STRUCTURAL AND GEOPHYSICAL MODELING OF THE BELUGA MOUNTAIN FAULT, SOUTHERN ALASKA

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.