DETERMINING HOLOCENE AND LATE PLEISTOCENE SLIP RATES ALONG THE DENALI FAULT USING COSMOGENIC 10BE ANALYSIS OF BOULDERS ON DISPLACED MORAINES

Preliminary results of in-situ cosmogenic ¹⁰Be analysis from boulders (n=16) and sediment (n=4) collected from one terminal and two lateral moraines offset by the Totchunda and Denali Faults, respectively, suggest ages that range from 12.2±1.3 to 16.7±1.8 ky. The offset moraines have broad crests with relatively few large boulders (> 1 meter in diameter) protruding above the surface. Most of the boulders are composed of quartz pegmatites, granite, and gneiss.

Two of the sampled moraines, which are offset ~150 meters, are located along the part of the Denali Fault that ruptured in November 2002. During sampling, we avoided boulders that were clearly pushed to the surface by permafrost and fault activity. The sediment samples consisted of hundreds of small (~1 cm) clasts from the surface in the area between the sampled boulders. On one moraine the sediment samples (n=2) yielded an average age of 11.9±1.3 ky, identical to the boulders (n=7; 12.2±1.3 ky). On the second moraine the sediment samples (n=2) yielded an average age of11.6±1.2 ky, slightly younger than the boulders (n=5; 14.1± 1.5 ky) although similar within 1s. We interpret this difference being the result of mixing of surface and depth material by bioturbation and freeze-thaw cycles. The four boulders that were sampled from the terminal moraine, offset ~100 m, yielded an average age of 16.7±1.8 ky.

All of the cosmogenic ages were corrected for snow cover of 1 meter over 8 months and boulder surface erosion (1 mm/ky). Age calculation based on cosmogenic analysis are moderately sensitive to the modeled snow cover, but are relatively insensitive to the modeled erosion rate as long as these values are within the accepted range for this region (1-3 mm/ky). Because these moraines were derived from very small drainage basins (1-5 km²), we assume that cosmogenic nuclide inheritance is low. Therefore, ages were not corrected for possible pre-deposition cosmogenic nuclide inheritance.

Preliminary ages of the offset moraines imply an average slip rate since the late Pleistocene of 9 to 13 mm/yr on the Denali Fault and 6 mm/yr on the Totchunda Fault. This difference in rates is expected because slip along the Denali Fault is partitioned between the Denali and Totchunda faults east of the Little Tok River. These rates are consistent with short term paleoseismic rates for the past ~1000 years.