A HYPOTHESIS FOR THE NEOGENE STRUCTURAL EVOLUTION OF SUSITNA BASIN, ALASKA

The Tertiary nonmarine Susitna basin (SB) lies at the intersection of several major tectonic elements in southern Alaska. It is adjacent to the Cook Inlet basin on the south, the Talkeetna Mountains on the east, the Denali massif of the Central Alaska Range on the north, and the Western Alaska Range (WAR) on the west and southwest. The region’s tectonic history was influenced by Paleogene subduction of a slab window and Neogene underthrusting by the Yakutat terrane. Based on seismic-reflection and potential-field data integrated with regional geology, we hypothesize that in Neogene to present time, the SB has occupied a footwall position relative to an arcuate reverse-fault system comprising (from north to south) the NE-striking Broad Pass fault (BPF), north-striking Skwentna fault (SF), and the NW-striking Beluga Mountain fault (BMF). These three reverse faults may form one continuous curved structure along which the WAR (plutonic and metamorphic basement to a Cretaceous and Tertiary arc) and Yenlo Hills (YH) area (low-grade metasedimentary rocks of the Jurassic-Cretaceous Kahiltna assemblage and overlying Tertiary strata) have overridden the SB and its Wrangellia terrane basement. Industry seismic reflection data show that the SB is cut by a series of N-to-NE-striking reverse faults of Neogene (to Quaternary?) age; the base of the Tertiary is about 4.4 km deep in the deepest fault block in the SE part of the basin. Although the detachment level for these faults is not well-constrained, one possibility is that they root into a shallow detachment in the underlying Wrangellia rocks, relative to deeper-rooted contractional structures (BPF, SF, BMF) that juxtapose the YH and WAR blocks against the SB along its southwestern and western flanks. There are several possible models for the geometry and kinematics of the large basin-bounding reverse faults. For example, we have interpreted potential-field data to infer a planar basement overthrust geometry for the BMF, yet seismic reflection data combined with local geologic constraints suggest a large fault-propagation fold offset by a more modest reverse fault. Similarly, we are weighing a planar BPF interpretation against geologic evidence more conducive to trishear fault-propagation folding. Our ongoing structural modeling may help reconcile these issues.