Identification and evolution of tectonic faults in the greater Fairbanks area, Alaska

Abstract

I have identified evidence for potentially active northeast-striking faults in the greater Fairbanks area and have developed a model for their evolution that defines the character of faults and timing of structural events. Seismicity, topographic data, and geological and geophysical maps show that potentially active faults correspond with previously mapped bedrock faults, topographic lows, and magnetic anomalies. Seismicity indicates that faults in Interior Alaska have low to moderate tectonic activity; geomorphic and structural analyses of mapped faults indicate they are dominantly left-lateral strike-slip, but have smaller normal or reverse components. The normal component of slip increases to the west toward the Nenana basin based on tilted fault block geometries observed in seismicity. Deformed Pliocene to Quaternary strata, placer deposits presumably related to tectonic-induced stream capture events, and geomorphic anomalies such as valley and basin asymmetry, barbed drainages, and changes in river morphology all suggest Pliocene-Quaternary tectonic activity along northeast-striking left-lateral faults. Northeast-striking faults have been at least episodically active since the late Cretaceous. Northeast-striking fault-hosted gold deposits related to ~90 Ma intrusions suggest that fault parallel extension fractures or tension veins formed at ~90 Ma during a period of northwest-southeast extension. Thermochronological data are consistent with exhumation at ~56-42 Ma, possibly resulting from dextral shearing between the Denali and Tintina faults. Northeast-striking faults were reactivated as normal faults along pre-existing extensional trends. Thermochronological data suggest that strike-slip displacement on the Tintina fault significantly decreased at ~42 Ma. However, dip-slip motion continued along northeast-striking faults after 42 Ma as a result of contraction related to northward plate convergence. Drainage restorations, high-levels of background seismicity, and focal mechanisms support the hypothesis that at ~6 Ma, significant left-lateral motion occurred along northeast-striking faults and has continued to the present.