HR: 0800h
AN: T11B-0380    [Abstracts]
TI: Structural Implications of new Geologic Mapping and a Detailed Gravity Traverse in the Brooks Range Foothills, Chandler Lake Quadrangle, Northern Alaska
AU: * Peapples, P R
EM: paige_peapples@dnr.state.ak.us
AF: Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709
AU: Saltus, R W
EM: saltus@usgs.gov
AF: United States Geological Survey, Box 25046 Mail Stop 964 Denver Fed Center, Denver, CO 80225
AU: Swenson, R
EM: robert_swenson@dnr.state.ak.us
AF: Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709
AU: Brown, P J
EM: pbrown@usgs.gov
AF: United States Geological Survey, Box 25046 Mail Stop 964 Denver Fed Center, Denver, CO 80225
AB: A fold and thrust belt in northern Alaska occupies the structural transition from the imbricate thrust sheets (or allochthons) of the Brooks Range to the North Slope coastal plain. Understanding this complexly deformed zone is critical to hydrocarbon assessment in the state-owned lands south of Prudhoe Bay. New geologic mapping and geophysical studies are focused on this important region. Detailed (1:63,360 scale) geologic mapping along the drainages of Tiglukpuk Creek and the Siksikpuk River documents exposures of deep structural levels and provides important constraints for a structural model of this region. A south-to-north structural transect encompasses the transition from the highly deformed thrust sheets of the Endicott Mountains Allochthon (EMA) at the mountain front to the inferred triangle zone at the Tuktu Escarpment 45 miles to the north. Mississippian Carbonates to Triassic siliciclastics make up the EMA north of the mountain front where there is an abrupt transition in structural style from north-vergent asymmetric overturned folding to gentle warping above shallowly dipping fault ramps. Tiglukpuk Anticline represents a fenster where the EMA is overlain in thrust contact by a melange of more distal Ipnavik River Allochthon (IRA) rocks. Parallel synclinoria contain the Okpikruak siliciclastics of the IRA assemblage and the Brookian syntectonic siliciclastics of the Fortress Mountain formation, carried atop the EMA thrust sheets during the latest phase of deformation. This complex structural style abruptly ends at the Tuktu Escarpment which likely represents a backthrust/triangle zone that places shallow north dipping coarse clastics of the Nanushuk Formation over the less competent and highly deformed Torok shale. The structural complexity and associated steeply dipping strata in the foothills belt were not well imaged by existing conventional seismic data in this region. To provide additional subsurface control, we collected ground station gravity data (500 m spacing with elevation control from precise differential GPS) along the structural transect. Complete Bouguer gravity values along this transect are dominated by a broad gradient caused by the thickening of the crust beneath the high Brooks Range and a major density boundary in the pre-Mississippian basement. Removal of the long-wavelength trend results in a residual gravity profile that is remarkably subdued given the mapped structural complexity - this implies relatively subtle density variations in the shallow rock units. In detail, residual gravity (with values spanning a range of only about 10 mGal) along the profile shows a positive gradient from the southern end of the profile, through the Tiglukpuk anticline, culminating near the intersection of Tiglukpuk Creek and Siksikpuk River. Continuing to the north, the gravity profile is relatively flat to the Tuktu escarpment. North of the escarpment, residual gravity values follow a negative gradient. Superimposed on this broad pattern are subtle gravity lows (1-3 mGal) that coincide with the anticlinal crests of both the Tiglukpuk and Siksikpuk anticlines, suggesting they are cored by strata with somewhat lower densities. The broad gravity high in the central portion of the profile is consistent with the possible presence of the higher-density, carbonate-bearing EMA rocks farther north in the subsurface than previously mapped.
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8108 Continental tectonics: compressional
DE: 8122 Dynamics: gravity and tectonics
DE: 8169 Sedimentary basin processes
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005