To gain insights into the processes governing the thrust-truncation of anticlines, we conducted a field study of the thrust-truncated folds in the remote Brooks Range of northern Alaska, where there is a transition in fold style from symmetric detachment folds to thrust-truncated asymmetric folds. In order to document the detailed geometry of the km-scale folds exposed in cliff-forming, largely inaccessible outcrops, a new surveying technique was developed that combines data from a theodolite and laser range finder. The field observations, survey profiles, and cross section reconstructions, indicate that late-stage thrust breakthrough of the anticlines within the mechanically competent Lisburne Group carbonates accommodated continued shortening when other mechanisms became unfeasible, including fold tightening, forelimb rotation, and parasitic folding in the anticline forelimbs. These results provide constraints on the processes that govern the transition from buckle folding to thrust truncation in fold-and-thrust belts worldwide.