THE KINEMATIC EVOLUTION OF THRUST-TRUNCATED DETACHMENT (?) FOLDS, EXAMPLES FROM THE EASTERN BROOKS RANGE FOLD-AND-THRUST BELT, ALASKA

In the eastern Brooks Range fold-and-thrust belt, Alaska, the Carboniferous Lisburne Limestone is separated, by the locally east-west trending continental divide thrust front, into two structural domains. The first domain is characterized by symmetric, upright, east-northeast-trending detachment folds and is located to the north of the thrust front. East-northeast-trending asymmetric folds and thrust-truncated folds characterize the second domain, located south of the thrust front. This research focuses on the latter structural domain where both the truncated and untruncated folds in the Lisburne Limestone are spectacularly exposed. Geometric analyses of the map-scale folds and thrust sheets in the eastern Brooks Range suggest a kinematic development from asymmetric detachment folds in the Lisburne Limestone into thrust-truncated folds. Novel surveying methods were utilized to collect quantitative fold geometry data and thus enabled detailed documentation of several stages in the evolution of thrust-truncated folds. The thrust faults appear to preferentially truncate the folds within the anticline forelimb and/or adjacent syncline hinge. This is evidenced by the prevalence of truncated anticline forelimbs in contrast to the long, flat, and relatively undeformed anticline backlimbs. Furthermore, numerous parasitic folds in the anticline forelimb region suggest that the accumulation of strain is greater in the anticline forelimb than in the backlimb, thus rendering the anticline forelimb prone to thrust-truncation. The overall structural character of the truncated anticlines reflects a superimposed fault-bend-fold geometry that formed as the truncated folds were translated over the upper footwall flat. In addition, steeply dipping normal faults appear to crosscut the hangingwall anticlines and associated thrust sheets. The normal faulting may be a consequence of late-stage extension oriented orthogonal to the north-northwest shortening responsible for thrust-emplacement and fold truncation.