HR: 1340h
AN: S13B-0189    [Abstracts]
TI: Age of most Recent Motion on the Western Denali Fault from Lichenometric Dating of a Large Rockfall Avalanche and Offset Moraines
AU: Keskinen, M
EM: ffmjk@uaf.edu
AF: Dept. of Geology and Geophysics, University of Alaska, Fairbanks, Ak. 99775-5780 United States
AU: * Beget, J
EM: ffjeb1@uaf.edu
AF: Dept. of Geology and Geophysics, University of Alaska, Fairbanks, Ak. 99775-5780 United States
AB: No major historic earthquakes are known to have occurred on the portion of the Denali Fault lying west of the 7.9 M 2002 rupture zone. The 2002 earthquake produced large rockfall avalanches at several locations within a few kilometers of the 2002 fault trace. Panorama Mountain lies 60 km west of the limit of the 2002 fault zone, and just 4 km off the trend of the western Denali Fault. The west face of Panorama Mountain was the source of a large prehistoric rockfall avalanche that traveled 2 km across the Nenana River, covering an area of ca. 8 km2. The rockfall avalanche deposit consists of fractured blocks as much as 7 m high in a finer, comminuted matrix. Lichenometric dating of the rockfall avalanche based on the central Alaska Range lichen curve of Beget (1991) indicates this rockfall occurred about 300-500 years ago. This is similar to a lichenometric date on an ice-cored moraine offset >5m by the Denali Fault 40 km to the west near Mt. McKinley. The "Bains Creek fault" lies only 1 km south of Panorama Peak, and although Pleistocene glacial deposits are offset by this fault in an exposure along the Parks Highway, we found no evidence this fault was active in the Holocene. In contrast, there is good agreement between the age of offset moraines on the main, active strand of the Denali Fault near Mt. McKinley and the rockfall avalanche deposit near Panorama Mountain. This suggests the most recent large earthquake on the western Denali Fault occurred ca. 300-500 years ago. If the long-term slip rate on the western Denali Fault is ca. 1 cm/yr, i.e. similar to the slip rate inferred for the portion that broke in the 2002 event, then at least several meters of strain have accumulated since the last seismic event.
DE: 1130 Geomorphological geochronology
DE: 7221 Paleoseismology (8036)
SC: Seismology [S]
MN: Fall Meeting 2005