GEOMETRY AND KINEMATICS OF ASYMMETRIC, THRUST-TRUNCATED FOLDS, UPPER MARSH FORK AREA, EASTERN BROOKS RANGE, ALASKA

The three main types of thrust-related folds (fault-bend folds, fault-propagation folds, and detachment folds) are commonly difficult to distinguish based solely on incomplete field exposures. This problem is exacerbated if the folds themselves are faulted. Exceptionally well-exposed examples of asymmetric thrust-truncated folds outcrop along the upper Marsh Fork (UMF) of the Canning River in the eastern Brooks Range, Alaska. This study addresses the geometry and kinematics of the folds, reconstructs how the folds were modified by truncation and displacement, and tests existing models for thrust-related folds against natural examples.

Based on careful structural mapping and use of previously collected survey data, four detailed serial cross sections have been constructed of the UMF area. As folds commonly display different stages of evolution along strike, this detailed documentation of three-dimensional fold geometry provides constraint on the process of fold deformation. Most existing two-dimensional models for fault-propagation folds and detachment folds were created first and then compared to field examples. This study's approach is fundamentally different because it uses detailed three-dimensional observations of natural fold examples to test and modify existing models to match the natural folds.

The folds in the UMF area formed in ~1000 meter thick Lisburne Group carbonates. They have long, gentle backlimbs and short, overturned forelimbs that are cut by thrusts. In most cases, the footwalls are not exposed. The folds formed above the thick, incompetent Kayak Shale, which suggests that most of the folds in the UMF originated as detachment folds. However, termination of one thrust upward into a syncline suggests a fault-propagation fold origin in one location. Significant variation in interlimb angle and forelimb rotation along strike allows the kinematic evolution to be modeled. The main fold hinges can be traced between serial sections, indicating that they remained fixed during fold growth, although additional minor hinges appeared as the folds were displaced. Many of the folds in the UMF area show thinning in the forelimb of the fold, and thickening in less competent intervals.