Structural Style—Brooks Range Mountain Front, Alaska

The Brooks Range mountain front between the Sagavanirktok River and Kurupa Lake is characterized by thrust sheets of Lisburne rocks, which dominantly have stratigraphic tops to the north and either dip northward or are overturned to the south. The early (Jurassic-Neocomian) thrust belt strikes obliquely (N70°W) into the mountain front and can be traced on seismic sections into the foothills. Individual thrust structures die out westward into overturned folds and ultimately into plunging noses as the thrust belt plunges to the west and successively higher structural levels are exposed. The total thrust displacement remains essentially constant because of transfer of motion to higher thrusts. Most of the west occurs along narrow zones of instant plunge with essentially zero occurring along trend. The present east-west mountain front is oblique to the original thrust vergence (N20°E) of Jurassic-Neocomian age and is due to later (Albian and younger) Brooks Range core uplift, folding, and comparatively minor thrusting. The Lisburne folds at the mountain front respond almost plastically to the late core uplift and gravitationally slide downward on rotated north-dipping thrust faults to form cascading folds with easiest relief upward and northward. Core uplift has rotated original thrust sheets 90° in some instances so that geologic map patterns are plunge-projection cross sections of thrust plates. The pattern at Eskimo Creek (Table Mountain quadrangle, eastern Brooks Range) illustrates how a view can reveal the true structure of the thrust belt. Examples of mountain-front structures are given at Killik River, Kurupa Lake, Kaikshak Hill, Akmagolik Creek, Atigun River, and Ivishak River. End_of_Article - Last_Page 660------------