GEOMETRY AND EVOLUTION OF THE FRONTAL PART OF AN OROGENIC WEDGE, CENTRAL BROOKS RANGE FOOTHILLS, ALASKA

WALLACE, Wesley K.¹, DUNCAN, Alec S.², PEAPPLES, Paige R.³, SWENSON, Robert F.³, WARTES, Marwan A.⁴, O'SULLIVAN, Paul B.⁵, and FINZEL, Emily S.⁴, (1) Department of Geology and Geophysics, University of Alaska, Fairbanks, Fairbanks, AK 99775, [email protected], (2) Department of Geology and Geophysics, University of Alaska Fairbanks, 900 Yukon Dr, Fairbanks, AK 99775, (3) Alaska Division of Geologic and Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, (4) Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, (5) Apatite to Zircon, Inc, 1075 Matson Rd, Viola, ID 83872-9709

Transects across the southern Brooks Range foothills in the Cobblestone and Tiglukpuk Creek areas illustrate the structural geometry and evolution of the frontal part of the Brooks Range orogenic wedge. To the south, a thick allochthonous wedge tapers northward from the range front into the subsurface of the southern foothills. The thickest and latest of the allochthons, the Endicott Mountains allochthon (EMA), was displaced onto little-deformed rocks of the North Slope. Large-displacement imbricate thrust faults in the lower, more competent part of the allochthon commonly are folded in local antiformal stacks. The upper part of the allochthon consists of turbidites (Okpikruak Fm.) and mélange containing blocks of diverse age and lithology derived from EMA and higher allochthons. We interpret the blocks to be of olistostromal origin, but complex syn- to post-depositional faulting and folding now overprint the rocks.

The leading edge of the allochthonous wedge was buried by clastic foreland basin deposits (Fortress Mt., Torok, and Nanushuk Fms.). Abrupt and complex variations in thickness and facies probably reflect paleogeography across the ancient range front-basin interface. Later folds and faults in these rocks vary in character according to their stratigraphically controlled mechanical competency. Local breakthrough thrust faults cut across both the foreland basin deposits and pre-existing structures in the underlying EMA. North-vergent structures are bounded to the north by a backthrust zone in the incompetent Torok Shale that marks the roof thrust of a triangle zone. Cuspate anticlines and flat-bottomed synclines formed in the competent Nanushuk Formation above and to the north of this roof thrust. Shorter-wavelength folds and thrust faults in Fortress Mt. and Torok below and south of the roof thrust may be analogous to structures that core the Nanushuk detachment anticlines in the subsurface.

EMA was emplaced during Early Cretaceous time, followed by structural modification of the wedge and formation of structures in the foreland basin deposits. Local evidence exists for mid-Cretaceous syn-tectonic deposition in Fortress Mt. and Nanushuk, but apatite fission-track ages suggest that breakthrough thrusting, backthrusting, and detachment folding occurred mainly during events at ~60 and ~45 Ma.