LARGE SCALE THRUST-BREAKTHROUGH OF FOLDS IN THE BROOKS RANGE, ALASKA

Spectacular examples of thrust-related folds occur throughout the remote Brooks Range of northern Alaska, where folds with wavelengths of a kilometer and greater are exposed. In the northeastern Brooks Range, the Lisburne Group, a large fold-forming carbonate unit, is characterized by intact, upright, symmetric detachment folds. In contrast, to the south in the central and eastern Brooks Range, the Lisburne Group carbonates typically form north-vergent, asymmetric thrust-truncated folds, and less commonly, upright non-truncated folds. To gain insights into the processes governing the thrust-truncation of anticlines, we present results from field studies at the transition in fold style from symmetric detachment folds to thrust-truncated asymmetric folds. We present field observations, survey profiles, and cross section reconstructions, that 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.