DENALI FAULT GEOLOGIC RELATIONS AT GUNSIGHT PASS, DENALI NATIONAL PARK, ALASKA

The Denali fault (DF) of the central Alaska Range has been mapped by others as a dextral transcurrent fault from the area of Nov. 2002 M~8 seismic events westward across Gunsight Pass and ~800 km beyond to the Bering Sea, with offset estimates as much as 400 km. The previously proposed dislocation of a granodioritic body along DF near Gunsight Pass to form two discrete 38-Ma-dated bodies is widely cited to show >40-km post-Oligocene dextral offset. Our 1997 study of the well-exposed relations at the pass clearly show an intrusive relation between the granodiorite and metasedimentary rocks we correlate with the unit DOs of Csejtey and others (1992). Steep striae of a nearby shear zone only relate to nontranscurrent movements, such as by Alaska Range uplift.

New major- and trace-element studies show that the two granodioritic bodies are two discrete intrusions we call McGonagall and Foraker plutons (MP, FP), rather than a single disrupted body. In major elements, FP is more silicic and distinctly more potassic than MP (SiO2- FP, 71%, MP, 67%; alkali index- FP, 42, MP, 36). Trace elements show stronger differences and multielement plots suggest variations in tectonic setting or fractionation history: Nb (FP, 13 ppm; MP, 7 ppm); Rb (FP, 155 ppm; MP, 48 ppm); Sr (FP, 77 ppm; MP, 522 ppm); K/Ba (FP, 77; MP, 22). MP shows greater light REE-enrichment than FP (La/Yb, respectively, 15 and 10), with positive Eu anomaly (avg. Eu/Eu*=1.62) compared to strongly negative ones for FP (avg. Eu/Eu*=0.44). Such differences show that MP and FP are different intrusions though coeval.

The DF does not cross Gunsight Pass as a dextral transcurrent fault as mapped by others. Instead, if the fault continues westward from areas of the Nov. 2002 seismic events it changes character from dextral transform to dominanly dip-slip just west of the westernmost epicenter.