Eastern Denali Fault Slip Rate and Paleoseismic History, Kluane Lake Area, Yukon Territory, Canada

In 2002, the central part of the dextral-slip Denali fault (DF) system generated a M 7.9 earthquake in central Alaska. This rupture included the section of the Denali fault with the highest measured late Pleistocene slip rate, of 12.1±1.7 mm/yr, and the Totschunda fault, with a slip rate of 6.0±1.2 mm/yr. Immediately east of the Denali-Totschunda fault juncture, the slip rate on the eastern Denali fault (EDF) decreases to 8.4±2.2 mm/yr. We present observations of Holocene fault activity on the Yukon part of the EDF (Shakwak segment), which is located about 280 km southeast of the Denali-Totschunda intersection in the vicinity of Kluane Lake. Aerial reconnaissance in 2007, from the Denali-Totschunda fault juncture to the Kluane Lake region revealed a nearly continuously identifiable fault trace, which is occasionally obscured where it is subparallel to glacial landforms. In addition to geomorphic features associated with strike-slip faults, such as shutter ridges and sag ponds, the fault is commonly expressed by a chain of elongate mounds, likely tectonic pushups, 20-70 m in length, 10-50 m wide, and locally up to 10 m high. These appear to have formed by shortening between en echelon left-stepping fault strands that developed in layered glacial sediments. At one location (61°18'30.12" N, 139°01'02.54"W) we measured on the ground a channel offset of 20-25 m. An aerial view showed that other channels in the vicinity, as well as the margins of two mounds, were offset by similar amounts. These channels likely developed after deglaciation 10-12 ka. Using this age and the offset yields a slip-rate range of 1.7-2.5 mm/yr, a minimum value but one that may be close to the actual rate. However, because of uncertainties in age relations between construction of the uplift mounds and channel incision the offset could be younger and we estimate an upper limit of about 5 mm/yr. Adjacent to and south of the Duke River, an approximately 2-km-long section of the fault is expressed by an upslope southwest-facing 9-m-high scarp. In 2008 we targeted this location (61°21'54.36"N,139°07'59.34"W) with three cross-fault excavations that exposed deformed sediments. Evidence for at least 3 paleoearthquakes was recognized in fissure fills, folding, and scarp- derived colluvium. Preliminary age control is provided by the two White River volcanic ashes (1.2 ka and 1.9 ka) exposed in the trenches and by unfaulted paleo-shorelines of Alsek and Kluane Lakes, which others suggest have an age of 0.3-0.5 ka, respectively. The most recent event post-dates the 1.2 ka White River ash and occurred prior to formation of the 0.3 ka and 0.5 ka shorelines. The older events pre-date the 1.9 ka White River ash. Radiocarbon will be analyzed to further improve the chronology. Although the ages of the two older events are uncertain, trench and field relations suggest that the most recent event (MRE) occurred close in time to deposition of the 1.2 ka ash. The observations of a lower slip rate on the eastern Denali fault in the Yukon are consistent with previous regional tectonic models. The occurrence of at least three surface- rupturing paleoearthquakes in late Holocene time, and a significant elapsed time since the MRE, emphasizes the need for proper seismic design of proposed natural gas pipelines in this region.