THE CENOZOIC STRUCTURAL EVOLUTION OF A FOLD-AND-THRUST BELT, NORTHEASTERN BROOKS RANGE, ALASKA

A Cenozoic fold-and-thrust belt in the eastern structural province of the northeastern Brooks Range exposes polydeformed low-grade metasedimentary and metavolcanic rocks of the pre-Mississippian basement and its sedimentary cover immediately adjacent to much younger foredeep deposits. Analysis of mesoscopic and map-scale structures in the range-front region suggests that at least one pre-Mississippian deformational event was recorded in the basement sequence by north-vergent fold-and-thrust structures and associated penetrative structures. Most of later Cenozoic shortening of the pre-Mississippian rocks was accommodated by thrust duplication, with little development of penetrative mesoscopic structures. Although separated from the underlying basement rocks by a major regional decollement horizon, Cenozoic deformation in the overlying Mississippian through Lower Cretaceous cover sequence also was primarily by thrust duplication. Although local and regional structural trends within the cover sequence suggest that Cenozoic deformation was north-northwest directed, east-west Cenozoic structural trends within the pre-Mississippian rocks may reflect an inherited pre-Mississippian structural grain and/or pre-Mississippian-age structures reactivated during Cenozoic deformation. A regional balanced cross section of the eastern structural province was constructed by integrating the detailed structural data from the range-front region with subsurface data from the foredeep basin to the north and reconnaissance surface data from the interior of the range. This balanced cross section indicates that Cenozoic shortening across the region was 101 km (63 mi) over an undeformed length of 220 km (137 mi), or 46%.