GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 124-9
Presentation Time: 3:45 PM


POTTER, Christopher J., U.S. Geological Survey, 610 Taylor Road, Rutgers Univ., Piscataway, NJ 08854, SALTUS, Richard W., Cooperative Institute for Research in Environmental Sciences and NOAA's National Centers for Environmental Information, University of Colorado, Boulder, CO 80035, LEWIS, Kristen A., U.S. Geological Survey, MS 939, Denver Federal Center, Denver, CO 80225-0046, HAEUSSLER, Peter J., U.S. Geological Survey, 4210 University Dr, Anchorage, AK 99508, STANLEY, Richard G., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, SHAH, Anjana K., U.S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225-0046 and PHILLIPS, Jeffrey D., U.S. Geological Survey, M.S. 964, Denver Federal Center, Denver, CO 80225-0046,

The northwest-striking Beluga Mountain fault (BMF) is a major structure separating Cretaceous and Tertiary crystalline rocks of the Beluga Mountain block (on the SW) from Tertiary nonmarine sedimentary rocks of the Susitna basin (SB; on the NE). Its geometry and movement history reflect the region’s Neogene tectonic evolution and influence the petroleum potential of the SB. Exposure is poor, but high-quality aeromagnetic data track the concealed boundary between crystalline and sedimentary rocks. Vintage reflection-seismic data reveal that the SB is cut by several north-striking reverse faults that are east-vergent along its west margin and west-vergent in the east, producing a 4-km deep structural low near the basin’s western margin.

Along the SW margin of the SB, seismic data indicate the BMF is a SW-dipping ramp-on-ramp reverse or thrust fault, with NE-dipping reflectors in the hanging wall. A well-defined gravity gradient that straddles the mountain front (Beluga Mtn. to Little Mount Susitna) is shifted to the SW, consistent with dense basement thrust over a less dense footwall sedimentary wedge along the BMF. An earthquake fault-plane solution (May 1991, M 3.5, depth 4.8 km; N. Ruppert, Alaska Earthquake Ctr.) just SE of Beluga Mtn. suggests a thrust geometry for the BMF. Using structural modeling software, we constructed a restorable cross section that honors reflection-seismic and potential-field data, incorporates a footwall sedimentary wedge beneath a gently dipping BMF, and uses trishear deformation kinematics to produce a NE-dipping Paleogene panel that unconformably overlies crystalline rocks in the hanging wall. We also evaluated steeper dipping reverse-fault solutions and associated gravity models.

North of Beluga Mtn., the gravity and seismic data indicate that a major north-striking splay [Skwentna reverse fault (SF)] intersects the BMF and continues northward as the main basin-bounding fault. East-dipping Tertiary reflectors (including a Miocene growth wedge) in the SF hanging-wall are likely part of a fault-propagation fold. There is probably no significant footwall sedimentary wedge at this latitude. The basin-bounding BMF-SF system and the intra-SB reverse faults collectively record 5-10 km of east-west Neogene shortening, possibly driven by subduction of the Yakutat microplate.