Structural Provinces of the Northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska (1)

Abstract

The dominant Cenozoic structures of the northeastern Brooks Range are anticlinoria with cores of sub-Mississippian rocks, reflecting a regional north-vergent duplex with a floor thrust in the sub-Mississippian sequence and a roof thrust in the Mississippian Kayak Shale. The number of horses forming each anticlinorium and the structural style of the overlying Mississippian and younger cover sequence varies regionally, providing a basis for dividing the northeastern Brooks Range into structural provinces. In the western province, each anticlinorium contains a single horse, and shortening above the Kayak Shale was accommodated mainly by detachment folds. To the north in the Sadlerochit Mountains, the Kayak Shale is depositionally discontinuous and rocks elsewhere separated b this detachment deformed together. In the eastern province, each anticlinorium contains multiple horses, and shortening above the Kayak Shale was accommodated largely by thrust duplication of Mississippian through Triassic rocks. In the narrow central province, the Devonian Okpilak batholith was detached from its roots, internally shortened along shear zones and by penetrative strain, and transported northward. Because the Kayak Shale is locally absent, the Mississippian and younger cover sequence deformed in part penetratively along with the batholith. East-northeast trends formed where sub-Mississippian rocks were not involved in deformation, and probably are normal to the direction of Cenozoic tectonic transport. East trends formed where sub-Mississippian rocks were involved in deformation, and probably reflect a pre-Mississippian structural grain. At any given location, east trends generally post-date east-northeast trends, reflecting a drop over time of the basal detachment into sub-Mississippian rocks. INTRODUCTlON Well-exposed structures in the northeastern Brooks Range fold and thrust belt (Figure l) may provide insights into the evolution of similar structures elsewhere in the world, as well as offering clues to the factors that control their geometry. In addition, the northeastern Brooks Range includes the nearest well- exposed analogs to structures that may underlie the Arctic coastal plain immediately to the north, the most promising area for onshore hydrocarbon exploration remaining in North America. The stratigraphy of the northeastern Brooks Range has had a significant influence on the geometry of structures formed during deformation, as is true in many other fold and thrust belts (Woodward and Rutherford, 1989). The interlayering of strata of differing thicknesses, lithologies, and structural competencies has resulted in a structural stratigraphy in which particular stratigraphic intervals display a specific structural style. Several different structural provinces can be defined in the northeastern Brooks Range based upon lateral variations in structural style (Figure 2). These lateral variations commonly correspond with lateral variations in stratigraphy. Recent discussions of the structural geometry and evolution of the northeastern Brooks Range have dealt mainly with the western part of the region (for example, Kelley and Foland, 1987; Leiggi, 1987; Oldow et al., 1987a). In this paper, we illustrate the variations in structural geometry that exist over a much larger region, and argue that lateral changes in stratigraphy influence the style of deformation. Our objective is to provide a regional overview of the structure of the northeastern Brooks Range, and to interpret the influence of variations in stratigraphy on the structural geometry of the fold and thrust belt. This overview and interpretation are based primarily on our own detailed geologic studies throughout the northeastern Brooks Range, complemented by studies by graduate s udents at the University of Alaska on specific structural problems. However, End_Page 1100------------------------------ we do not intend to do more than summarize the results of these studies here. Rather, we seek in this paper to provide a conceptual and testable regional structural interpretation that will serve as a framework for future, more detailed papers and for further detailed structural and stratigraphic studies.