285-4 Structure of the northern Brooks Range Foothills, Arctic Alaska

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part II

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Abstract:

The northern foreland of the western and central Cenozoic Brooks Range orogen exhibits a classic thin-skinned fold-and-thrust belt architecture.  Interpretation of modern 2D and 3D seismic reveals new insights into the complexity of this belt.  Contractional structures in the foothills are predominately imbricate, north-vergent fault-bend folds, and less commonly fault-propagation folds, accommodated by substantial north-directed thrusting.  A stratigraphically localized concentration of organic-rich mudstone in Triassic–Upper Cretaceous strata controls the development of multiple detachment levels, localized thickening in incompetent units, and distinct structural styles. 

In the western part of the northern foothills, the fundamental basal detachment in the foreland occurs within a composite condensed mudstone section of Triassic Shublik Formation, Upper Jurassic-Lower Cretaceous Kingak Shale, and Cretaceous pebble shale unit and Hue Shale. A less well-developed upper detachment lies in slope facies of the Lower to middle Cretaceous Torok Formation.  Local wedging resulted in south-directed backthrusting.  In the central part of the northern foothills, greater Kingak Shale thickness and spatially variable facies in Cretaceous strata contributed to a more complex structural style.  The Kingak Shale and overlying strata are commonly duplexed above a detachment at the top of the Shublik Formation and below upper detachment surfaces in the Lower to middle Cretaceous Torok Formation and composite Cretaceous pebble shale unit and Hue Shale.  Above this imbrication, the Lower to middle Cretaceous Nanushuk Formation and Upper Cretaceous Tuluvak and Schrader Bluff Formations are folded primarily by flexural slip.  Local folds that superficially could be described as detachment folds, show visible internal geometries indicating discrete imbrication with roof thrusts.  These geometries and a lack of deformation to the north that should be associated with throughgoing upper detachments indicate these structures are more accurately described as passive-roof duplexes.  This characterization is consistent with the presence of associated, local south-directed back thrusts and related folds in the foreland. Additionally, vertically and laterally varying internal imbrication within the Torok Formation resulted in the development of multistory duplexes related to lateral variations in the original basin geometry of these weaker units.  In places, imbricate stacking caused steep-limbed, upright folds to form above deeper duplexes of more continuous lateral extent.  Local breakthroughs manifest at the present-day land surface are common within these structures. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9694

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