Paleomagnetism of the Fort Knox Stock, Alaska, and rotation of the Yukon-Tanana terrane after 92.5 Ma

摘要

The 92.5 Ma Fort Knox granodiorite stock, near the western end of the Fairbanks Belt in the Yukon-Tanana terrane (YTT) of central Alaska, hosts a world-class gold mine. The stock has been analysed paleomagnetically using thermal and alternating-field step demagnetization and isothermal remanence methods. This pluton retains a primary thermoremanent magnetization at 18 sites (232 specimens) that resides mainly in single-to pseudosingle-domain magnetite with a direction of D=228.8 degrees, I=84.3 degrees (N=18, k=130, alpha(95)=3.0), giving a paleopole at 56.5 degrees N, 197.1 degrees E (dp=5.9 degrees, dm=5.8 degrees). The pluton's host rock, the Fairbanks schist, does not retain a stable coherent remanence. Relative to the North American craton, the stock's paleoinclination indicates that the Fairbanks Belt has undergone nonsignificant poleward (northwesterly) translation of 25 +/- 750 km only. Analysed in concert with the few available paleoinclinations available for the YTT in Yukon, the paleoinclination suggests further that the YTT has undergone only similar to 250 to 450 km of dextral displacement along the Tintina fault in the past similar to 100 Ma and, therefore, is parautocthonous since the mid-Cretaceous. The stock's paleodeclination records 121 +/- 35 of counterclockwise rotation relative to the North American craton. Consideration of models published for Alaska's tectonic evolution suggests that this paleodeclination discordance is caused by rotations associated with the opening of the Canada Basin, with dextral displacement on the Tintina fault, and with development of the western Alaskan orocline. Thus the paleomagnetic results for the Fort Knox stock support a thin-skin tectonic model for the accretion of the YTT and Intermontane Belt terranes to the northern Cordillera. (c) 2006 Elsevier B.V. All rights reserved.