Tectonic evolution and subsidence history of the Nenana Basin, Interior Alaska: Preliminary results from seismic-reflection, electric logs and gravity data

The Nenana basin is an elongated Tertiary structural half-graben located in Interior Alaska, between the Denali fault to the south and the Tintina fault to the north. Although the basin has been explored for oil, gas and coal episodically over the past 40 years, the timing and mechanisms that are responsible for its formation remain unclear. Our preliminary work offers new insights into the tectonic subsidence history and structural history of the basin. Seismic-reflection and gravity data indicate that the Tertiary sedimentary fill of the southern Nenana basin is up to 19,500 ft deep, resulting in a Complete Bouguer gravity anomaly with a low of -50 mgal. The southeast margin of the basin is formed by the Minto fault, a major, steeply dipping, east-northeast striking fault. The fault shows evidence of both significant sinistral strike-slip and down-to-the-west normal faulting, with metamorphic rocks of the Yukon-Tanana terrane exposed to the east and Quaternary deposits to the west. Secondary active normal faults in the basin are oriented west-northwest and east-northeast and indicate a probable ongoing sinistral transtension across the Minto fault zone. Our preliminary interpretation of these geometries suggest that the Nenana basin is superimposed on a crustal block rotating clockwise within a dextral shear zone bounded by the regional Denali and Tintina fault systems, which is probably the direct driver of present tectonic subsidence in the basin. Further details as to the subsidence history of the basin can be derived from the geometry, thickness and seismic character of the Tertiary basin fill. The basin experienced three phases of subsidence and two uplift events during this time, possibly due to regional tectonic events during the history of Interior Alaska. The most important tectonic control that may have resulted in periods of basin subsidence was probably Tertiary strike-slip faulting of the Denali and Tintina fault systems, and subsequent transtension across the Minto fault zone. Growth of the basin during the Tertiary widened the basin to the west. Other far-field driving mechanisms that may have controlled basin subsidence to lesser degrees include subduction of a spreading center along the former coast of southern Alaska (61 Ma- 50 Ma) and resulting oroclinal bending of western Alaska in response to the northwestward shift in plate convergence (60 Ma - 42 Ma). Burial history models further indicate that the basin experienced two different inversion events, possibly in response to increased northward compressive stresses. These compressive regimes may be due to Kula-Pacific plates reorganization (42 Ma - 23 Ma) and/or ongoing flab-slab subduction of Yakutat block beneath south-central Alaska (26 Ma to present day). Our preliminary study accounts for the structure of the southern Nenana basin primarily as the result of combination of Tertiary transtension, transpression and strike-slip faulting, and suggests that the present-day geometry of the basin resembles a pull-apart structure.