A geometric analysis of thrust-truncated asymmetric folds, Upper Marsh Fork area, eastern Brooks Range, Alaska

Abstract

A new surveying technique was developed to document the geometry of eight km size thrust-related folds in the eastern Brooks Range, Alaska. This method combines data from a theodolite and reflectorless laser rangefinder to construct cross sections that are more precise than those based on field map data. In the study area, weakly to non-metamorphosed Carboniferous carbonates typically form northeast-trending, northwest-vergent, asymmetric thrust-truncated folds. The geometry of most of the folds is consistent with a detachment fold model that allows local thickness changes. The anticlines typically display interlimb angles of less than 90,̊ structurally thickened hinge zones, and overturned forelimbs that dip steeply to moderately, suggesting the folds are over-tightened. Furthermore, structural disruption of bedding in the anticlinal forelimbs suggests that strain was localized in this region of the fold and is interpreted to be a record of the transition from folding to thrust faulting within each fold.